Participants

We discuss the potential of optically levitated SiO2 microdisks to be used as force sensors in gravitational wave detection with the advantage of reduced photon recoil heating. We discuss analytical and numerical models of the motional dynamics of these microdisks, as well as initial experimental trapping results. Similar disks will be used in the Levitated Sensor Detector (LSD) project at Northwestern to detect high frequency (10-100kHz) gravitational waves above the region previously probed by LIGO.

Optically levitated micro-scale sensors within an optical cavity can be used to detect gravitational waves in the 10-300 kHz band, which is beyond the optimal sensitivity range of ground-based detectors like Advanced LIGO. Motivated sources in this high frequency band include binary coalescence of sub-solar-mass primordial black holes and annihilation of gravitationally-bound QCD axions around astrophysical black holes. We present the concept of Levitated Sensor Detector (LSD), the methods of trapping and cooling micro-discs, and the experimental progress of the 1-meter prototype that is under development at Northwestern University.

To conduct wideband (WB) timing of millisecond pulsars (MSPs), especially at larger bandwidths, it is important to model frequency-dependent profile evolution to resolve pulse-broadening effects caused by interstellar scattering. The current method for doing this is to use a data portrait of the pulse profile in terms of both rotational phase and observing frequency and simultaneously measure a phase time of arrival of a pulse and a dispersion measure. Using WB timing techniques on four MSPs in the NANOGrav 15yr Dataset, we investigated how binary timing, particularly the measurement of post-Keplerian parameters, and astrometry benefit from the high cadence observing campaigns on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope.

The Argentine Institute of Radioastronomy (IAR) is equipped with two single-dish, 30-m radio antennas. The Pulsar Monitoring in Argentina (http://puma.iar.unlp.edu.ar/) collaboration in using these to perform daily observations of pulsars in the southern hemisphere. We aim to contribute to pulsar timing studies related to short time-scale interstellar scintillation and searches for sources of continuous gravitational waves. In particular, we used observations of PSR J0437-4715 to perform single pulsar searches of continuous gravitational waves, setting constraints in the nHz-μHz frequency range. More recently, we have applied machine learning techniques to analyze and cluster single-pulses of the Vela Pulsar, thereby obtaining an insight into the pulsar’s magnetosphere and emission mechanism. As a result of this work, we demonstrate IAR's potential for contributing to increase the sensitivity of the existing pulsar timing arrays. References: Gancio, G., Lousto, C. O., Combi, L., et al. 2020, A&A, 633, A84. Sosa Fiscella V., et al., 2021a, Astrophys. J., 908, 158. Carlos O. Lousto et al. 2021. e-Print: 2108.13462 [astro-ph.HE]

Eccentric black hole – neutron star binaries pose not only a challenge for gravitational wave detectors, but also provide a probe into the nuclear equation of state. The tidal interactions at close pericenter passage excite f-modes on the star, which emit their own gravitational waves. We derive an analytic time-domain waveform for these oscillations to leading post-Newtonian order. Decomposing the f-modes in orbital harmonics, we re-sum the infinite series into a simple decaying harmonic oscillator whose amplitude depends not only on the stellar structure, but also on the orbital frequency. We find matches >0.98 between our re-summation and numerical integrations of the response for highly eccentric orbits (e>0.8). Finally, we develop a timing model to track pericenter passages under the effect of radiation reaction and thus build a sequence of f-mode excitations.

Gravitational waves emitted by inner binaries in hierarchical triples are interesting astrophysical candidates for space-based detectors like LISA. In the presence of a third body, the inner binary undergoes oscillations in eccentricity and inclination angle due to the Kozai-Lidov (KL) mechanism. In this talk, I will discuss how to construct analytic gravitational waveforms in the Fourier domain, with this effect included. Using multiple-scale analysis, we make use of the separability of timescales in the system to combine the effects of KL oscillations and radiation reaction. The imprint on the waveform, due to this combined evolution, can then be analytically computed in the stationary phase approximation. I will also discuss our analysis of the parameters of the hierarchical triple which can produce a detectable (by LISA) imprint of KL oscillations on the waveform and outline potential implications for gravitational wave data analysis and tests of GR.

We describe new strategies adopted by our research group to improve the development, performance and accuracy of Effective-One-Body (EOB)-based spin-precessing waveform models. We accomplish this by developing EOB models through an intuitive, python-based implementation that meticulously documents the EOB physics and the corresponding code in Jupyter notebooks. We then build on this implementation by using our own python package Nrpy+ to boost accuracy and performance of the current state-of-the-art approximant SEOBNRv4P. Strategies to accomplish this include the generation of analytical expressions for derivatives on the right-hand sides of the evolution equations and in the initial conditions, as well as the generation of highly optimized C-code to significantly boost performance. We discuss our progress in accomplishing these goals, and present results comparing our implementation to that in the LIGO Algorithms Library. We will also discuss future strategies to vastly improve performance by adopting elegant physically motivated integration techniques and merger models.

Accurate analytical waveform models are crucial for gravitational-wave data analysis, and since spin has significant effect on the binary dynamics, it is important to improve the spin description in these models. In this talk, I will present results for the spin-orbit (SO) coupling at the fifth-and-a-half post-Newtonian (5.5PN) order. The method used splits the conservative dynamics into local and nonlocal-in-time parts, then relates the local-in-time part to gravitational self-force results by exploiting the simple mass-ratio dependence of the post-Minkowskian expansion of the scattering angle. We calculate the nonlocal contribution to the 5.5PN SO dynamics to eighth order in the small-eccentricity expansion for bound orbits, and to leading order in the large-eccentricity expansion for unbound orbits. For the local contribution, we obtain all the 5.5PN SO coefficients from the redshift and spin-precession invariants, except for one unknown at second order in the mass ratio. However, by incorporating these results in the effective-one-body formalism and comparing its binding energy to numerical relativity, we find that the remaining unknown has a small effect on the dynamics.

After the joint detection of GW170817-GRB 170817A, experiments have led to greater constraints on the Hubble constant, speed of gravity, and GRB jet models. Much effort has been made to find similar candidates, including improving the search statistics used. We test the performance of various search statistics based on their ability to distinguish true coincidences from those formed by random chance.

Pulsar timing arrays (PTAs) can detect low-frequency gravitational waves by looking for correlated deviations in pulse arrival times. Current Bayesian searches using PTAs are hampered by the large number of parameters needed to be sampled concurrently with Markov Chain Monte Carlo methods. As the data span increases, this problem will only worsen. An alternative Monte Carlo sampling method, Hamiltonian Monte Carlo (HMC), utilizes Hamiltonian dynamics to produce sample proposals informed by first-order gradients of the model likelihood. This in turn allows it to converge faster to high dimensional distributions. We implement HMC as an alternative sampling method in our search for an isotropic stochastic gravitational wave background, and present the accuracy and efficiency of the algorithm for this analysis. We also discuss implications of tailoring this algorithm to additional gravitational wave searches.

Pulsar Timing Arrays (PTA) are attempting to detect a Stochastic Gravitational Wave Background (SGWB) by measuring the deviations and correlations it causes in pulsar time-of-arrivals. However post-detection, attention will turn towards studying astrophysical phenomena from PTA data. Currently, analysing a single astrophysical model given the data can take days or weeks, and future data sets will feature even more timing data from more pulsars, slowing down analysis. Given that most of the SGWB’s spectral information is carried in the PTA’s autocorrelation terms, factorising the Bayesian likelihood of a PTA would speed-up this work. In this talk, I will introduce the Generalised Factorised Likelihood (GFL), a flexible and accurate method to analyse PTA data within 3 minutes.

The development of reusable artificial intelligence (AI) models for wider use and rigorous validation by the community promises to unlock new opportunities in multi-messenger astrophysics. To that effect, we first train an ensemble of independent neural networks that simultaneously process strain data from multiple detectors. The output of these networks is then combined and processed to identify significant noise triggers. We apply this methodology in O2 and O3 data finding that deep learning ensembles clearly identify binary black hole mergers in open source data available at the Gravitational-Wave Open Science Center. We then develop a workflow that connects the Data and Learning Hub for Science, a repository for publishing AI models, with the Hardware-Accelerated Learning (HAL) cluster, using funcX as a universal distributed computing service. Using this workflow, the ensemble of our four openly available AI models can be run on HAL to process an entire month’s worth (August 2017) of advanced Laser Interferometer Gravitational-Wave Observatory data in just seven minutes, identifying all four binary black hole mergers previously identified in this dataset and reporting no misclassifications. This approach combines advances in AI, distributed computing and scientific data infrastructure to open new pathways to conduct reproducible, accelerated, data-driven discovery.

The optimal statistic is a frequentist method to estimate the amplitude of the stochastic GWB. In presvious analyses, we have calculated the optimal statistic for single, cross-correlated signals. To test the ability of the optimal statistic to retrieve multiple cross-correlated signals with different ORFs, we have created simulated timing residuals that contain a Hellings-Downs signal, or a GW-like monopole signal. We compare the recovered amplitudes and confidence intervals for these simulated data sets using different numbers of cross-correlated signals

Intermediate mass ratio inspiral (IMRI) binaries -- containing stellar-mass black holes coalescing into intermediate-mass black holes (M>100M⊙) -- are a highly anticipated source of gravitational waves (GWs) for Advanced LIGO/Virgo. Their detection and source characterization would provide a unique probe of strong-field gravity and stellar evolution. Due to the asymmetric component masses and the large primary, these systems generically excite subdominant modes while reducing the importance of the dominant quadrupole mode. Including higher order harmonics can also result in a 10%−25% increase in signal-to-noise ratio for IMRIs, which may help to detect these systems. We show that by including subdominant GW modes into the analysis we can achieve a precise characterization of IMRI source properties. For example, we find that the source properties for IMRIs can be measured to within 2%−15% accuracy at a fiducial signal-to-noise ratio of 25 if subdominant modes are included. When subdominant modes are neglected, the accuracy degrades to 9%−44% and significant biases are seen in chirp mass, mass ratio, primary spin and luminosity distances. We further demonstrate that including subdominant modes in the waveform model can enable an informative measurement of both individual spin components and improve the source localization by a factor of ∼10. We discuss some important astrophysical implications of high-precision source characterization enabled by subdominant modes such as constraining the mass gap and probing formation channels.

Following the release of GWTC-2 and the strain data from the first half of the third observing run of the Advanced LIGO and Virgo detectors (April - October 2019), the LVC presented GWTC-2.1, a deep catalog of compact binaries observed during this period. I will discuss the probability of astrophysical origin of these events and the expected number of signals in different source categories contained in the catalog. I will also present the source properties of a subset of the new events in GWTC-2.1.

Binary black holes (BBHs), as observed by the Laser Interferometer Gravitational-Wave Observatory(LIGO) and the Virgo Interferometer, undergo a high mass x-ray binary (HMXB) phase during their binary evolution. However, there are no well-constrained x-ray observations of HMXBs that are predicted to be likely progenitors of BBH gravitational wave sources. In this project, we seek to quantify the relationships between HMXB and BBH populations in order to explain this discrepancy in observations. We use the Compact Object Synthesis and Monte-Carlo Investigation Code (COSMIC) to obtain representative local (z ≤ 0.05) and Universe (z ≤ 20) populations of binaries. We find that all merging BBHs in both populations evolve from HMXB progenitors. We show that detectable LIGO–Virgo sources and detectable HMXBs originate from populations of stellar binaries with different redshift and metallicity distributions. When we factor in observability, we find that no HMXB progenitors of BBHs are observable in x-ray in the universe population, and less than 1% are observable in the local population. Similarly, we calculate that the probability of observing an HMXB that will become an observable merging BBH in the future is less than 1% for the local universe population and 0% for the full universe population. These results demonstrate that observing an HMXB that is a BBH gravitational wave source progenitor is very unlikely, which explains the discrepancy in observations

M-dwarfs, or red dwarfs, are the smallest stars on the main sequence in the universe. M-dwarfs are the most common star in the Milky Way, and have become increasingly popular because they have been found to often harbor exoplanets. This is likely because M-dwarfs are stable, living the longest lives of all the stars on the main sequence. This project is interested specifically in M-dwarf binary systems. Using stellar catalogs to determine the fundamental range of parameters of M-dwarfs and M-dwarf binary systems, our goal is to simulate the tidal interactions between two stars as they orbit close to each other. Specifically, we wish to observe how mass transfer occurs as one star fills its Roche Lobe, and whether or not the stars can remain in close stable orbit. The interactions are simulated using smoothed particle dynamics software.

Analyzing all detections made by LIGO/Virgo to date as a single population of compact binary coalescences, we search for features in the mass distribution at the transition between neutron stars and black holes. We find a deviation from a single power law describing both neutron stars and low-mass black holes at $2.99 ^{+1.29}_{-0.74} M_{\odot} $, which is consistent with many neutron star equation-of-state predictions for the maximum neutron star mass. We search for evidence for the purported lower mass gap and find that it cannot be entirely empty and has edges at $ 2.50 ^{+0.34}_{-0.49} M_{\odot}$ and $ 6.31^{+1.60}_{-1.67} M_{\odot}$, if it exists. We then put events that have been deemed ``exceptional" by the LIGO-Virgo-KAGRA collaboration in the context of these features. The models presented in this analysis are the first to be able to accommodate the ambiguously classified event GW190814, which is an outlier with respect to the binary black hole population. We additionally present rates of neutron star-black hole mergers, and predict the fraction of such events that will result in an electromagnetic counterpart.

Gravitational lensing of gravitational waves is expected to be detected in the upcoming observing runs. We investigate how non-lensed events could mimic lensed events because of overlap of parameters due to astronomical coincidence and what type of event could provide a conclusive detection of strong lensing. In particular, we construct a mock catalog of lensed and non-lensed events and estimate their parameter overlap in the chirp mass, sky location, and coalescence phase, as expected of the lensing hypothesis. We find that the probability of a false alarm based on overlaps of the chirp mass, sky location, and coalescence phase are approximately 13%, 10%, and 10% per pair, respectively. Combining the three, we arrive at a false alarm probability per pair of ∼ 1/1000 based on the simultaneous overlap of parameters. We cross-validate our simulation against the O1, O2, and O3a data, arriving at a false alarm probability per pair of 16% and 15% based on overlaps of the chirp mass and total mass respectively – consistent with our simulation. Overall, we find that high signal-to-noise ratio events with more than two detected images may better discriminate lensed events. Low-mass events could also be better at discriminating lensed events, owing to their more accurate chirp mass measurements. The probability that we find at least one false alarm in a population of O(10) and O(100) events is ∼ 6% and ∼ 99%, respectively. When we expect O(1000) events at design sensitivity, the probability that we find at least one false alarm is approximately equal to 1 – showing how the false alarm probability rapidly grows with the increasing number of events.

Strong gravitational lensing of gravitational wave sources offers a novel probe of both the lens galaxy and the binary source population. In particular, the strong lensing event rate and the time delay distribution of multiply-imaged gravitational-wave binary coalescence events can be used to constrain the mass distribution of the lenses as well as the intrinsic properties of the source population. We calculate the strong lensing event rate for a range of second (2G) and third generation (3G) detectors, including Advanced LIGO/Virgo, A+, Einstein Telescope (ET), and Cosmic Explorer (CE). For 3G detectors, we find that ∼ 0.1% of observed events are expected to be strongly lensed. We predict detections of ∼ 1 lensing pair per year with A+, and ∼ 50 pairs per year with ET/CE. These rates are highly sensitive to the characteristic galaxy velocity dispersion, σ∗, implying that observations of the rates will be a sensitive probe of lens properties. We explore using the time delay distribution between multiply-imaged gravitational-wave sources to constrain properties of the lenses. We find that 3G detectors would constrain σ∗ to ∼ 21% after 5 years. Finally, we show that the presence or absence of strong lensing within the detected population provides useful insights into the source redshift and mass distribution out to redshifts beyond the peak of the star formation rate, which can be used to constrain formation channels and their relation to the star formation rate and delay time distributions for these systems.

Rapid binary population synthesis codes are often used to investigate the evolution of compact-object binaries. They typically rely on analytical fits of single-star evolutionary tracks and parameterized models for interactive phases of evolution (e.g., mass-transfer on thermal timescale, determination of dynamical instability, and common envelope) that are crucial to predict the fate of binaries. These processes can be more carefully implemented in stellar structure and evolution codes such as MESA. To assess the impact of such improvements, we compare binary black hole mergers as predicted in models with the rapid binary population synthesis code COSMIC to models ran with MESA simulations. In this talk I will discuss how the predictions of these two codes differ in terms of formation paths, the orbital periods and mass ratios of merging binary black holes, and consequently merger rates. Depending upon the black hole donor mass, and mass-transfer and common-envelope physics, at sub-solar metallicity COSMIC overproduces the number of binary black hole mergers with a significant fraction of them having merger times orders of magnitude shorter than the binary black holes formed when using detailed MESA models. Therefore we find that some binary black hole merger rate predictions from rapid population syntheses of isolated binaries may be overestimated. We conclude that the interpretation of gravitational-wave observations requires the use of detailed treatment of these interactive binary phases.

Common envelope evolution (CEE) is thought to play an important role in the formation of many systems of astrophysical interest, including X-ray binaries, type Ia supernovae, double neutron stars and double black holes. In spite of its importance, our knowledge of CEE remains limited by difficulties in theoretical modeling as a result of the number of different physical processes involved. In this talk, I discuss the integration of new physics into 3-D numerical simulations of CEE using the moving-mesh code MANGA. CEE simulations to date have modeled the companion object as a point particle, but the accuracy of this approximation is unclear since it neglects hydrodynamic interactions between the envelope and the surface of the companion. To investigate the effect of such interactions, the surface of the companion can instead be treated as a hard boundary. To this end, I describe the implementation of moving boundary conditions into MANGA and their application to simulations of CEE. I find that the orbital mechanics of the spiral-in phase are affected by the size of the companion, suggesting that hydrodynamic interactions between the companion and envelope affect the transport of angular momentum. I have also implemented a magnetohydrodynamics algorithm into MANGA, using a divergence-cleaning scheme to ensure that the divergence of the magnetic field is zero (or at least small). Preliminary results show that the magnetic field strength is rapidly amplified near the companion at the onset of CEE, creating a toroidal field which is thought to assist in creation of nebulae at late times.

Over the last decade, observations of a class of optical transients called, Intermediate Luminosity Optical Transients(ILOTs) that are brighter than novae, but dimmer than supernovae, suggest that at least a subset of them are the results of stellar mergers. However, a direct connection between these observed systems and numerical simulations of common envelope evolution has not been made. In an effort to make this connection, we develop a simplified framework to compute the observational appearance of common envelope evolution simulations. Here, we assume radial leakage of radiation from the radiative cooling layer in the thin-shell approximation. In this way, we can utilize purely hydrodynamic simulations of CEE without resorting to radiative hydrodynamics simulations of CEE, which is currently beyond the state of the art. We present a few example calculations as part of this ongoing work.

The tidal response of a compact object is a key gravitational-wave observable encoding information about its interior. This link is subtle due to the nonlinearities of general relativity. We show that considering a scattering process bypasses challenges with potential ambiguities, as the tidal response is determined by the asymptotic in- and outgoing waves at null infinity. As an application of the general method, we analyze scalar waves scattering off a nonspinning black hole and demonstrate that the low-frequency expansion of the tidal response reproduces known results for the Love number and absorption. In addition, we discuss the definition of the response based on gauge-invariant observables obtained from an effective action description, and clarify the role of analytic continuation for robustly (i) extracting the response and the physical information it contains, and (ii) distinguishing high-order post-Newtonian corrections from finite-size effects in a binary system. Our work is important for interpreting upcoming gravitational-wave data for subatomic physics of ultradense matter in neutron stars, probing black holes and gravity, and looking for beyond-standard-model fields.

The tidal response of compact stars in a binary system is a useful probe of the stellar interior. The tide can be decomposed into a quasi-static component, the so-called equilibrium tide, and a dynamical component that depends on the resonant response of the normal modes of the star. While the effect of equilibrium tide dominates the matter response for systems with a large orbital separation, the dynamical tide starts to become important as the separation becomes smaller and the orbital period gets closer to the matter response time. In this presentation, I will focus on the effect of dynamical tides on the gravitational wave signals from eccentric binary white dwarf systems. These systems are one of the target sources of the proposed space-based gravitational wave detector (LISA). We show numerically that for orbits with high eccentricities and small separations, the dynamical tide can cause a chaotic growth of the normal mode amplitudes and cause the orbit to evolve in a random manner, leading to a chaotic waveform. Meanwhile, for systems with lower eccentricities and larger separations, the dynamical tide affects the orbit slightly by introducing extra dependence on the normal mode frequencies. This causes slight amplitude modulations of the gravitational wave emitted, which implies that normal mode oscillations can potentially be detected with the inspiral signal.

Ultra-compact binaries in short period orbits can have strong tidal interactions between the individual stars. Resonant tidal forcing occurs when the tidal forcing frequency of a binary matches a quadrupolar oscillation mode of one of the binary members and energy is transferred from the orbit to the resonant mode of the star. At small binary separations during tidal lock, it is possible a significant fraction of binary orbital energy can be deposited quickly into a resonant mode and the orbit decays faster than it would by gravitational radiation alone. We are considering a population synthesis of the Milky Way and considering whether these resonant interactions are important for any of the close white dwarf binaries in the model, in order to evaluate whether or not LISA will be able to place bounds or constraints on the tidal interactions.

Gravitational Waves(GW's) observed from the Binary Neutron Star(BNS) coalescence GW170817 have been used to probe the nature of extreme matter inside neutron stars. Inferring the Neutron Star (NS) Equation of State (EoS) from GW data involve multiple nested sampling or Parameter Estimation (PE) runs per event, making it computationally expensive. Given the number of expected BNS detections in the 4th observing run of LIGO-VIRGO (O4), this problem can only worsen. An alternative rapid EoS model comparison technique: GWXtreme (Ghosh et. al 2021) that reuses mass and tidal-deformability posterior samples, has been shown to significantly speed up EoS model comparison requiring only one PE run per event. In this work, we generalize this technique to incorporate inference of phenomenologically parametrized EoS models, motivated by the superiority of using such models in constraining the properties of NS matter from GW data, over known named EoS models. We perform our new, computationally cheaper analysis on both real and simulated sources, and demonstrate the accuracy of our results in comparison to full PE runs and injected EoS parameters respectively.

With recent observations of gravitational wave signals from binary neutron star(BNS) mergers and observations by NICER, the nuclear equation of state(EoS) is becoming increasingly testable by numerical simulations. Numerous simulations currently exist exploring the equations of state at different density regimes for the constituent neutron stars. In this work we perform full GR three-dimensional hydrodynamics simulations of BNS mergers for parameterized EoSs based on quark matter at the highest nuclear densities. We construct our initial data using Lorene followed by simulating the merger with Einstein Toolkit. The goal of this study is to extract the effects on the observed GW waveform as the merger happens caused by quark matter.

We investigate the quark deconfinement phase transition in the context of binary neutron star (BNS) mergers. We employ a new finite-temperature composition-dependent equation of state (EOS) with a first-order phase transition between hadrons and deconfined quarks to perform numerical relativity simulations of BNS mergers. The phase transition is imprinted on the postmerger gravitational wave (GW) signal duration, amplitude, and peak frequency. However, this imprint is only detectable for binaries with sufficiently long-lived remnants. We also study the impact of the phase transition on dynamical ejecta, remnant accretion disk masses, r-process nucleosynthetic yields, and associated electromagnetic (EM) counterparts.

It is believed that 23% of our universe is composed of dark matter, and 5% of normal matter. The dark matter is often to be modeled as a scalar field. However, given the complexity of Stand Model, why should dark matter be so simple even though it occupies a larger portion of the universe? We considered Twin Higgs mirror matter, which is a dark matter model that is a copy of Stand Model with its own interaction and particle species but weakly interacting with Stand Model particles. We consider a neutron star that is entirely made of mirror matter and discuss the possibility of detection through gravitational wave observation.

Gravitational-wave cosmology began in 2017 with the observation of the gravitational waves emitted in the merger of two neutron stars, and the coincident observation of the electromagnetic emission that followed. Although only a 30% measurement of the Hubble constant was achieved, future observations may yield more precise measurements either through other coincident events or through cross correlation of gravitational-wave events with galaxy catalogs. Here, we implement a new way to measure the Hubble constant without an electromagnetic counterpart and through the use of the binary Love relations. These relations govern the tidal deformabilities of neutron stars in an equation-of-state insensitive way. Importantly, the Love relations depend on the component masses of the binary in the source frame. Since the gravitational-wave phase and amplitude depend on the chirp mass in the observer (and hence redshifted) frame, one can in principle combine the binary Love relations with the gravitational-wave data to directly measure the redshift, and thereby infer the value of the Hubble constant. We implement this approach in both real and synthetic data through a Bayesian parameter estimation study in a range of observing scenarios. We find that for the LIGO/Virgo/KAGRA design sensitivity era, this method results in a similar measurement accuracy of the Hubble constant to those of current-day, dark-siren measurements. For third generation detectors, this accuracy improves to ≲10% when combining measurements from binary neutron star events in the LIGO Voyager era, and to ≲2% in the Cosmic Explorer era.

The gravitational wave ringdown spectra of compact objects provide unique opportunities to probe the most extreme objects in the Universe. In this talk, I present recent work on how to constrain black holes from quasi-normal modes. It will be demonstrated how theory-agnostic approaches, starting from a parametrized metric or parametrized perturbation equations, can be combined with Markov chain Monte Carlo (MCMC) analysis or principal component analysis (PCA) to infer possible deviations from general relativity.

While Einstein’s theory of general relativity (GR) has been extremely successful at describing our astrophysical observations for the past 100 years, there are a variety of contending theories of gravity that can help solve some long standing issues in physics. In this search for physics beyond GR, gravitational wave (GW) astronomy is proving to be an effective tool at constraining these other theories of gravity. To do this, one must extract information about the dynamics of merging compact object binaries through a parameter estimation analysis, exposing which regions in parameter space are consistent with our universe for these theories beyond GR. One can combine information from multiple sources of GW, so as catalogs of these GW sources grow, the power of large numbers of sources will become increasingly important. In this talk, I will discuss the techniques and results of a recent work we have published, in which we combined information from multiple GW sources to attempt to place tight, robust constraints on a few specific theories of gravity that incorporate higher order corrections to the Einstein-Hilbert action, Einstein-dilaton-Gauss-Bonnet (EdGB) and dynamical Chern Simons (dCS) gravity. I will first outline the methods used to analyze the data from each source and how the sources in question were selected. Following this discussion on the methodology, I will present some of the analysis we carried out to verify the robustness of our constraints, including the lack of information about higher order corrections to our waveforms due to these modified theories of gravity and the choice in waveform model within GR. Finally, I will discuss the conclusions of our work, including the extent to which we could constrain EdGB and our continued inability to place meaningful constraints on dCS.

GW190521 in a class of its own is a different signal, being the most massive BBH event observed to date. The exceptionally dominant ringdown of this event and its large mass makes it an optimal candidate to search for GW echoes (if they exist). In this talk I will describe an unprecedented for echoes that makes use of two well-established platforms: matched-filtering (based on the PyCBC algorithm) and a burst search (based on the coherent WaveBurst pipeline - cWB). During the talk, I will show that not only do the results from these two independent techniques agree in every respect but also are in harmony with theoretical predictions.

Our ability to test gravity with gravitational wave detectors is limited by our ability to accurately construct a gravitational waveform template. Thus, it is important to construct such templates for modified theories of gravity. Einstein-æther theory is a particularly interesting modified theory of gravity because it is the most generic Lorentz-violating theory one can construct with one additional vector field and its first derivatives. As well as modifying the amplitude and phase of the gravitational wave, this theory contains scalar and vector polarizations in addition to the tensor polarizations of general relativity. Therefore, any waveform model that can describe gravitational waves in this theory must include extra polarizations in the detector response function, which can travel with speeds different than that of light. In this talk, I will describe the construction of such a waveform through modification of the general relativity IMRPhenomD_NRTidalv2 model (used by the LIGO/VIRGO Collaboration) and explain how recent calculations of the sensitivity for neutron stars in the theory make this possible. I will outline the current constraints on the theory and describe our plan for placing further constraints with gravitational wave data.

In models that allow for additional spacetime dimensions, energy from gravitational waves can leak into these extra spacetime dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and are thus a source of potential systematics in the inferred luminosity distances to gravitational wave sources. Since binary black hole (BBH) mergers are standard sirens, we use the PISN mass gap and its expected features to set a mass-scale and thus be able to break the mass-redshift degeneracy. We simultaneously fit for the BBH population and extra spacetime dimensions parameters using existing BBH observations with a mass model that takes into account the PISN mass gap. We set constrains on the number of spacetime dimensions using different gravitational leakage models and find constraints on the BBH population parameters as inferred previously in recent work.

Relations between the neutron star moment of inertia, tidal Love number and quadrupole moment are known to be insensitive to the nuclear equation of state (the so-called I-Love-Q relations). Such universal relations are powerful for testing general relativity and beyond in the strong-field regime with neutron star observations. Ho\v rava-Lifshitz gravity is one such alternative theory of gravity which has interesting properties such as ultraviolet completion of gravity while also inducing a preferred time direction. This theory is characterized by three coupling constants; two of them have been constrained stringently from existing neutron star observations, such as GW170817, while the remaining parameter is only weakly constrained. We thus studied how the I-Love-Q relations depend on this third parameter. We found that this sole parameter disappears from the field equations in Ho\v rava-Lifshitz gravity. Therefore, the I-Love-Q relations are universal against not only the nuclear physics uncertainty but also the gravitational physics uncertainty within Ho\v rava-Lifshitz gravity.

Gravitational wave observations of compact binary coalescences provide precision probes of strong-field gravity. There is thus now a standard set of null tests of general relativity (GR) applied to LIGO-Virgo detections and many more such tests proposed. However, the relation between all these tests is not yet well understood. We start to investigate this by running a set of standard tests on simulated observations of binary black holes in GR and with phenomenological deviations from GR. The phenomenological deviations include self-consistent modifications to the energy flux in an effective-one-body (EOB) model, the deviations used in the TIGER and FTA parameterized tests, and the dispersive propagation due to a massive graviton. We consider four types of tests: residuals, inspiral-merger-ringdown consistency, parameterized (TIGER and FTA), and modified dispersion relation. We also check the consistency of the unmodelled reconstruction of the waveforms with the waveform recovered using GR templates. These tests are applied to simulated observations with both large and small deviations from GR. We find that while very large deviations from GR are picked up with high significance by almost all tests, more moderate deviations are picked up by only a few tests, and some are not recognized as GR violations by any test at the moderate SNRs we consider. Moreover, we find that the tests that find the largest deviation from GR are not necessarily the expected ones, and the TIGER and FTA 2PN (1PN) tests recover much smaller deviations than the true values in the modified EOB (massive graviton) cases.

Employing the free energy landscape, we study the phase transition and its dynamics for a class of regular black holes in Anti-de Sitter spacetime governed by the coupling of non-linear electrodynamics, which reduces to Hayward and Bardeen solutions for particular values of spacetime parameters. The Fokker-Planck equation is solved numerically by imposing the reflecting boundary condition and a suitable initial condition, using which, we investigate the probabilistic evolution of regular AdS black holes. In this approach, the on-shell Gibbs free energy is treated as a function of the radius of the event horizon, which happens to be the order parameter of the phase transition. The numerical solution is also obtained for the absorbing boundary condition. The dynamics of switching between the coexistence small black hole phase and large black hole phase due to the thermal fluctuation is probed by calculating the first passage time. The effect of temperature on the dynamical process is also investigated.

The gravitational-wave millihertz band that LISA is sensitive to will contain a diverse range of signals, both from within and outside our galaxy. Many of these signals will be unresolvable and will produce a stochastic confusion noise by incoherently overlapping with each other. The angular structure of the noise contains information about the distribution of the sources in the sky, their astrophysics, and their evolution over cosmological time scales. We present a Bayesian algorithm to infer the angular distribution of the confusion noise with a spherical harmonic basis. A novel technique based on Clebsch-Gordan coefficients is developed to ensure that the spherical harmonic expansion is physical and describes only distributions of non-negative power. This is demonstrated through a series of simulations of distributed and localized LISA sources, including a simplified model of the galactic white dwarf foreground. This method will be vital for simultaneously inferring the properties of resolvable sources along with the foreground.

The classical Einstein-Hilbert Lagrangian derivation of the Einstein equation leaves the form of the classical mass-energy tensor unspecified. We show how the Dirac Lagrangian can also be used to derive an Einstein equation in a way that specifies the form of the mass-energy tensor in terms of the Dirac wavefunctions and their derivatives.

Testing strong gravity regimes such as the vicinity of black holes is likely to be attainable with the future developments of observing technology. In this talk, adopting a theory-agnostic approach, we first propose a class of Kerr-like rotating black holes, whose Z_2 symmetry is generically broken. We focus on the possibility that such a violation of Z_2 symmetry is induced by the spin of the black hole. This class of Kerr-like spacetimes could be a good approximation to general black hole solutions in effective low-energy theories of a fundamental quantum theory of gravity. In the model, the violation of the Z_2 symmetry can be parametrized by a single parameter. Then, we discuss how the Z_2 asymmetry in the spacetime could give interesting astrophysical consequences which may be observable.

The LIGO and Virgo collaborations have observed the coalescence event GW190814 involving a 2.6 solar masses object with a 23 solar masses black hole. Thorne-Zytkow objects (TZOs) can eventually become low mass gap black holes. I will show how those black holes end up in binaries with other stellar mass black holes and compare to the reported rate for GW190814-type of events (1-23 $\Gpc^{-3} \yr^{-1}$). TZOs in dense stellar clusters can not alone explain the LIGO-Virgo rate. However, TZOs formed within hierarchical triple systems in the field with the third more distant star being the progenitor of a stellar mass black hole, may be able to give a rate comparable to that of GW190814-type events. If TZOs are connected to the GW190814-type events, then future observations should discover mergers between stellar mass and low mass gap black holes, with the lower mass spanning the entire low mass gap range.

LIGO/Virgo detects gravitational wave strain signals and fits a range of theoretical waveforms to each signal to constrain the range of possible parameters for the compact objects. We have recently developed a Bayesian method to interpret the formation of binary black holes whose mergers produce the gravitational waves detected by LIGO/Virgo using a model of assumed astrophysical and cosmological priors, publicly available parameter estimation samples and our code dart_board. In a recently published paper, we present results for the first gravitational wave system, GW150914, determining the range of possible progenitors and their final states as compact objects, including updates on the black hole masses. In this talk I will describe this method and then provide our preliminary findings when we apply our method to the entire set of binary black hole detections from the LIGO/Virgo O3A dataset. For example, the dart_board posterior of compact objects accounts for isolated binary stellar evolution and cosmology to place tighter constraints on the masses of these compact objects than the publicly available parameter estimation samples.

We show that Dark Matter consisting of ultralight bosons in a Bose-Einstein condensate induces, via its quantum potential, a small positive cosmological constant which is close to the observed value. This explains why the densities of Dark Matter and Dark Energy are approximately equal.

In unimodular gravity the stress-energy tensor of matter is not necessarily conserved, and so the theory offers a natural framework for interacting dark energy models, where dark energy nonetheless has a constant equation of state w = −1. We derive the equations for cosmological perturbations, focusing on the scalar sector. Then we focus on a specific model where the energy-momentum transfer potential is proportional to the energy density of cold dark matter; this transfer potential has the effect of inducing a non-zero effective equation of state for cold dark matter. We analyze in detail the evolution of perturbations during radiation domination on super-Hubble scales in the tight-coupling regime, showing that the well-known non-adiabatic large-scale instability that affects a large class of interacting dark energy models is absent in our model. Also, during matter domination the evolution of the gravitational potential and density perturbations outside the sound horizon is the same as in the generalized dark matter phenomenological model.

The mass, spin, and kick velocity of a black hole formed from the collision of two smaller black holes can be calculated with general relativity. For stellar mass remnants, these remnants will persist for much longer than the age of the universe, assuming no interactions with other astrophysical bodies. In this work, we use the inferred population of in-spiraling black hole masses and spins to infer the population properties of the remnant black holes and their current day number density. We find that a Milky-Way-equivalent galaxy at redshift zero should contain O(50,000) remnant black holes.

The origin of the energy behind the observed relativistic jets in several accreting objects has been, so far, best explained by the Blandford-Znajek process. This process may serve as a way to probe the gravity in the strong regime, and it has been extensively studied in general relativity. How does the Blandford-Znajek process change in alternative theories of gravity? Will the changes help with the test of these theories? This talk will present some basics for the discussion of Blandford-Znajek process in quadratic gravity theories, a brief review two well-motivated quadratic gravity theories: scalar Gauss-Bonnet and dynamical Chern-Simons gravity, and summarize the characteristics of their black hole solutions under the small-coupling, slow-rotation approximation. To make these solutions useful in the analysis of Blandford-Znajek process, we have developed a resummation strategy to recover the exact Kerr solution in their general relativity limit and to shift their coordinate singularities to the corresponding perturbed event horizons. This resummation strategy may also support other applications in which the near-horizon physics needs to be resolved.

The local universe expansion rate is one of the most fundamental and essential cosmological parameters. This value which is known by the name of Hubble’s Constant is scientifically measured by electromagnetic sources called distance ladder. Surprisingly, using Gravitational Wave (GW) analysis this value can be measured making GW sources another significant method to act as standard sirens with their electromagnetic counterparts from their host galaxy. The gravitational wave event GW 170817 was the outcome of the merger of two different neutron stars. The electromagnetic event was recorded from the host galaxy NGC4993. The GW170817 has been a considerable success in this direction measuring the value of universe acceleration H0 = 70.0+12.0 −8.0 kms−1 Mpc−1 The results obtained from this GW event is analyzed and compared with the other observations done with traditional methods of CMB and Cosmic Distance Ladder. Another event in this series GW190817 is Compact Binary Coalescence involving a 22.2 – 24.3 Solar Masses Blackhole and a compact object with a mass of 2.50 – 2.67 Solar Masses. The EM counterpart of this event is unknown so far and hence the event is named Dark Siren. The detection of gravitational waves from merger event GW190814 reveals a different picture. The Hubble’s value proposed with this even is close to H0 = 75+59 −13 km s−1Mpc−1 .The source GW190814 which involves a massive black hole and the other compact object as the lightest black hole or the heaviest neutron star was localized to 18.5 deg2 at a distance of Mpc. The diversity of the two events can be used to measure the expansion rate of the universe. This can also be used to verify this rate in different directions of the universe. Key Words Gravitational Waves; Universe Acceleration; Hubble's Constant; Standard Siren; Dark Siren; GW170817; GW190814; General Relativity; Black Hole; Binary Mergers

We will address the existence of a new symmetry for an imperfect fluid by introducing local four-velocity gauge-like transformations for the case when there is vorticity [1]. A similar tetrad formulation as to the Einstein-Maxwell spacetimes formalism presented in previous manuscripts [2,3] will be developed in this manuscript for the imperfect fluids. The four-velocity curl and the metric tensor will be invariant under these kind of four-velocity gauge-like local transformations. While the Einstein-Maxwell stress- energy tensor is locally gauge invariant under electromagnetic gauge transformations, the perfect fluid stress-energy tensor will not be invariant under four-velocity gauge- like local transformations. We will dedicate our analysis to the imperfect fluid stress-energy tensor that will be invariant under local four-velocity gauge-like transformations when additional transformations are introduced for several variables included in the stress-energy tensor itself. We will also pay special attention to the construction of a vorticity stress-energy tensor invariant under local four-velocity gauge-like transformations. An application on neutron stars will be developed in order to show the simplications brought about by these new tetrads [4-6]. REFERENCES [1] A. Garat, New symmetry for the imperfect fluid, Eur. Phys. J. C, 80 4 (2020) 333. https://doi.org/10.1140/epjc/s10052-020-7887-9 [2] A. Garat, J. Math. Phys. 46, 102502 (2005). A. Garat, Erratum: Tetrads in geometrodynamics, J. Math. Phys. 55, 019902 (2014). [3] A. Garat, Einstein-Maxwell tetrad grand unification, Int. J. Geom. Methods Mod. Phys., (2020) 2050125. DOI: S021988782050125X. [4] A. Garat, Euler observers in geometrodynamics, Int. J. Geom. Meth. Mod. Phys., Vol. 11 (2014), 1450060. arXiv:gr-qc/1306.4005 [5] A. Garat, Covariant diagonalization of the perfect fluid stress-energy tensor, Int. J. Geom. Meth. Mod. Phys., Vol. 12 (2015), 1550031. arXiv:gr-qc/1211.2779 [6] A. Garat, Euler observers for the perfect fluid without vorticity, Z. Angew. Math. Phys. (2019) 70: 119.

We consider scenarios where a mass gap black hole is formed as the end state of a Thorne-Zytkow object: a red giant with a neutron star in its core. We estimate the rate for such mass gap objects to subsequently merge with a black hole.

General relativistic numerical mangeto-hydrodynamics simulations are use to simulate the merger of neutron stars, disks around black holes as well as supernovae. The Einstein Toolkit is a community driven framework for numerical astrophysics simulations that is used by many group throughout the world. CarpetX is an in development adaptive mesh refinement driver for the Einstein Toolkit which leverages the AMReX mesh refinement library that is being developed as part the Exascale project. It improves on the current mesh refinement driver, Carpet, by supporting fine grained mesh refinement to track features in the simulations and native support for GPU accelerated computing. In this talk I will present an outline of CarpetX's design and current capabilities.

There exists a fifth spacetime dimension where the universe expands along two degrees of freedom, parallel and normal to light. A tetherball path around its pole (the origin of the universe) is the parallel direction. The radial path, away from the pole, increasing by one radius per revolution, is the normal path. These two paths alter the equations of gravity - gravitational constant, deleting the cosmological constant, and adding a dimension to the Riemannian manifold. They may redefine how time, and space are perceived and help address other unexplained phenomena. At the Universe’s inception, near infinite mass density created near infinite spacetime curvature. Since then, the universe has expanded through the fifth dimension, decreasing mass density, thus unwinding spacetime, the illusion of dark energy. This is one example of the many enigmas answered by the fifth dimension. Gravity acts in the axis normal to light, whereas light on this axis only moves through quasi tunneling. Parallel expansion holds standard gravity where normal expansion holds composite gravity. This would result in different rates of time with equilibration events that convert time directly into energy. This might explain certain high energy events like gamma-ray bursts possibly down to time crystals accounting for unchanging entropy with no energy input. Further fifth spacetime implications may help develop the theory of everything. It would advance precision of measurement key to developing practical quantum computers, fusion reactors, and better health care.

From a recent Blue Waters PEP internship, computational data is available from thousands of LAMMPS MD simulations. Analysis of the temperature and pressure dependencies of sound wave velocities and frequency spectra in the highly symmetric (hexagonal and fcc) lattices provides the basis for deriving effective/acoustic metrics. Petascale parallel processing aided the analysis over a large parameter space of varied temperatures, pressures, and interatomic potentials (LJ and related). While reviewing only an initial case study, the long-term goal of this work is to test the feasibility of reverse-engineering interatomic potentials to produce effective metrics of interest to researchers in analog general relativity.

Merging compact binaries that form in isolation need to survive two supernova explosions and one or more common-envelope phases. As a compact object undergoes common-envelope evolution, it accretes from the surrounding stellar plasma and gravitational drag drives its orbital inspiral. If enough energy is dissipated into the common envelope, the envelope is ejected, leaving the binary at a smaller separation. In addition, the accretion rates are expected to exceed the Eddington limit due to the high densities and temperatures of the envelope. Does accretion onto the compact object result in outflows that contribute to common-envelope ejection? We use the Cosmos++ code to perform 2D axi-symmetric general-relativistic M1 radiation hydrodynamics simulations of hyper-Eddington black-hole accretion. We characterize the outflow rate and its angular dependence over a range of black-hole masses, inflow rates, and flow angular momenta. I will also discuss the prospects for using these general-relativistic simulations to inform subgrid feedback models in global common-envelope simulations.

Platonic symmetries as responsible for fermion generations; the six quark flavors come from duality. A qualitative analysis, similar to the Ricci tensor picture in GR, of the interaction between the fractional charges of quarks, at Platonic nodes, of two EM-neutral neutrons for example, yields a candidate for Gravity. In conclusion Gravity is not a fundamental interaction and can in principle be controlled via dynamic nuclear orientation.

We consider a scenario of spontaneous baryogenesis in a frame-work of warm quintessential inflation where the residual inflaton field, left out after warm inflation, plays the role of quintessence field at late times and is coupled to a non-conservedbaryonic current. Assuming a four fermion (B−L) violating effective interaction, we have demonstrated that the required baryon asymmetry can be produced successfully in this case.We show that the post-inflationary evolution, with the underlying scalar field potential, V(φ) \propto V_0 exp (−αφ^n/),n >1 well suited to warm inflation, exhibits scaling behaviour soon after a brief kinetic regime. We show that the coupling of the scalar field to massive-neutrino matter can give rise to exit from the scaling regime to cosmic acceleration at late-times as massive neutrinos turn non-relativistic. The proposed model is shown to success-fully describe the cosmic history from inflation to late time acceleration, with the evolution independent of initial conditions, along with the the generation of baryon asymmetry during the post-inflationary era.

We perform scattering experiments of initially spinning black holes (BHs) using numerical relativity. We find the critical impact parameter for scattering versus non-scattering binary black holes (BBHs). In the case of BHs scattering off each other, we find a spin-up of the outgoing BHs. Using Canuda and the Einstein Toolkit on the Blue Waters supercomputer, we numerically solve the field equations. We present our latest results of BH scattering simulations which can be used to inform the construction of waveform templates for LIGO or LISA, perform population studies, and estimate spin distribution in BH binaries.

Recently, this author proposed a novel interpretation of quantum mechanics called the Heisenberg Interpretation. Its principal difference from the textbook formalism is a mathematical distinction between things which exist merely as possibilities and those which exist as actualities. This is achieved by means of representing the latter as elements of a set separate from the Hilbert space, called the classical states set, and associated with quantum measurements. The distinction imposes a hard boundary between the quantum and classical domains, which under identification of the latter with the domain of general relativity leads to the prediction of novel phenomena because under the current paradigm, there is no such boundary. As a first step toward testable precision predictions, this paper characterizes the predictions generated from the existence of such a boundary mainly at a conceptual level and classifies them by the testability.

The origin of the energy behind the observed relativistic jets in several accreting objects has been, so far, best explained by the Blandford-Znajek process. This process may serve as a way to probe gravity in the strong-field regime, and it has been extensively studied in general relativity. How is the Blandford-Znajek process modified in quadratic gravity theories? In this talk, I will show analytical solutions to the Blandford-Znajek process in two well-motivated quadratic gravity theories: scalar Gauss-Bonnet and dynamical Chern-Simons gravity. The mechanism was found assuming the split-monopole configuration and under the small-coupling and slow-rotation approximations. I will finish by comparing our results with the general relativity prediction, and discuss what they imply to tests of quadratic gravity theories through the observations of active galactic nuclei.

Heisenberg’s uncertainty principle is the principle in quantum mechanics. Very roughly, it states that we know everything about where a particle is located but we know nothing about its momentum and vice versa. Versions of the uncertainty principle also exist for other quantities as well, such as energy and time, the uncertainty principal is only working for the subatomic particle. According to the uncertainty principle, if the position is known then the momentum is more uncertain and vice versa. the uncertainty principle is not correct because we can calculate the momentum and the velocity at a time. Mr. Heisenberg said imagine there's an object and you can’t measure its position and velocity at a time but I say we can measure its position and velocity at a time. suppose you have to measure the position and velocity of an object which does not exist at a time but you can't, because the uncertainty principle says you can't measure the position and velocity at a time, But I say we can measure their position and velocity at a time let’s see how. Imagine there are 2 subatomic particles, Particle A which is a real particle, and particle B which is just an imaginary particle. A Once you start imagining the particles it will give you a better understanding and you can easily measure the particle's position and velocity at a time. Measure their mass and stick them together. A B Mass of A particle should be equal to mass of particle B and particle B mass should be equal to particle A mass. Then only we can find the position and velocity of 1st particle and simultaneously of 2nd Imaginary particle. After measuring the mass and sticking the particles, we will spring them apart in a different direction. A B After springing apart Particles A and B, find the position of particle A and after finding the position then find the velocity of particle B. Then, after you got the position of A particle and velocity of B it means you got the position and velocity of both the particles simultaneously. Now you can stop imaging particle B because you have found the position and velocity of particle A with the help of the imaginary particle which was B. Hence, the uncertainty principal is proven wrong.

Binary neutron star mergers can be sources of gravitational waves coincident with electromagnetic counterpart emission across the spectrum. To solidify their role as multimessenger sources, we present fully general relativistic, magnetohydrodynamic simulations of binary neutrons stars modeled by nuclear equations of state (SLy and H4) on quasicircular orbits that merge and undergo delayed collapse to a black hole. Each star is initially threaded by a dynamical unimportant interior dipole magnetic field, which is either confined in the stellar interior or extended into the exterior where a nearly force-free magnetosphere resembles that of a pulsar. In all cases, we find that a magnetically-driven jet emerges from the poles of the BH remnant, though the jet launching time strongly depends on the equation of state (EOS) and the initial seed magnetic field configuration. The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent with short gamma-ray bursts, as well as with the Blandford--Znajek mechanism for launching jets and their associated Poynting luminosities. Also, our numerical results also suggest that the dynamical ejection of matter can be enhanced by the magnetic field, and hence GRMHD studies may be required to explain kilonova signals from GW170818-like events. Our results can be potentially used to constraint both the EOS and the stellar magnetic field configuration through multimessenger observations.

The spacetime surrounding compact objects provides an excellent place to study gravity in the strong, non-linear, dynamical regime. Here, the effects of strong curvature can leave their imprint on observables which we may use to study gravity. Recently, the Neutron Star Interior Composition ExploreR (NICER) provided mass and radius relations of an isolated neutron star (NS). These measurements, combined with tidal deformability gleaned from GW170817 have aided in the understanding of neutron stars (NS) in general relativity (GR). However, modified theories of gravity may present different results due to their behavior in the strong field regime, leading to results which may differ from those in Einstein's theory. Here, we focus on comparing the results mentioned with new theoretical corrections to GR obtained from scalar-Gauss-Bonnet (sGB) gravity. Our goal is to determine whether the mass-radius relations as well as the Love-compactness relations can help constrain sGB given the observations from NICER and LIGO. In this talk, I will present the results of our study of sGB gravity and how these compare with the information gathered from NICER and LIGO. In addition to this, I will discuss the usefulness of this approach in placing constraints on sGB theory.

The Kerr black hole hypothesis can be tested by using two approaches namely the top-bottom approach and bottom-up approach. The first one involves introducing the deviations in the Kerr metric through a theoretical model. The second approach involves introducing the deviations in terms of parameters. The metric proposed by Johannsen and Psaltis is one such parametrically deformed Kerr spacetime. It reduces to the Kerr metric when one sets the deviation parameters to zero. We construct some generalizations of this spacetime including the charged and accelerated versions and discuss their horizon structure and thermodynamics.

Re-visiting Einstein's original derivation of E =mc2 Ajay Sharma Fundamental Physics Society. His Mercy Enclave, Post Box 107 GPO Shimla 171001 HP India Email: ajoy.plus@gmail.com Mobile 0091 94184-50899 Einstein derived ΔL= Δmc 2 ( inter-conversion of light energy -mass) in Sep. 1905 paper (2.5 pages) in a speculative way. In the thought experiment, Einstein considered two systems i.e. system at rest (SR), where the luminous body ( emanating light energy L) , and another system (SV) moving with uniform velocity v. The corresponding change in mass and energy of the body is measured in SV. Thus equation ΔL= Δmc 2 is derived. ΔL= Δmc 2 is speculated as true for all possible energies ( heat, chemical, invisible, cosmological, co-existing in many form, etc. ) as ΔE = Δmc 2. It is never justified The luminous body emanates only two light waves, of the equal magnitude of energy (L/2), in opposite directions ( angle ï¦ , 180° ). The choice of the equation for variation in light energy is in relativistic form and interpretation in classical form ( Binomial theorem, v<< Δmc 2 or ΔL > Δmc 2 (or equivalently for ΔE= Δmc 2 ). Thus derivation is full of speculations and arbitrariness, thus the equation needs to be derived by a different method. The general equation is ΔL = A Δmc 2 or ΔE = A Δmc 2 . The plenty of abandoned experimental data may justify the same. The data of the atom bomb “Little Boy” was declassified in 1965, which implies that energy emitted is only 2% . The 98% energy lost can be explained with ΔE = A Δmc2 (A

The origins and environments of stellar-mass black hole (BH) binaries remain a mystery. One of the proposed binary formation mechanisms is manifest in dynamical interactions between multiple BHs. A resulting framework of these dynamical interactions is the so-called hierarchical triple-merger scenario, which happens when three BHs become gravitationally bound, causing two successive BH mergers to occur. In such successive mergers, the BHs involved are directly related to each other. Therefore, this channel can be directly tested from the properties of the detected binary BH mergers. Here we present a search for hierarchical triple mergers among detected BH merger events within the first and second gravitational-wave transient catalogs of the Laser Interferometer Gravitational-Wave Observatory/Virgo, the eccentric localization of GW190521, and those found by the IAS-Princeton group. We discuss the parameters of the search as well as present the resulting individually significant merger pairs. The search yields interesting candidate triple merger events and hints at its future impact.

Evidence of dark energy found on earth is common place. Just strike a match made of ancient atoms and light photons are emitted traveling at 186,282 miles per second. That power source is an energy currently undetected by any sensing instruments known beyond our own eyesight. Further, that energy exercise is displayed in firework displays, dynamite explosions, chain reactions, atomic bombs, and in the furnace of stars. Following that energy trail provides a completely new understanding of our universe allowing me to answer all the most perplexing questions of cosmology including how the Big Bang Banged, how galaxies with individual orientation are formed, what is the physics of the CMB, how to visualize the geometry of our universe, and how to compute the age of our universe and its component – atoms.

The merger events being detected by gravitational wave observatories proceed in three stages: inspiral, merger and ringdown. The ringdown signal encodes the properties of the remnant and in case of it being a Kerr black hole, the ringdown frequencies depend solely on its mass and spin. Thus the ringdown part of the signal is a fertile ground for tests of general relativity, verifying no-hair theorem and measurement of black hole spins. Now, it has long been posited based on physical arguments that the so-called “high frequency” regime of the quasinormal modes corresponds to the dynamics of the unstable photon orbits around the black hole. This analysis largely relies on the WKB/geometric optics framework and has been extensively studied for the Schwarzschild black hole with appreciative success. However, for the Kerr black hole, the situation is rather precarious. Even for the slow rotation case, the correspondence between the dynamics of photon orbit and the gravitational quasinormal modes has a numerical uncertainty of an uneasy 30% for the astrophysically relevant (2,2) modes. Furthermore, the discrepancy of results of this correspondence for prograde and retrograde orbital motion has put into question whether such a correspondence even exists in the first place. In this work, I shall talk about my attempts with collaborators in trying to address this problem. Firstly, I shall discuss the success of the techniques of complex angular momentum (CAM) and Regge Poles in producing with high fidelity the gravitational quasinormal modes via an analytic framework for the entire parameter space of interest, while also discuss the challenges in incorporating the photon orbits into the framework. Secondly, I shall discuss how a gauge invariant treatment of the Lyapunov exponent, which is used to characterise orbital instability, might be useful in reducing this numerical discrepancy. The implications of this results for detection of supermassive binary black hole mergers by LISA as well as photon ring resolution by the next generation EHT will also be discussed.

Theories of Quantum Gravity predict a minimum measurable length and a corresponding modification of the Heisenberg Uncertainty Principle to the so-called Generalized Uncertainty Principle (GUP). However, this modification is non-relativistic, making it unclear whether the minimum length is Lorentz invariant. We formulate a Relativistic Generalized Uncertainty Principle, resulting in a Lorentz invariant minimum measurable length and the resolution of the composition law problem. This proved to be an important step in the formulation of Quantum Field Theory with minimum length. We derived the Lagrangians consistent with the existence of minimal length and describing the behaviour of scalar, spinor, and U(1) gauge fields. We calculated the Feynman rules (propagators and vertices) associated with these Lagrangians. Furthermore, we calculated the Quantum Gravity corrected scattering cross-sections for a lepton-lepton scattering. Finally, we compared our results with current experiments, which allowed us to improve the bounds on scale at which quantum gravity phenomena will become relevant.

We find a new homogeneous solution to the Einstein-Maxwell equations with a cosmological term. The spacetime manifold is R × S^3. The spacetime metric admits a simply transitive isometry group G = R × SU(2) of isometries and is of Petrov type I. The spacetime is geodesically complete and globally hyperbolic. The electromagnetic field is non-null and non-inheriting: it is only invariant with respect to the SU(2) subgroup and is time-dependent in a stationary reference frame.

We show that considering the spherical modes of gravitational waves and how a center-of-mass velocity acts on them, the motion of the source can be detected by only using gravitational waves. This result does not only break the well known mass-redshift degeneracy of gravitational waves, that appears when only considering the effect of velocity on the frequency of the wave, but also provides a way to obtain information about the source and its environment. As an example we consider an extreme mass-ratio inspiral moving due to the peculiar velocity of its host galaxy. We show that for such a source the velocity can be detected for a signal-to-noise ratio of only 20. Using a Fisher matrix analysis, we show that the velocity of the source can be measured to a precision of just a few percent for a signal-to-noise ratio of 100. In contrast, ignoring the velocity of the source will lead to a bias in the parameter estimation of the source. This talk is based on the papers: https://doi.org/10.1103/PhysRevLett.127.041102 & https://arxiv.org/abs/2010.15856

We perform magnetohydrodynamic simulations in full general relativity of rotating neutron stars with mixed poloidal and toroidal magnetic fields. The initial uniformly rotating neutron star models are computed self-consistently assuming perfect conductivity, stationarity, and axisymmetry. The hydromagnetic stability of these equilibria is assessed and their final fate is presented.

We present binary neutron star (BNS) merger simulations that use both IllinoisGRMHD and HARM3D to maximize physical realism while minimizing numerical errors. We have recently enhanced IllinoisGRMHD -- a rewrite of the dynamical-spacetime general relativistic magnetohydrodynamics (GRMHD) code of the Illinois group -- with a new equation of state (EOS) infrastructure that supports realistic, tabulated nuclear EOSs. We use this version to model BNS inspirals through merger and black hole (BH) formation on a Cartesian AMR grid structure. After the remnant spacetime has become nearly stationary, evolving the BH accretion disk on a moderate-resolution 3D Cartesian AMR grid over long timescales would be suboptimal, as angular momentum is spuriously lost due to numerical errors associated with GRMHD flows obliquely crossing coordinate lines. So instead we transfer the simulation data to an EOS-enhanced, spherical-coordinate GRMHD code, HARM3D, which specializes in modeling BH accretion disks over long timescales. This enables us to efficiently and reliably model the accretion disk over the relatively long time scales required to generate theoretical predictions of the early EM counterpart.

A study of the Cowling Approximation is presented with rigorous results involving Hilbert-space methods and integrodifferential equations. Methods are developed to exploit the self-adjoint form of operators involved in our formulation of a standard linear Newtonian model of stellar pulsation. These are, in turn, used to compare solutions to their approximations: Confirmation in a variety of settings, involving large parameters, are discussed well as some counter-examples. We also demonstrate methods of symbolic computation in forming asymptotic estimates in order to exploit the form of dominant terms.

We study the cosmological propagation of gravitational waves (GW) beyond general relativity (GR) across homogeneous and isotropic backgrounds. We consider scenarios in which GWs interact with an additional tensor field and use a parametrized phenomenological approach that generically describes their coupled equations of motion. We analyze four distinct classes of derivative and non-derivative interactions: mass, friction, velocity, and chiral. We apply the WKB formalism to account for the cosmological evolution and obtain analytical solutions to these equations. We corroborate these results by analyzing numerically the propagation of a toy GW signal. We then proceed to use the analytical results to study the modified propagation of realistic GWs from merging binary black holes, assuming that the GW signal emitted is the same as in GR. We generically find that tensor interactions lead to copies of the originally emitted GW signal, each one with its own possible modified dispersion relation. These copies can travel coherently and interfere with each other leading to a scrambled GW signal, or propagate incoherently and lead to echoes arriving at different times at the observer that could be misidentified as independent GW events. Depending on the type of tensor interaction, the detected GW signal may exhibit amplitude and phase distortions with respect to a GW waveform in GR, as well as birefringence effects. We discuss observational probes of these interactions with both individual GW events, as well as a population studies for both ground- and space-based detectors.

The detection of gravitational waves has allowed us to test general relativity (TGR) in various regimes of gravity. LIGO/VIRGO collaboration tests the deviations of the post-Newtonian coefficients derived from general relativity. Doing so requires the evaluation of millions of waveforms, which is computationally expensive. We present our work on fast estimation of TGR post-Newtonian coefficients using the multibanding method (Morisaki 2021).

Gravitational waves have become a powerful tool in constraining theories of modified gravity. The propagation speed of gravitational waves $c_T$ has been constrained to be $|c_T/c-1| \lesssim 10^{-13}$ by GW170817 and its electromagnetic counterpart. However, the frequency window of LIGO-Virgo, in which this binary neutron star merger was detected, is very close to, if not higher than, the potential cutoff scale of many effective low-energy modified gravity models. These models predict that $c_T$ can be different from the speed of light below the cutoff scale, which corresponds to lower frequencies of gravitational waves. The variation of $c_T$ would leave an imprint on the waveforms from binary super-massive black hole mergers, which can then be constrained by LISA observations. We derive the modified waveform with frequency-dependent $c_T$, and forecast the constraints on the modified gravity parameters for different binary systems. We identify the most ideal systems to detect the potential modification effect.

Accurate and efficient modeling of binary black holes (BBHs) is crucial for the detection of gravitational waves (GWs) emitted by them. Closed-form solutions to these systems when they are in the initial inspiral phase are highly sought after and have been worked out by many groups in the post-Newtonian (PN) approximation. Most of these solutions are valid only in certain limits (small eccentricity, no spins, equal mass, etc). Establishing the integrable nature of these systems opens up the possibility of constructing closed-form solutions, since integrability precludes chaos and guarantees the existence of action-angle variables. In this talk, against the backdrop of the PN Hamiltonian framework, I will discuss our series of efforts in establishing the integrable nature of the most general BBH system (arbitrary masses, spins, and eccentricity) as per the Liouville-Arnold theorem at 2PN order. I will also discuss the recently derived action-angle variables for these systems at 1.5PN order.

While recent observations by the LIGO/Virgo have given us the ability to test general relativity in the extreme gravity regime, they are still blind to a large swath of phenomena that is outside the sensitivity curve of these instruments. Future detectors such as LISA will enable us to probe longer-duration, lower-frequency events, and in particular enable us to search for chaos in the trajectories of an extreme mass-ratio inspiral (EMRI), when a small compact object falls into a supermassive black hole. In this paper, we discuss the possibility of detecting chaos in EMRIs, and how they can be used to constrain modified gravity theories. We show that these theories are likely to exhibit chaos. We present a framework and code-base used to search for chaos, and discuss the implications for finding chaos in EMRI observations with LISA.

Rotating black holes in general relativity are notoriously simple objects. They can be fully fixed by measuring a single dominant (complex) characteristic frequency. Having determined this 'quasinormal mode' (QNM) frequency for the endstate of a binary merger, any additional information, from earlier binary dynamics or other QNMs, can serve as a test for our understanding of general relativity and/or black holes. I will discuss how quasinormal modes of rotating black holes are modified by higher derivative curvature corrections to the Einstein-Hilbert action, up to quartic order and including parity violating terms. After introducing the problem, its relevance and its difficulties, I will schematically describe a convenient perturbative approach. I conclude with a trick, a wish and a caveat.

The detection of gravitational waves from compact binary mergers by the LIGO/Virgo collaboration has, for the first time, allowed us to test relativistic gravity in its strong, dynamical and nonlinear regime, thus opening a new arena to confront general relativity (and modifications thereof) against observations. We consider a theory which modifies general relativity by introducing a scalar field coupled to a parity-violating curvature term known as dynamical Chern-Simons gravity. In this theory, spinning black holes are different from their general relativistic counterparts and can thus serve as probes to this theory. Having studied the linear gravito-scalar perturbations of black holes in dynamical Chern-Simons gravity at leading-order in spin, in this talk I will (i) present the perturbed field equations describing the evolution of perturbed gravitational and scalar fields, (ii) present the quasinormal mode frequencies obtained by numerically solving these equations and (iii) discuss the stability of black holes in this theory and how these results pave the way towards future tests of this theory with gravitational wave ringdown observations.

Quasinormal modes (QNM) of black holes in Anti-de Sitter space correspond, according to AdS/CFT, to poles of thermal correlators in the dual theory. Using the powerful results of 4d black hole perturbation theory — namely the Newman-Penrose formalism, Teukolsky’s equations and Hertz’s reconstruction map, I will derive master equations and holographic boundary conditions for gravitational perturbations of NUT-AdS black branes. This leads, for the first time, to the QNM frequency spectrum of a NUT-charged space. The results provide definite holographic predictions regarding the hydrodynamic behaviour of the dual plasma. I will conclude by discussing stability against scalar and gravitational perturbations.

Black holes in certain alternative theories of gravity containing a scalar field are known to exhibit a two parameter family of solutions, one where the scalar field is regular at the event horizon and the other where the scalar field diverges at the event horizon. In this talk, we outline a proof that shows that this behaviour is true in a large class of alternative theories where a scalar field linearly couples to arbitrary curvature scalars. We then address the question of end point of stellar collapse in these theories. By an extension of Kay-Wald theorem, we outline a proof that shows that to first order in perturbation theory, the scalar field always settles to a solution which is regular on the event horizon. To explain why we settle to a regular solution, we consider two examples of dynamical collapse. In the first example, we use the Oppenheimer-Snyder spacetime as our background spacetime and we find the \emph{exact} solution for the scalar field in this spacetime. Using this exact solution, we answer why the scalar field regularises as the star collapses to a black hole. In the second example, we consider a numerical collapse of a real star and then evolve the scalar field in this background. Our results from Oppenheimer-Snyder collapse agree very well with the numerical result. In answering why the scalar field regularises, we also explain the evolution of scalar hair in large class of theories including shift symmetric theories like Scalar Gauss-Bonnet theory.

The metric of a spacetime can be greatly simplified if the spacetime is circular. In this talk I will show that in generic effective theories of gravity, the spacetime of a stationary, axisymmetric, and asymptotically flat solution must be circular if the solution can be obtained perturbatively from a solution in the general relativity limit. This result applies to a broad class of gravitational theories that include arbitrary scalars and vectors in their light sector, so long as their nonstandard kinetic terms and nonmininal couplings to gravity are treated perturbatively.

The gravitational waves emitted when a small compact object falls into a supermassive black hole depend sensitively on the orbital trajectory of the small object, which in turn depends on the properties of the spacetime generated by the supermassive black hole. The ability to test general relativity in this extreme gravity regime using gravitational wave observations from future space-based detectors such as the Laser Interferometer Space Antenna motivates the mathematical study of the symmetries of black holes in modified theories of gravity. In this talk, I will focus on the quadratic gravity modified theories dynamical Chern-Simons gravity and scalar Gauss-Bonnet gravity. While the Kerr metric for a spinning black hole in general relativity possesses two distinct principal null directions, the still unknown analogous exact solutions in both modified theories are each thought to possess four. I will present all principal null directions and Weyl scalars of such modified black holes in the slow rotation approximation. Additionally, for each modified theory I will investigate the existence of a Killing tensor that would generate a fourth constant of the motion.

In this talk I will give an overview of recent and ongoing work regarding rotating black holes in dynamical Chern-Simons (dCS) gravity. dCS gravity is a well motivated modified theory of gravity which has been extensively studied in gravitational and cosmological contexts. I will first discuss unique geometric structures, `the Chern-Simons caps,’ which slowly rotating black holes in dCS gravity were recently found to possess. Motivated by the dCS caps, I will then discuss superradiance in the context of slowly rotating dCS black holes and show that there are corrections to the usual solution for a Kerr black hole. Lastly, I will comment on the observable implications for these corrections and point towards avenues for future work.

It has been shown that, for certain modified theories in which a scalar field is coupled with the Gauss-Bonnet curvature invariant, black hole spacetimes can undergo a tachyonic instability which culminates in a non-trivial stationary scalar configuration. This mechanism is known as spontaneous scalarization, and its onset is controlled by the coupling strength, mass and spin of the black hole. Thus, dynamical binary systems allow for a variety of configurations that depend on these parameters. For example, two scalarized black holes can merge to form a larger remnant that has no scalar profile due to the weaker spacetime curvature near the horizon, a mechanism we termed "dynamical descalarization". In this talk, I will be discussing my work on modelling binary black hole systems in scalar-Gauss-Bonnet gravity to identify the possible configurations. Knowledge of these configurations can assist in the identification of the gravitational waveform modifications due to the scalar field and can provide estimates of the constraints on the theory.

Gravitational theories differing from General Relativity may explain the accelerated expansion of the Universe without a cosmological constant. However, their viability crucially depends on a “screening mechanism” needed to suppress, on small scales, the fifth force driving the cosmological acceleration. I will discuss a scalar-tensor theory with first-order derivative self-interactions exhibiting such a mechanism, and present screened solutions in this theory for both non-relativistic and relativistic stars. I will also show how the screening mechanism enables this theory to pass Solar System tests up to 1PN order.

Knowledge of the geodesics on a manifold is insufficient to determine uniquely the metric. In fact, this only endows a determination of the connections up to a projective equivalence class. Additionally, it has been found that in (1+1) dimensions, the coadjoint orbits of the Virasoro algebra promote the existence of an associated gauge field, the diffeomorphism field Dab, and that it acts geometrically. The Thomas-Whitehead gravitational action is the currently investigated theory for incorporating these features for 4d spacetime. This talk will discuss what progress is being made for the static spherically symmetric solutions in the Thomas-Whitehead Theory.

The complete paper can be found here: https://arxiv.org/abs/2108.09900. Using numerical algebra tools, new classes of monopole solutions are obtained to the static, spherically-symmetric vacuum field equations of five-dimensional general relativity. First proposed by Kaluza, 5D general relativity unites gravity and classical electromagnetism with a scalar field. These monopoles correspond to bodies carrying mass, electric charge, and scalar charge. The Reissner-Nordstrom limit allows us to constrain the signature of the fifth component to be spacelike, but valid solutions are obtained for either sign of the scalar field. We nd that Kaluza vacuum solutions imply the scalar field energy density is the negative of the electric field energy density, so the total electric and scalar field energy of the monopole is zero. Yet the new solutions provide reasonable Reissner-Nordstrom and Coulomb limits in mathematical form, with varying possibilities for the scalar field. The vanishing of the total electric and scalar field energy density for vacuum solutions seems to imply the scalar field can be understood as a negative-energy foundation on which the electric field is built.

The interaction between black holes and ultralight bosonic fields, that represent popular dark matter candidates or axion-like particles, has opened new avenues to probe for beyond-standard model physics. In particular, the gravitational wave signal of a black hole binary may be modified. We present preliminary results of the dynamical evolution of a massive scalar field around (i) a Newtonian binary and (ii) a fully relativistic black-hole binary. In the Newtonin binary, composed of point particles, we find a strong amplification of the scalar field. The fully relativistic binary, simulated in 3+1 numerical relativity, gives rise to a complex mode mixing of an initially monopolar scalar field and the excitation of higher multipoles when the Compton wavelength of the field is comparable to the characteristic size of the binary.

If ultralight scalar fields exist, they may give rise to gravitational wave (GW) signals observable by current and next-generation detectors. Bound states of such fields can grow exponentially in the vicinity of a Kerr black hole due to superradiant amplification, and the resulting bound state, known colloquially as an "axion cloud", sources monochromatic GW's. We are investigating the detectability of these GW signals in conjunction with the development of the Levitated Sensor Detector (LSD), an experiment to measure GW's in the 10 - 300 kHz band using optically-levitated nanoparticles as test masses. A key quantity for assessing the astrophysical relevance of axion clouds is their growth timescale, which is the reciprocal of their superradiance rate. In turn, the superradiance rate is related to the imaginary part of the bound state's eigenfrequency. While these complex eigenfrequencies have closed-form solutions in limiting cases, there are also numerical techniques for solving the eigenvalue problem in general. In this talk, we will present results for growth timescales using both analytical approximations and numerical techniques, and we will discuss implications for the relevance of axion clouds as sources for the LSD.

Rotating boson stars - a promising laboratory to study the dynamical behavior of a large class of ultra compact objects in the strong gravity regime - have been shown to be dynamically unstable to non-axisymmetric perturbations in linear scalar models. We show numerically that for sufficiently strong scalar self-interactions, and independent of the type of scalar potential, however, the growth timescales of the unstable modes are tamed, and ultimately, the instability is quenched altogether. This provides a pathway to study the inspiral and merger of rotating ultra compact objects using rotating binary boson stars.

Self-gravitating non-topological solitons whose potential admits multiple vacua are promising candidates for exotic compact objects. Such objects can arise in several extensions of the Standard Model and could be produced in the early Universe. In this talk, we focus on objects made from complex scalars (gravitating Q-balls/soliton boson stars), deriving analytic solutions in spherical symmetry and comparing them with fully numerical ones, illuminating their connection with the flat-space time limit and scanning the parameter space, including several potentials that allow for such configurations. Moving beyond the isolated configurations, we study the merger of two soliton boson stars by numerically evolving the fully relativistic Einstein-Klein-Gordon equations, discussing the parameter space of such mergers, including possibility of a rotating remnant and other possible outcomes such as scalar ejecta and the cloud formation. Finally, we comment on the gravitational wave signal from the binary and distinguishability with other compact objects in the present LIGO data.

Ultra-relativistic collisions of black holes are ideal settings where to investigate how general relativity behaves under extreme conditions. A large fraction of the allowed parameter space has been explored over the past 13 years (including mass, spin, impact parameter), with the notable exception of charge. The inclusion of charge is significant because it adds non trivial degrees of freedom to the system. For example, charge constitutes another way to reach extremality (together with spin) and opens up a new channel to radiate away energy (through electromagnetic waves). Moreover, charge makes the study more directly relevant to particle physics and the production of microscopic black holes in cosmic rays and in particle accelerators. In this talk, I am going to present the first numerical-relativity simulations of head-on collisions of black holes with the same charge and mass. In particular, I am going to discuss what they can teach us about a variety of conjectures, including cosmic censorship and the role of the inner structure of bodies colliding at relativistic speed.

Lorentz violation has been a popular topic in recent years. We construct a comprehensive effective field theory to analyze experiments testing diffeomorphism invariance and local Lorentz symmetry in gravity. We analyze experiments in different gravitational potentials, free-dropping experiments, gravitational interferometry experiments, and gravitational bound-state experiments. First constraints on certain coefficients for Lorentz violation and spin-gravity couplings are extracted.

We show a new way of generating initial data for binary Boson Stars, which reduces initial excitations significantly. Furthermore, we show that it will dramatically improve the quality of the extracted gravitational waves and reduce violations by one order of magnitude. Using this new method, we merge boson stars with an impact pact parameter b and an initial velocity v to study the behavior of angular momentum in the resulting boson star remnant.

I will present extreme mass ratio inspirals (EMRIs), during which a small body spirals into a supermassive black hole, in gravity theories with additional scalar fields. No-hair theorems and properties of known theories that manage to circumvent them introduce a drastic simplification to the problem: the effects of the scalar on supermassive black holes, if any, are mostly negligible for EMRIs in vast classes of theories. I will show how to exploit this simplification to model the inspiral perturbatively and demonstrate that the scalar charge of the small body leaves a significant imprint on gravitational wave emission. This result is particularly appealing, as this imprint is observable with LISA, rendering EMRIs promising probes of scalar fields.

As their spacetime points approach coincidence, the n-point functions of local quantum field theories can be approximated to arbitrary precision by their so-called operator product expansions (OPEs). The coefficients of OPEs are ordinary c-number distributions which contain key information about both the quantum fields and their physical states. Under variations of the coupling parameters, Hollands et al. have shown the OPE coefficients of renormalizable Euclidean QFTs satisfy "flow equations". The Hollands flow equations have been rigorously proven to hold order-by-order in perturbation theory, but remain mathematically well defined under very general assumptions for any value of the coupling parameters and, thereby, potentially provide a non-perturbative approach to obtaining OPE coefficients. However, there exist serious obstacles to deriving flow relations on curved Lorentzian spacetimes in a manner compatible with locality and covariance. In this talk, I describe these issues and our resolutions to them for a solvable toy model: Klein-Gordon theory with the mass viewed as an "interaction parameter". Our approach to obtaining local and covariant flow relations for the Klein-Gordon OPE coefficients on Lorentzian spacetimes generalizes to QFTs with nonlinear interactions.

It has been argued that low-energy experiments say nothing about quantum gravity. The following thought experiment suggests that quantum aspects of gravity must play an important role: Alice maintains a mass in a superposition of locations, and attempts an interference experiment. In a spacelike-separated region, Bob may attempt to measure the superposed gravitational field using a test particle. If Alice emits no decohering radiation, any observed decoherence must result from Bob's measurement, giving Bob an apparent channel for superluminal communication to Alice. In linear quantum gravity, however, an order-of-magnitude argument showed that gravitational vacuum fluctuations can resolve this paradox by limiting the accuracy with which Bob can localize his particle. This, and the requirement that Alice emits no decohering radiation, apparently resolves the paradox. However, it might appear that Bob's accuracy could be improved by making multiple uncorrelated field measurements, which, when averaged, could make his uncertainty of the field arbitrarily small. Now, we provide a rigorous proof that the paradox is resolved in quantum field theories in stationary spacetimes, including in linear quantum gravity, when interactions are confined to within the causal diamond of Alice's experiment. In this general framework we show that Bob's ability to measure any observable in the causal complement of Alice's experiment is bounded by the decohering radiation emitted by Alice. The paradox and its resolution in linear quantum gravity provide strong evidence that the gravitational field must be quantized, and that low-energy experiments can probe its quantum nature.

A linear analytical solution is derived for the gravitational shock wave produced by a particle of mass M that decays into a pair of null particles. Formulae are derived for the perturbation as a function of polar angle, as measured by an observer at the origin observing clocks on a sphere at distance R. The effect of the shock is interpreted physically as an instantaneous displacement in time and velocity when the shock passes the clocks. The time displacement is shown to be anisotropic, dominated by a quadrupole harmonic aligned with the particle-decay axis, with a magnitude δτ∼GM/c^3, independent of R. The velocity displacement is isotropic. The solution is used to estimate the angular distribution of gravitational perturbations from a quantum state with a superposition of a large number of randomly oriented, statistically isotropic particle decays. This approach is shown to provide a well-controlled approximation to estimate coherent, nonlocal, spacelike correlations of weak-field gravity from systems composed of null point particles up to the Planck energy, including macroscopic quantum coherence of causal quantum-gravitational fluctuations.

The "infrared problem" is the generic emission of an infinite number of low-frequency quanta in any scattering process with massless degrees of freedom. In particular, this problem plagues QED as well as any theory of quantum gravity. That the ``out'' state contains an infinite number of such quanta implies that it does not lie in the standard Fock representation. Consequently, the standard S-matrix is undefined as a map between "in" and "out" states in the standard Fock space. This fact is due to a low-frequency tail of the radiation field (i.e. the memory effect) as well as the existence of an infinite number of conserved charges at spatial infinity. In the case of massive QED, there exists an "I.R. finite" S-matrix known as Kulish-Faddeev representations which consist of eigenstates of the conserved charges. We prove that such constructions in massless QED and in quantum gravity fail to contain any physically reasonable states except the vacuum. Furthermore, we also argue that "non-Kulish-Faddeev" representations are similarly not suitable for scattering. In totality, our results indicate that an "infrared finite" scattering theory should be formulated as a map between "in" and "out" correlation functions without reference to a particular "in/out" Hilbert space.

Primordial inflation produces a vast ensemble of cosmological scale gravitons which can affect both the force of gravity and the propagation of gravitational radiation. These effects can be studied by using the graviton self-energy to quantum correct the linearized Einstein equations. We will first give an analysis of the structure of the graviton self-energy and then present an explicit result for one loop corrections to the propagation of gravitons. Although suppressed by a minuscule loop-counting parameter, these corrections are enhanced by the square of the number of inflationary e-foldings. This makes the one loop corrections to the tensor power spectrum potentially observable, in the far future, after the full development of 21 cm cosmology.

The problem of the observed baryon asymmetry in the Universe is a long standing problem in physics, with no satisfactory explanation so far. To explain the observed baryon asymmetry, three Sakharov conditions must be met. A study of Quantum Gravity effects in cosmology, and in particular that of the Generalized Uncertainty Principle on the Friedmann equations is presented. To include Quantum Gravity effects to cosmological scales, the holographic principle is used. An interaction term which couples space-time and the baryon current satisfies the first two Sakharov conditions and it is shown that the Generalized Uncertainty Principle (GUP) induces variations of the energy density and pressure in the radiation-dominated era which satisfies the third Sakharov condition and thus provides a viable explanation for the observed baryon asymmetry in the Universe.

In the non-relativistic theory of quantum mechanics the Born rule determines the probability of detecting one or several particles in a given region of configuration space at an instance of time. Despite its foundational status within quantum theory, its generalization to the special or even general relativistic setting is still a subject of debate in the mathematical physics literature. A major hurdle has been the so called `problem of time''---a popularized term highlighting the concern that quantum mechanics rests on a prerelativistic conception of time. In this talk I first present a naive generalization of the Born rule to curved spacetime for the particular case of one body. This `naive solution'', though not attributable to any particular author, is firmly based in the literature. I will then provide three conceptual arguments against the naive solution, thus motivating a mathematical generalization thereof. Borrowing terminology from continuum mechanics, this generalization is shown to admit a Lagrangian as well as an Eulerian description, the Lagrangian one being arguably more fundamental. The respective mathematical theory is almost analogous for the conservation of other scalar integral quantities, such as charge and mass, and rests on earlier contributions due to C. Eckart and J. Ehlers. In this context the `problem of time'' is resolved by showing that the relativistic continuity equation implies the existence of an absolute invariant in the sense of Poincaré-Cartan, making the total probability (resp. mass or charge) independent of the particular choice of ''simultaneity hypersurface''. The co-author is Dr. Bill Poirier from Texas Tech University.

The privileged position of 3+1 dimensional spacetime has been explored from many aspects in the literature. In this talk, I will discuss this topic from the dynamical instability of a self-gravitating fluid in (N+1) dimensional spacetime.

In order to describe gravity in General Relativity we need an evolution system for the metric. Here we consider the strongly hyperbolic first order conformal and covariant Z4 (FOCCZ4) system. We implement this system in the new Nmesh code which uses a discontinuous Galerkin method to evolve astrophysical systems. For the implementation of the FOCCZ4 system we use a code generator. This generator, called Cpi, uses the Python module SymPy to translate tensor equations into an optimized C-routine. Here, we present some preliminary results when evolving with FOCCZ4. We also discuss how Cpi can is used for such tasks.

We describe our new radiative transport module that we implemented in the Illinois GRMHD code. The module employs the general relativistic truncated moment ("M1”) formalism, which uses an analytic expression to close the radiation moment equations and smoothly interpolates between optically thin and thick limits. We tested the scheme by simulating thermal Oppenheimer-Synder collapse to a black hole and find we can reliably reproduce the results in previous studies that could only treat transport in the optically thick interior. However, our M1 scheme can also track the emitted radiation into the optically thin exterior and correctly determines the emission as viewed by a distant observer. We then employ our code to treat the full GRMHD-neutrino transport evolution of a merging binary neutron star system. Two versions are considered: a "simple" one that evolves a single neutrino species (electron anti-neutrino) and considers only charged-current processes and another "complete" one that evolves all three species (electron neutrino, electron anti-neutrino, and heavy-lepton neutrino) with all relevant interactions. Preliminary results will be presented.

I will introduce NRPyElliptic, an elliptic solver for numerical relativity (NR) built on the NRPy+ infrastructure. As its first application, it sets up conformally flat, binary puncture initial data on prolate-spheroidal-like grids, similar to the widely used TwoPunctures code. However, NRPyElliptic employs a hyperbolic relaxation scheme, whereby an elliptic equation is transformed into a hyperbolic equation. Our implementation of this scheme implements new performance optimizations that speed up the solver by orders of magnitude over the original approach, making it fast enough to generate compact binary initial data for NR. NRPyElliptic is easily extensible to other nonlinear elliptic PDEs, and we intend to generate conformal thin-sandwich binary neutron star initial data with it in the near future. It has been developed as an Einstein Toolkit thorn and as a stand-alone code, both of which are documented in pedagogical Jupyter notebooks.

Based on the open source IllinoisGRMHD, GiRaFFE, and NRPy+ codes, CurviGiRaFFE is an in-development, dynamical spacetime GRFFE code aimed at studying the magnetospheres of compact object binaries. Unlike IllinoisGRMHD and GiRaFFE however, CurviGiRaFFE does not make use of Cartesian AMR grids, instead opting for highly efficient bispherical-like coordinate grids composed of overlapping spherical-like and Cartesian-like grid patches. As these grids exploit near-symmetries in binary compact object spacetimes, the memory overhead is reduced by orders of magnitude. This efficiency gain will enable CurviGiRaFFE to model interacting compact binary magnetospheres on high-end, consumer-grade desktop computers, so the vast parameter space of magnetosphere configurations can be explored with minimal computational expense. I will report on CurviGiRaFFE’s covariant GRFFE formulation and numerical implementation, which adopts similar strategies to those of spherical-coordinate BSSN and GRMHD codes (e.g., SphericalNR) to enable robust and reliable simulations in spherical coordinates. However, CurviGiRaFFE leverages NRPy+ and BlackHoles@Home to extend to a much broader class of multi-coordinate, bispherical-like grids.

Using the Einstein Toolkit, we have generated waveform templates of eccentric, spinning binary black hole inspirals. The simulation library consists of 3 different eccentricities, 3 different mass ratios (q = 2, 4, 6) and dimensionless spin configurations of ±0.6 and ±0.3 aligned with the orbital angular momentum. In this talk I will present preliminary results of investigating the effect of including higher order modes up to l=4 in the waveform on signal detection and parameter recovery in LIGO when compared to using only l=2 modes. I will also talk about possible degeneracies between these waveforms and those of different quasi-circular binary black hole inspirals, which has implications for the inference of system parameters in gravitational wave observations.

We explore the promise of physics informed deep learning to capture the physics of subgrid-scale magnetohydrodynamics turbulence of simulations of the magnetized Kelvin-Helmholtz instability (KHI). The KHI creates general relativistic magnetohydrodynamic turbulence that amplifies the magnetic field at a smaller scale than binary neutron star simulations are capable of resolving. We develop a physics informed artificial neural network model known as DeepONetPI and evaluate its ability to resolve similar subgrid effects. Specifically, we use DeepONetPI to reproduce the results of high resolution simulations given low resolution data. We compare DeepONetPI's performance to that of large eddy simulation based methods which have also been proposed as a means of resolving this KHI induced subgrid-scale turbulence. We discuss the feasibility of using such methods to capturing subgrid-scale general relativistic magnetohydrodynamic turbulence in numerical relativity simulations of binary neutron star mergers.

The LIGO-Virgo gravitational wave detectors have observed 4 events involving neutron stars: two binary neutron star (BNS) mergers (GW170817 and GW190425), and two neutron star-black hole mergers (GW200105 and GW200115). However, our theoretical understanding of the remnant properties of such systems is incomplete due to the complexities related to the modeling of matter effects and the very high computational cost of corresponding numerical relativity simulations. An important such property is the recoil velocity, which is imparted onto the remnant due to the anisotropic emission of gravitational radiation and the dynamical ejection of matter in the post-merger kilonova. In this work, we combine gravitational radiation as well as dynamical ejecta distributions, computed by the Computational Relativity (CoRe) numerical simulations, to get accurate estimates for BNS remnant recoil velocities. Based on this, we also obtain a numerical relativity fit for recoil velocity as a function of BNS parameters. BNS remnant recoil velocities play an important role in determining if the remnant is retained by its environment for future hierarchical mergers in a dynamical formation scenario which, in turn, can populate black holes within the lower mass gap of $\sim 3-5 M_{\odot}$.

Tidal disruption events such as ASASSN-14li and XMMSL1 J0740-85 have recently been observed in the radio. To better understand these observations, we modeled the data from our numerical simulations as synchrotron emission associated with the bow shock driven through an external medium by the unbound debris. We find higher velocities of the unbound debris, resulting in higher expected flux than previously thought. The impact parameter, beta, showed to follow a linear relationship with the velocity of the unbound debris, and therefore the flux as well. This relationship gives us a constraint on beta for the system. Analysis of the radio data using the synchrotron model showed us that the radio-emitting region expands with a constant speed. We computed analytic density profiles for several observed TDEs which proved to be similar to the expected, along with higher velocities than previous work predicted. We explore the reasons for this.

The production of relativistic jets by accreting black holes requires the presence of strong poloidal magnetic fields near the event horizon. However, most known physical processes are thought to strengthen magnetic fields in the toroidal directions. Recently, numerical GRMHD simulations were used to show that accretion disks with a purely toroidal magnetic field can create poloidal magnetic fields strong enough to launch jets via a large-scale dynamo (Liska et al. 2020). Prior to this analysis, it seemed likely that this dynamo functioned similarly to the alpha-omega dynamo, which generates poloidal fields in the Sun. In this presentation, I will summarize work that I conducted to determine the dynamo's mechanism. The dynamo creates four loops of alternating polarity. To determine if the alpha-process could account for these changes in polarity, I analyzed how the movement, expansion, and vorticity of the fluid vary throughout the disk, and how they relate to each other. I found that the behavior of the fluid is inconsistent with the alpha-process. Intriguingly, changes in the sign of the fluid velocity and expansion nonetheless correspond to changes in loop polarity providing strong clues to the dynamo's actual mechanism.

We revisit Bondi accretion - steady-state, adiabatic, spherical gas flow onto a Schwarzschild black hole at rest in an asymptotically homogeneous medium - for stiff polytropic equations of state (EOSs) with adiabatic indices $\Gamma > 5/3$. A general relativistic treatment is required to determine their accretion rates, for which we provide exact expressions. We discuss several qualitative differences between results for soft and stiff EOSs - including the appearance of a minimum steady-state accretion rate for EOSs with $\Gamma \geq 5/3$ - and explore limiting cases in order to examine these differences. Our results are useful, for example, to estimate the accretion rate onto a mini-black hole residing at the center of a neutron star.

The collapse of a rotating massive star forming a black hole remnant system may lead to r-process material outflow whose astrophysical origin remains a mystery. LIGO has recently reported binary black hole mergers with very massive black holes (> 50 Msun) that reside in the pair-instability supernova (PISN) mass gap. We speculate that similar black holes may be formed by these rapidly spinning stars above the mass gap, which as collapsars evolve into the mass gap by losing large amounts (>~10 Msun) of material to massive accretion disk outflows. We estimate the amount of r-process material synthesized in such events and argue that such extreme versions of collapsars, though much rarer, might produce a similar amount of r-process nuclei compared to their ordinary counterparts below the PISN mass gap. The radioactive decay of the nuclei present in such outflows may result in extreme versions of kilonovae, the electromagnetic radiations released due to such decays. If such kilonovae exist, they maybe targeted by future follow-up observations of long gamma-ray bursts or in blind surveys with the Vera Rubin Observatory or the Nancy Grace Roman Space Telescope. In addition, the massive disks formed in these systems would be susceptible to gravitational instabilities leading to gravitational wave emissions which may be picked up by future gravitational wave detectors.

In this talk, I will outline how bulk viscosity driven by Urca processes is likely to affect the gravitational wave signal of a neutron star coalescence. Based on post-processing data from a state-of-the-art equal-mass binary neutron star merger simulation, I will illustrate that bulk viscosity can reach high values in regions of the merger. I will present several estimates of how much it might directly affect the global dynamics of the considered merger scenario. In the second part of the talk, I will describe a new numerical scheme that includes all (first-order) dissipative effects (heat conduction, bulk- and shear viscosity). This 14-moment based closure can seamlessly interpolate between the highly collisional limit found in neutron star mergers and heavy-ion collisions, and the weakly coupled Braginskii-like limit of extended MHD appropriate for the study of accretion disks.

In this talk, I review the massive gravity theory and I present the problems and solutions which had been introduced throughout the years. In the next stage, I point out a black hole in dRGT massive gravity and its horizons, in comparison with the horizon of the Schwarzschild black hole. In the following, I show the calculation of equations of motion and I talk about the specific energy, specific angular momentum, and angular velocity of the particles which move in circular orbits. In other words, I indicate all parameters of the accretion disk of the black hole in dRGT massive gravity and their changes in terms of the different values of the components of the theory. Finally, I discuss the possible results and the range of thin accretion disk parameters.

The observation of the shadow of the supermassive black hole M87* by the Event Horizon Telescope (EHT) is sensitive to the spacetime geometry near the circular photon orbit and beyond, and it thus has the potential to test general relativity in the strong field regime. Obstacles to this program, however, include degeneracies between putative deviations from general relativity and both the description of the accretion flow and the uncertainties on "calibration parameters", such as e.g. the mass and spin of the black hole. We introduce a formalism, based on a principal component analysis, capable of reconstructing the black hole metric (i.e. the "signal") in an agnostic way, while subtracting the "foreground" due to the uncertainties in the calibration parameters and the modelling of the accretion flow. We apply our technique to simulated mock data in a toy model for spherically symmetric black holes surrounded by a thick accretion disk. We show that separation of signal and foreground may be possible, with next generation EHT-like experiments.

K-essence is a scalar-tensor theory with first-order derivative self-interactions that can screen interactions with the scalar field in local scales, while allowing for sizeable deviations from General Relativity on cosmological scales. In this talk, I will present our results about the effect of this screening mechanism in non-linear stellar oscillations and gravitational collapse by using numerical relativity simulations in spherical symmetry. Towards the end of the talk, I will discuss how one can study this kinetic screening in binary neutron stars.

The early universe is likely filled with a large number of interacting fields with unknown interactions. How can we quantitatively understand particle production (for example, during inflation and reheating after inflation) when such fields undergo a sufficient number of non-adiabatic, non-perturbative interactions? A recent proposal of a precise mapping between stochastic particle production events during inflation and re-heating in cosmology to conduction phenomena in disordered quasi one-dimensional wires provide a powerful statistical framework to resolve such seemingly intractable calculations. In our work, we use this precise correspondence to present a derivation of the quantum corrected Fokker-Planck equation without dissipation responsible for studying the dynamical features of the stochastic particle creation events during the inflationary and re-heating stage of the universe. We also present a computation for the measure of the stochastic non-linearity arising in the inflationary and reheating epoch of the universe, often described by the Lyapunov Exponent. Doing so, we quantify this quantum chaos arising in our set-up by a stronger measure, known as the Spectral Form Factor from the principles of Random Matrix Theory. We also discuss hints of emergent universality from Random Matrix Theory in early universe cosmology using this correspondence. Finally, we propose a bound on the Spectral Form Factor arising due to the presence of stochastic non-linear dynamical interactions in the early universe in a model-independent way.

In a class of single field models of inflation, the idea of Primordial Black holes(PBHs) production is studied. In this case, the dynamics on small cosmological scales differ significantly from that of the large scales probed by the observations of cosmic microwave background(CMB). This difference becomes a virtue in producing the correct physical ambiance for the seeds required to produce PBHs. Thus, once the perturbed scales renter the horizon of our Universe during the later epochs of radiation domination and subsequent matter domination, these seeds collapses to produce PBHs. We have shown, in this class of model, depending on the model parameters and the class defining set parameters, one can have PBHs formed for a vast mass ranges from $10^{-18}$ to $10^{-6}$ solar mass(\(\textup{M}_\odot\)). We have also shown, for a particular class of model, the total dark matter density today can be attributed to the PBHs density. The vast range of the mass depending on the class parameter, gives ample opportunity to study enriched phenomenological implications associated with this model to probe the nascent Universe dynamics.

We consider a scenario of modified gravity, which is generic to late-time acceleration, namely, acceleration in the Jordan frame and no acceleration in the Einstein frame. The possibility is realized by assuming an interaction between dark matter and the baryonic com- ponent in the Einstein frame which is removed by going to the Jordan frame using a disformal transformation giving rise to an exotic effective fluid responsible for causing phantom cross- ing at late times. In this scenario, past evolution is not distinguished from ΛCDM but late time dynamics is generically different due to the presence of phantom crossing that causes a monotonous increase in the expansion rate giving rise to distinctive late-time cosmic fea- ture. The latter can play a crucial role in addressing the tension between the observed value of Hubble parameter by CMB (Cosmic Microwave Background) measurements and the local observations. We demonstrate that the Hubble tension significantly reduces in the scenario under consideration for the chosen scale factor parametrizations. The estimated age of the universe in the model is well within the observational bounds in the low and high red-shift regimes.

Production of Gravitational Wave (GW) during the epoch of preheating is a fascinating and active field of research. Several seminal works have been carried out on this front. After the end of inflation, during preheating, the GW production mechanism and the prospect of detecting them observationally is a prime focus of research. In our work, we wanted to study the effect of the presence of extra scalar fields in the GW spectrum. In the string theory motivated model of inflation, the presence of moduli fields is unavoidable. Moreover, the moduli field plays a key role after the end of inflation due to the fact that the energy density of the moduli field starts to dominate due to the volume misalignment. Thus in this work, we have studied the effect of this extra moduli field on the GW spectrum and its prospect of detection in the current and upcoming GW detectors.

We implement 3D numerical relativity simulations of an inhomogeneous universe using the dust perfect fluid approximation. We setup fully nonlinear initial conditions, corresponding to perturbations of the $Lambda$CDM model in the synchronous comoving gauge, by defining the comoving curvature perturbation $\mathcal{R}_c$ as a 3D single scale sinusoidal and then use the Einstein Toolkit code to evolve the system. Around the peak this represents a quasi-spherical inhomogeneity. We compare the evolution of this over-density with that of the homogeneous Top-Hat spherical model, which is used in cosmology as a simple model of the first collapsing structures and virialisation. We find that the Top-Hat is a very good approximation at the peak of the over density where the evolution is independent of it’s environment, while the surrounding matter collapses later. Work is in progress to establish virialisation criteria from this more general model and the role of specific relativistic effects such as gravitomagnetism. We also compare to E.Bentivegna and M.Bruni’s (2016) simulations where we find the presence of the decaying mode to significantly slow down the virialisation/simulation crash of the over-density.

Existing theories of gravitational-wave (GW)-matter coupling are not directly applicable to GWs that are inhomogeneous in space and have more general polarization than those in vacuum, thus leaving some important observable physical phenomena out of their scope. The standard approach to this problem has been to solve Einstein equations with matter and fields as source terms, but this has proven to be prohibitively cumbersome. We use an alternative, variational formulation [PRD 102, 064012 (2020)] to derive the gauge-invariant wave equation for collective oscillations of the self-consistent metric with a general polarization. In addition to the vacuum GWs, our formulation captures modes strongly coupled to matter, for example, the Jeans instability, which emerges as a GW with a non-transverse polarization. We will also briefly summarize findings from (arXiv:2105.04680) where we show how to find the gauge-invariant part of the metric perturbation in an arbitrary background metric, and (arXiv:2106.05062) where we propose a gauge-invariant adiabatic quasilinear theory and geometrical optics of dispersive GWs in matter. This material is based upon the work supported by National Science Foundation under the grant no. PHY-1903130.

We investigate gyroscopic precession in the vicinity of event-horizon. Our goal is to address the question should the gyroscopic precession frequency blow-up approaching an event-horizon. We employ Frenet-Serret formalism of gyroscopic precession which provides a complete covariant formalism. We analyse the precession close to spherically symmetric Schwarzschild and Schwarzschild in anti-de-Sitter blackhole. In order to eliminate the artefacts due coordinate singularity close to horizon, we also use horizon penetrating Kerr-Schild coordinates. Our study show there is no concrete evidence to suggest that the gyroscopic precession frequency should blow-up at event horizon. As a counter example, we construct a time-like curve passing through the event horizon along which the gyroscopic precession frequency is finite.