"Gravitational Wave Memory Beyond General Relativity"Jann Zosso , ETH Zurich; University of Illinois UrbanaChampaign [Host: Kent Yagi]
ABSTRACT:
The gravitational wave memory effect that manifests itself as a permanent distortion of space after a gravitational wave burst, is a prediction of GR, whose detection is expected to be around the corner. In this talk, I will present recent work, in which the memory effect of a broad class of beyond GR metric theories of gravity is shown to be modified primarily through the presence of additional propagating modes in the theory. This strongly suggests the possibility of using memory observations as a model independent test of the number of dynamical gravitational degrees of freedom. Such a general statement rests upon a reinterpretation of the memory effect as naturally arising within the broadly applicable Isaacson approach to gravitational waves. 
Gravity Seminar Thursday, April 27, 2023 2:00 PM , Room Virtual https://us02web.zoom.us/j/83990634739?pwd=SnQ4M09wd0NYQzFld2Zzc08raElRdz09 Meeting ID: 839 9063 4739 Passcode: 576874 
"Astrophysical Lessons from LIGOVirgoKAGRA's Black Holes"Maya Fishbach , University of Toronto [Host: Kent Yagi]
ABSTRACT:
The LIGOVirgoKAGRA Collaboration has observed over 70 gravitationalwave sources to date, including mergers between black holes, neutron stars, and mixed neutron star—black holes. Focusing on the black hole mergers, I will describe some recent lessons into how, when, and where black holes are made. These questions are connected to several astrophysical puzzles, including the deaths of massive stars, the growth of black holes across cosmic time, highredshift star formation, and properties of globular clusters. 
Gravity Seminar Monday, April 10, 2023 1:30 PM , Room Zoom https://us02web.zoom.us/j/86764277969?pwd=UnJDOVVlalU3bTdwZFptbzBqQkJnQT09 Meeting ID: 867 6427 7969 Passcode: 906268 
"Towards Gravitational Wave Asteroseismology with Compact Binary Inspirals"Geraint Pratten , University of Birmingham [Host: David Nichols]
ABSTRACT:
Gravitational wave observations of neutron star mergers are one of the most exciting prospects for constraining the incredibly uncertain equation of state of cold supranuclear matter. Signatures of the stellar interior are directly imprinted in the gravitationalwave signal through characteristic tidal interactions, providing a vital observational playground in our quest to understand fundamental physics interactions deep in the cores of neutron stars. In this talk I will discuss the underlying physics behind binary neutron star mergers and how recent developments are showcasing the importance of describing the dynamical response of neutron stars when studying gravitationalwave observations. Finally, I will highlight the prospects for performing gravitationalwave asteroseismology with binary neutron star inspirals in current and future detector networks. 
Gravity Seminar Monday, March 27, 2023 1:30 PM , Room Zoom Meeting ID: 979 7765 8329
https://virginia.zoom.us/j/97977658329?pwd=cDluSDJ0QlUzV3M0dWhHODVGc1FFQT09 
"A study of gravitational wave memory effects in radiative geometries and wormholes "Indranil Chakraborty , IIT Kharagpur [Host: David Nichols]
ABSTRACT:
Gravitational memory effects are persistent deformations caused due to the passage of a gravitational wave pulse. It is a nonlinear effect in General Relativity that remains yet to be detected. In this talk, primarily from a theoretical perspective, we study memory effects in two different settings: i) Radiative geometries, ii) Lorentzian wormholes. In radiative geometries, we show how geodesic and geodesic deviation equations encode the gravitational wave memory. In the latter case, we perform a BondiSachs analysis and try to show how the Bondi mass loss depends on the wormhole hair. 
Gravity Seminar Monday, March 20, 2023 1:30 PM , Room Zoom Join Zoom Meeting https://virginia.zoom.us/j/97977658329?pwd=cDluSDJ0QlUzV3M0dWhHODVGc1FFQT09
Meeting ID: 979 7765 8329 Passcode: 357957 
"Hypermassive neutron stars and short gamma ray bursts"Cecilia Chirenti , University of Maryland [Host: Kent Yagi]
ABSTRACT:
Gamma ray bursts (GRBs) are the most luminous electromagnetic events in the universe. Short GRBs, typically lasting less than 2 seconds, have already been associated with binary neutron star (BNS) mergers, which are also sources of gravitational waves (GWs). The ultimate fate of a BNS, after coalescence, is usually expected to be a black hole (BH) with 23 solar masses. However, numerical relativity simulations indicate the possible formation of a shortlived hypermassive neutron star (HMNS), lasting for tens to hundreds of milliseconds after the BNS merger and before gravitational collapse forms a BH. The HMNS is expected to emit GWs with kHz frequencies that will be detectable by third generation groundbased GW detectors in the 2030s. I will present results from a recent analysis that revealed evidence for HMNSs by looking for kHz qusiperiodic oscillations in gammaray observations obtained in the 1990s with the Compton Gamma Ray Observatory. 
Gravity Seminar Monday, March 13, 2023 1:30 PM Physics, Room 313 
"Gravitational Faraday rotation of gravitational waves by a Kerr black hole"Zhao Li , University of Science and Technology of China [Host: Kent Yagi]
ABSTRACT:
Gravitational Faraday Rotation (GFR) is a framedragging effect induced by rotating massive objects, which is one of the important characteristics of lensed gravitational waves (GWs). In my previous work, we calculate the GFR angle of GWs in the weak deflection limit, assuming it is lensed by a Kerr black hole (BH). We find that the GFR effect changes the initial polarization state of the lensed GW. Compared with the Einstein deflection angle, the dominant term of the rotation angle is a secondorder correction to the polarization angle, which depends on the lightofsight component of BH angular momentum. Such a rotation is tiny and degenerates with the initial polarization angle. In some critical cases, the GFR angle is close to the detection capability of the thirdgeneration GW detector network, although the degeneracy has to be broken. 
Gravity Seminar Monday, March 6, 2023 1:30 PM , Room Zoom https://us02web.zoom.us/j/86544012000?pwd=ckJpMTY5L0dIbHc5WTZxNFMvM1lkdz09 Meeting ID: 865 4401 2000 Passcode: 033748 
"Gravitational waves from a particle physics perspective"Jan Steinhoff , Max Planck Institute for Gravitational Physics [Host: David Nichols]
ABSTRACT:
Continuing the success of gravitational wave observations demands considerable improvements of their theoretical predictions in the next decade, in order to keep their accuracy on par with upgrades of the detectors. This urges innovations on the methods by which gravitational waves from compact binaries are calculated. In this talk, we focus on approaches to analytic, perturbative predictions for relativistic binaries inspired by highenergy physics. In this area, effective field theories are highly useful and scattering amplitudes (the primary observable) can be calculated very efficiently using novel tools. These methods can indeed be applied the classical binaries and their gravitational waves. We give a basic introduction to the ideas of these approaches and recent progress. 
Gravity Seminar Monday, February 27, 2023 1:30 PM Clark Hall  Virtual talk, Room 108 Join Zoom Meeting https://virginia.zoom.us/j/97977658329?pwd=cDluSDJ0QlUzV3M0dWhHODVGc1FFQT09
Meeting ID: 979 7765 8329 Passcode: 357957 
"NextGeneration Black Hole and Neutron Star Collision Simulations"Zachariah Etienne , University of Idaho [Host: Alexander Grant]
ABSTRACT:
Perhaps the most significant astronomical discovery of our lifetimes, codenamed GW170817, involved the collision of two neutron stars. The collision was detected both by gravitational wave observatories, and traditional electromagnetic telescopes. As neutron stars are made of the densest form of matter in our current Universe, this single "multimessenger" event was a watershed moment in our understanding as to how matter and gravity behave at their most extreme, far beyond what we can study in laboratories on Earth. For the most part, we compare observations against theoretical models to extract science from events like this. Unfortunately, these theoretical models are severely limited both in quality and quantity, leading to a critical need to improve them. Such improvements pose a key challenge to computational astrophysics, as our most detailed models require expensive supercomputer simulations that generate full, nonperturbative solutions of the general relativistic field equations (numerical relativity). After a gentle introduction to multimessenger astrophysics and the challenges associated with multimessenger source modeling, I will outline a new approach aimed at greatly reducing the cost of these simulations. With the reduced cost comes the potential to both perform colliding black hole simulations on the consumergrade desktop computer, as well as add unprecedented levels of physical realism to colliding neutron star simulations on supercomputers. 
Gravity Seminar Monday, February 13, 2023 1:30 PM Physics, Room Zoom Zoom link: https://virginia.zoom.us/j/99508589926?pwd=TUFNMDJZMjZRUTJnaFd3TDgxNi9KZz09 Meeting ID: 995 0858 9926 Password: 441596 
"Observational consequences of nonlinear memory of gravitational waves"Shubhanshu Tiwari , University of Zurich [Host: Alexander Grant]
ABSTRACT:
Nonlinear memory is one of the most intriguing predictions of general relativity which is generated by the passage of gravitational waves (GWs) leaving the spacetime permanently deformed. For example a GW signal from binary black hole (BBH) will have two parts the oscillatory part which is known as the “chirp” and a much fainter nonoscillatory (DC like) part which is nonlinear memory. A nonlinear memory is produced by all the sources of GWs and has the peculiarity that even if the oscillatory part of the source lies at high frequency the nonlinear memory will be available at low frequency. This property of nonlinear memory makes it a valuable resource for GW astronomy. In this talk I will provide and introduction to how we can use gravitational waves memory as a resource for the current and future ground based detectors. To do this I will show examples of how one can creatively use the nonlinear memory to probe seemingly inaccessible sources of GWs like ultra low mass compact binary mergers where the oscillatory part lies at outside the reach of any current detectors and only nonlinear memory could be detected if these sources exist. Another example will be the matter effects from binary neutron stars and black hole neutron star binaries which are at high frequency but the nonlinear memory is accessible. I will also discuss the postmerger neutron star memory and the prospects of its detection. 
Gravity Seminar Monday, January 30, 2023 1:30 PM Physics, Room Zoom Zoom link: https://virginia.zoom.us/j/96960494406?pwd=MFBMaDRvb1BQc1V3RVRpcm14eVdCQT09 Meeting ID: 969 6049 4406 Passcode: 287180 
"Machine Learning for Gravitational Wave Astronomy "Stephen Green , University of Nottingham [Host: Alexander Grant]
ABSTRACT:
Since 2015, the LIGOVirgoKAGRA Collaboration has detected 90 signals from merging compact objects such as black holes and neutron stars. Each of these is analyzed using Bayesian inference, employing a stochastic algorithm such as Markov Chain Monte Carlo to compare data against models—thereby characterizing the source. However, this is becoming extremely costly as event rates grow with improved detector sensitivity. In this talk I will describe a powerful alternative using probabilistic deep learning to analyze each event in orders of magnitude less time while maintaining strict accuracy requirements. This uses simulated data to train a normalizing flow to model the posterior distribution over source parameters given the data—amortizing training costs over all future detections. I will also describe the use of importance sampling to establish complete confidence in these deep learning results. Finally I will describe prospects going forward for simulationbased inference to enable improved accuracy in the face of nonstationary or nonGaussian noise. 
Gravity Seminar Monday, December 5, 2022 1:30 PM Physics, Room via Zoom (Zoom link and meeting ID provided below) Meeting link: https://virginia.zoom.us/j/94422619812?pwd=L3RCS1FoZXliZzJ0Q3FQNDl3c0RUdz09
Passcode: 507640

"Ringdown overtones in black hole binary mergers"Roberto Cotesta , John Hopkins University [Host: Kent Yagi]
ABSTRACT:
The gravitationalwave signal emitted by the blackhole remnant resulting from a binary merger such as GW190514 consists in a superposition of damped sinu soids known as quasinormal modes. Besides the “fundamental" mode (the one with the longest damping time), it is important to detect the socalled “over tones" (modes with shorter damping time), because a measurement of their frequencies could allow us to identify the remnant as a Kerr black hole. We discuss theoretical and observational issues in the analysis of ringdown over tones. We present theoretical arguments showing that the spacetime is not well described as a linearly perturbed black hole close to the peak of the waveform amplitude. Then we analyze GW150914 postmerger data to understand if recent ringdown overtone detection claims are robust. We find no evidence in favor of an overtone in the data after the waveform peak. Around the peak, the logBayes factor does not indicate the presence of an overtone, while the support for a nonzero amplitude is sensitive to changes in the starting time much smaller than the overtone damping time. This suggests that claims of an overtone detection are noisedominated. We perform GW150914like injections in neighboring segments of the real detector noise, and we show that noise can indeed induce artificial evidence for an overtone.

Gravity Seminar Monday, November 21, 2022 1:30 PM Physics, Room 313 
"Surrogate models, methods, and applications: Learning highfidelity gravitationalwave models from numerical relativity data"Scott Field , University of Massachusetts [Host: Alexander Grant]
ABSTRACT:
Recently there has been significant interest in building datadriven gravitationalwave models directly from numerically generated data. These surrogate (or reducedorder) models can faithfully reproduce a parameterized gravitational wave model specified through computationally expensive ordinary or partial differential equations with significant speedups. Surrogates can be used, for example, to accelerate the generation of effective onebody or numerical relativity (NR) waveform models, thereby reducing the overall runtime of a multiquery data analysis study. In this talk, I will summarize the key algorithms and approaches toward building surrogate models as well as survey recent models that cover more of the binary black hole parameter space, including precession, eccentricity, and large to extrememass ratio systems. For surrogates to be useful, it is necessary that they be publicly available, easytouse, and decoupled from the building codes which produce them. In this talk, I will also describe a lightweight opensource code, GWSurrogate, which addresses this issue and enables surrogate models to be used in gravitationalwave data analysis studies. Some preliminary results for reanalyzing GWTC3 will also be shown. 
Gravity Seminar Monday, November 7, 2022 1:30 PM Physics, Room 313 
"Infrared Finite Scattering Theory in QFT and Quantum Gravity"Gautam Satishchandran , Princeton University [Host: Alexander Grant]
ABSTRACT:
A longstanding problem in QFT and quantum gravity is the construction of an "IRfinite" Smatrix. In the gravitational case, the existence of these "infrared divergences" is intimately tied to the "memory effect" (i.e. the permanent displacement of test masses due to the passage of a gravitational wave) and the existence of an infinite number of conserved charges at spatial infinity. In this talk, I shall explain the origin of these connections and illustrate that the construction of an IRfinite Smatrix requires the inclusion of states with memory (which do not lie in the standard Fock space). In massive QED an elegant solution to this problem was provided by Faddeev and Kulish who constructed an incoming/outgoing Hilbert space of charged particles "dressed" with memory. However, we show that this construction fails in the case of massless QED, YangMills theories, linearized quantum gravity with massless/massive sources, and in full quantum gravity. In the case of quantum gravity, we prove that the only "FaddeevKulish" state is the vacuum state. We also show that "nonFaddeevKulish" representations are also unsatisfactory. Therefore, in full quantum gravity, it seems that there does not appear to be any (separable) Hilbert space of incoming/outgoing states that can accommodate all scattering states. Therefore we argue that, if one wants to treat scattering theory at a fundamental level, one must take an "algebraic approach" which does not require an a priori choice of Hilbert space. We outline the framework of such a manifestly IRfinite scattering theory. 
Gravity Seminar Monday, October 24, 2022 1:30 PM Physics, Room 313 
ABSTRACT:
The final phase of gravitational radiation from black hole binaries is the ringdown of the merged black holes, which occurs at a characteristic set of frequencies. Measurements of the ringdown spectrum can provide especially clean tests of the nature of the final black hole, potentially revealing violations of the famous "nohair" theorem. The large number of gravitational wave observations that will be made in the coming years together with recent advances in the analysis of ringdown indicate that precision tests will be possible in the near future. Going beyond null tests, however, requires predictions of the ringdown spectrum in theories beyond GR and in spacetimes beyond Kerr. In this talk I will outline a flexible approach for calculating the ringdown spectrum of spinning black holes in theories where deviations from GR are small, and discuss future prospects for precision ringdown tests. 
Gravity Seminar Monday, October 10, 2022 1:30 PM Physics, Room 313 
"Observational signatures of tidal deformability in compact objects"CarlJohan Haster , MIT Kavli Institute for Astrophysics and Space Research [Host: Alexander Saffer]
ABSTRACT:
After having observed gravitational waves from a population of nearly 100 compactobject binaries, we are now in a position where we can start probing their internal structure and how they can be deformed by the strong tidal fields in which the binaries exist. I will present current observational constraints of the Neutron Star Equation of State from gravitationalwave observations, and discuss some of the potential complications and limitations the observations over the coming years might bring as the sensitivity of the gravitationalwave detectors are improved. I will also showcase a study about what possibilities the next generation of gravitationalwave detectors will bring, and what being able to observe a population of neutron stars at cosmological distances will teach us. Finally, I will present a study focusing on the observed “nonNeutron Stars”, and if they can be described as compact objects other than Black Holes. This can be done by extending the Black Hole description to also allow for them to be tidally deformed following the behavior of a uniformdensity ultracompact star (Schwarzschild star). Assuming that the population of Binary Black Holes, as observed in gravitational waves instead can be described as binaries of these Schwarzschild stars I present strong constraints on their compactness and tidal deformability, both as individual objects and for the population at large. Through these constraints, I comment on the viability of other categories of Black Hole mimickers as constituents of the ultracompactobject binaries observed so far. 
Gravity Seminar Monday, May 2, 2022 1:30 PM Physics, Room 313 
"Gravitationalwave multimessenger observations: from the production of gold to the age of the Universe"HsinYu Chen , MIT  Massachusetts Institute of Technology [Host: Alexander Saffer]
ABSTRACT:
The detections of gravitational waves from compact binary mergers and the followup observations of electromagnetic emissions together provide a powerful and independent tool to explore the Universe. With successive upgrades to the LIGO and Virgo sensitivities, and hence a growing number of detections, we are prepared to address a number of major questions in astrophysics. In this talk, I will discuss two topics in which I expect critical progress will be made in the next few years: (i) How do we make precise and accurate Hubble constant measurements with gravitationalwave events? (ii) What can we learn about neutron star nuclear physics from gravitationalwave observations? I will close with my outlook on the immense scientific opportunities that the nextgeneration gravitationalwave detectors will provide. 
Gravity Seminar Monday, April 18, 2022 11:00 AM Physics Building, Room 313 
"Binaryblackhole systems in general relativity"Arwa Elhashash , University of Virginia  Department of Physics [Host: Prof. David Nichols]
ABSTRACT:
Studying binaryblackhole systems has long been of great interest in general relativity, and is even more so after the first observed event in 2015 discovered by LIGO. 
Gravity Seminar Monday, April 4, 2022 1:30 PM Physics, Room 313 
"Black Holes and Compact Objects in GravitationalWave Astronomy"Horng Sheng Chia , Institute for Advanced Study [Host: Alexander Grant]
ABSTRACT:
Gravitational wave astronomy offers us a unique probe onto the nature of astrophysical bodies in coalescing binary systems. As a binary approaches merger, its dynamics is sensitive to various socalled "finitesize effects" of the binary constituents, which leave measurable imprints on waveforms. In this talk, I will first discuss the finitesize effects of Kerr black holes, focusing on their tidal response to a perturbing companion where most of recent progress has been made. I will then describe recent efforts to use these finitesize effects to detect new types of compact objects in the data. 
Gravity Seminar Monday, March 28, 2022 1:30 PM Online, Room via Zoom Join Zoom Meeting: https://virginia.zoom.us/j/98187156266Meeting ID: 981 8715 6266 Passcode: 569653 
"The imprints of stellar oscillations in gravitational wave signals from binary compact stars"Shu Yan Lau , University of Virginia  Department of Physics [Host: Prof. Kent Yagi]
ABSTRACT:
In this talk, I will present two separate projects related to the imprints of stellar oscillations on the gravitational wave (GW) signals. In the first project, we consider the signals from binary hybrid star (HS) coalescence events. The HS is a special type of neutron star which core is made of deconfined quark matter. We show that the HSs have a special oscillation mode associated with the nuclear matterquark matter phase transition that can potentially resonate during the inspiral stage, producing effects in the GW signal measurable by the groundbased detectors. In the second project, we study the effect of dynamical tide on eccentric binary white dwarf systems. We show that the nonresonant buildup of the oscillation modes can produce an orbit precession resolvable by the upcoming spacebased GW detector. For orbits with high eccentricities and small pericenter separations, it can even cause the waveform to become chaotic. 
Gravity Seminar Monday, March 21, 2022 1:30 PM Physics, Room 313 
ABSTRACT:
Gravitational waves (GWs) allow us to probe the content of the Universe and the behaviour of gravity on cosmological scales, through information contained in their propagation. In this talk, I will discuss particular cosmological scenarios where GWs interact with another tensorlike field, such as in the theory of massive bigravity. I will illustrate explicitly how the GW signal from a coalescence of black holes evolves during cosmological propagation, which could exhibit phase distortions and echoes of the GW signal emitted. These strong features suggest that stringent constraints on interacting GWs can be placed with current and future GW detectors. 
Gravity Seminar Monday, March 14, 2022 1:30 PM Physics, Room 313 
"Probing subatomic physics with gravitational waves from neutron star binary inspirals"Dr. Tanja Hinderer , Utrecht University [Host: Alexander Saffer]
ABSTRACT:
The gravitational waves from merging binary systems carry unique information about the internal structure of compact objects. This is of key interest for neutron stars, whose material is compressed by their tremendous gravity to supranuclear densities. Under such extreme conditions we expect novel, unexplored phases of matter to emerge. In this talk, I will describe examples of how the matter properties and their interplay with strongfield gravity leads to characteristic signatures in the gravitational waves. I will also highlight new insights gained from recent gravitationalwave discoveries, and conclude with an outlook onto the remaining challenges and exciting prospects for the next years, as gravitationalwave science continues to move towards an era of precision physics. 
Gravity Seminar Monday, February 28, 2022 1:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/97128942029Meeting ID: 971 2894 2029 Passcode: 200012 
ABSTRACT:
TBA 
Gravity Seminar Monday, February 14, 2022 1:30 PM Physics, Room 313 
"Gravitational waveforms for compact binaries from secondorder selfforce theory "Barry Wardell , University College Dublin [Host: Alexander Grant]
ABSTRACT:
Extreme mass ratio inspirals (EMRIs) are expected to be a key source of gravitational waves for the LISA mission. In order to extract the maximum amount of information from EMRI observations by LISA, it is important to have an accurate prediction of the expected waveforms. In particular, it will be necessary to have waveforms that incorporate effects that appear at second order in the mass ratio. In this talk I will present the latest progress towards this goal, including recent results for the secondorder gravitationalwave energy flux and for the gravitational waveform. 
Gravity Seminar Monday, January 31, 2022 1:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/97270745862Meeting ID: 972 7074 5862 Passcode: 567149 
"Test of black hole firewall hypothesis by electromagnetic emissions"Sudipta Hensh , Silesian University [Host: Prof. Kent Yagi]
ABSTRACT:
A recent "firewall" proposal claims to have settled the black hole information paradox. It suggests that an infalling observer is expected to encounter highenergy quanta in a region near the horizon. The fate of the observers or the very existence of the ‘’firewall’’ itself is still up for debate. Leaving those aside, we are interested in understanding observational signatures of the ‘’firewall’’ at the classical level. With this motivation, we consider a reflective ‘’firewall’’ just outside the event horizon of a Kerr black hole. I will discuss the results of our model and imprint of the ‘’firewall’’ leaves on the emissionline profile of a radiating Keplerian disk. The optical appearance of the black holeaccretion disk system will also be discussed in this context. 
Gravity Seminar Monday, January 24, 2022 1:30 PM Online, Room via Zoom Join Zoom Meeting:https://us02web.zoom.us/j/84365378162Meeting ID: 843 6537 8162 Passcode: 881928 
"Probing neutron star matter with gravitational waves"Philippe Landry , Canadian Institute for Theoretical Astrophysics [Host: Alexander Grant]
ABSTRACT:
Neutron stars harbor the densest matter in the Universe. Its precise composition and structure are unknown because laboratory experiments cannot reproduce the extreme conditions in neutron star cores. Gravitational waves from neutron star mergers, on the other hand, directly probe the core matter as they carry imprints of tidal interactions that depend on the stars' internal structure. In this talk, I will describe how gravitationalwave observations, complemented by radio and xray pulsar measurements, are shedding light on the neutron star interior, revealing hints about nuclear interactions, phase transitions and exotic matter. I will also discuss applications of improved dense matter knowledge towards open questions in astrophysics. 
Gravity Seminar Monday, December 6, 2021 1:00 PM Online, Room via Zoom Join Zoom Meeting: https://virginia.zoom.us/j/95625108281Meeting ID: 956 2510 8281 Passcode: 864174 
"Symmetries and Charges of Gravitational Subsystems"Venkatesa Chandrasekaran , Institute for Advanced Study [Host: Alexander Grant]
ABSTRACT:
Gravitational subsystems, such as black holes, are important objects of study in both classical and quantum gravity. Insight into the gravitational degrees of freedom of a subsystem can be gained by analyzing the symmetries and charges of gravity in the associated spacetime subregion. In this talk, I will use the covariant phase space formalism to make progress on this problem in general relativity, with a particular focus on subregions bounded by null hypersurfaces, such as event horizons or causal diamonds. Surprisingly, on null surfaces the gravitational field will turn out to have infinitely many symmetries, akin to the BMS symmetries at null infinity. In a completely general setting in which the subsystem can emit or absorb radiation, I will then derive an infinite set of charges and conservation laws on the null surface, and explain their significance for physics deep in the gravitational field. Finally, I will describe progress towards an understanding of black hole entropy through this formalism. 
Gravity Seminar Monday, November 15, 2021 1:00 PM Online, Room via Zoom Join Zoom Meeting: https://virginia.zoom.us/j/92573320085 Meeting ID: 925 7332 0085 Passcode: 208444

"Aspects of Rotating Black Holes in Dynamical ChernSimons Gravity"Leah Jenks , Brown University [Host: Kent Yagi]
ABSTRACT:
In this talk I will give an overview of recent and ongoing work regarding rotating black holes in dynamical ChernSimons (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 ChernSimons 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. VIDEO:

Gravity Seminar Monday, November 8, 2021 1:00 PM Online, Room via Zoom Join Zoom Meeting: https://us02web.zoom.us/j/89487769062Meeting ID: 894 8776 9062 Passcode: 287799 
"Black hole hair: from nohair theorems to scalarization"Professor Thomas Sotiriou , The University of Nottingham [Host: Alexander Saffer]
ABSTRACT:
In general relativity black holes are fully characterised by their mass, spin, and electromagnetic charge. Nohair theorems indicate that scalar fields cannot affect black hole spacetimes. However, the devil is in the details and, in practice, nohair theorems allow us to identify a list of interesting exceptions in which scalar field leave their imprint on black holes. Such scenarios are of particular interest to gravitational wave searches for new fundamental physics. I will give an overview of how new fundamental scalars affect black hole spacetimes and of how this can be imprinted on gravitational wave observations. 
Gravity Seminar Monday, November 1, 2021 1:00 PM Online, Room via Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/94853461679Meeting ID: 948 5346 1679 Passcode: 780696 
"Gravitational radiation from a binary black hole coalescence in EinsteinscalarGaussBonnet gravity"FelixLouis Julie , Johns Hopkins University [Host: Alexander Saffer]
ABSTRACT:
I will show how to derive analytic gravitational waveforms associated to the coalescence of a “hairy” black hole binary in EinsteinscalarGaussBonnet gravity (ESGB). I will present the ESGB postNewtonian (PN) Lagrangian and gravitational wave fluxes. The PN framework relies on reducing the black holes to point particles with scalarfielddependent masses. In light of the first law of thermodynamics of ESGB black holes, I will show that this procedure amounts to fixing their Wald entropies. As a consequence, inspiraling ESGB black holes can grow scalar “hair” until they turn into naked singularities. I will then extend the scope of the PN approximation to the strong field regime near merger by generalizing the EffectiveOneBody (EOB) formalism to ESGB gravity. 
Gravity Seminar Monday, October 18, 2021 1:00 PM Online, Room via Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/98368369604Meeting ID: 983 6836 9604 Passcode: 130144 
"Resonances in black hole spacetimes"Dr. BÃ©atrice Bonga , Radboud University [Host: Alexander Grant]
ABSTRACT:
Resonances are ubiquitous in nature. In this talk, I will focus on resonances due to the interaction of two black holes orbiting a central massive black hole. Such tidal resonances will generically occur for EMRIs if nearby compact objects exist. By probing their influence on the phase of the EMRI waveform, we can in principle extract information about the environmental tidal field of the EMRI system, albeit at the cost of a more complicated EMRI waveform model. I will also describe mean motion resonances, which are a mechanism that can create the necessary conditions for tidal resonance to occur. 
Gravity Seminar Monday, October 4, 2021 1:00 PM Online, Room via Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/92281876173Meeting ID: 922 8187 6173 Passcode: 606888 
"Highperformance Gravitational Wave Extraction in SpECTRE"Jordan Moxon , California Institute of Technology [Host: Alexander Grant]
ABSTRACT:
Numerical relativity stands as the primary method of generating precise waveform predictions for comparablemass compact binary coalescences used in gravitational wave data analysis pipelines. To produce those predictions, the spacetime in the vicinity of the binary system must be simulated to high precision, and then the waveform information must be extracted from the dynamical spacetime near the binary. Cauchy Characteristic Evolution (CCE) offers a wave extraction method capable of efficiently determining a complete, asymptotically wellbehaved metric at Scri+. For our implementation of CCE in SpECTRE, we have developed a number of mathematical refinements of the CCE system that improves spectral convergence and eases the computation of detailed dynamical spacetime information at Scri+. Our implementation in SpECTRE is far faster than previous CCE codes, is free from logarithmic pathologies, and computes full waveform information: news, strain, and all five Weyl scalars. I will discuss the numerical methods used to improve the performance of SpECTRE CCE, as well as several projects that have used the system to study gravitational wave memory and other subtle waveform effects that become available with full access to waveform data at Scri+. VIDEO:

Gravity Seminar Monday, April 26, 2021 1:00 PM Online, Room Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/98166651049Meeting ID: 981 6665 1049 Passcode: 726235 
"Formation and dynamics of extreme mass ratio inspirals with environmental effects"Huan Yang , Perimeter Institute and University of Guelph [Host: Alexander Grant]
ABSTRACT:
In this talk I will discuss relevant environment effects (i.e., accretion disk, tidal gravitational field from close objects) that influence the formation and dynamics of extreme mass ratio inspirals (EMRIs), which are important sources for LISA. I will show that diskassisted EMRIs may be more commonly seen by LISA. They can be distinguished from EMRIs formed through cluster multibody scattering by eccentricity measurements. The disk force and tidal gravitational field from nearby objects may also leave observable imprints on the waveform of the EMRIs, which provide new opportunities in probing the stellar distributions and constraining the accretion physics at galactic centers.
VIDEO:

Gravity Seminar Monday, April 5, 2021 1:00 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/98166651049Meeting ID: 981 6665 1049

"Adventures in Gravitationalwave Astronomy: testing for hair, memory, and eccentricity"Paul Lasky , Monash University [Host: Alexander Saffer]
ABSTRACT:
Since the first gravitationalwave detection of a binary black hole merger in 2015, the LIGO and Virgo detectors have observed gravitational waves from more than 50 merging systems. That number is expected to increase significantly over the coming years as these experiments become even more sensitive. The increased number of detections, and the improved sensitivity of these instruments, allows us to probe the ultrastrong regime of gravity, as well as the formation history of these systems. I will discuss ongoing efforts to test general relativity in the ultra strongfield regime, including tests of the nohair theorem and searches for gravitationalwave memory  a permanent deformation of spacetime following the passage of a gravitational wave. I will also discuss efforts to detect orbital eccentricity in these systems, which has led to potentially the first observation of a secondgeneration black hole merger. 
Gravity Seminar Monday, March 22, 2021 6:00 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/97049511556Meeting ID: 970 4951 1556

"Recent developments in the postMinkowskian approach to the spinning black hole binary problem"Justin Vines , Max Planck Institute for Gravitational Physics (Albert Einstein Institute) [Host: Alexander Grant]
ABSTRACT:
The detection and analysis of gravitational wave signals from coalescing binary systems crucially relies on analytic perturbative approaches to the twobody problem in general relativity (as well as on numerical approaches). While the postNewtonian (weakfield and slowmotion) approximation is most directly relevant to observations by LIGO et al., recent developments have revived interest in the more inclusive postMinkowskian (weakfield but arbitraryspeed) approximation  particularly in relation to highly advanced techniques developed by particle physicists for computing relativistic quantum scattering amplitudes and associated classical observables. This interplay between highenergy quantum physics and gravitationalwave science has led to several new results and useful insights, particularly regarding relationships between complimentary approximation schemes; this importantly also includes the "selfforce" or "posttestbody" approach, treating small mass ratios but arbitrary field strengths and speeds. We will review some of these developments, focusing on the postMinkowskian treatment of the spinning black hole binary problem. VIDEO:

Gravity Seminar Monday, March 8, 2021 1:00 PM Online, Room Zoom Join Zoom Meeting:https://virginia.zoom.us/j/98166651049Meeting ID: 981 6665 1049

"Testing the Schwarzschild/Kerr black hole hypothesis with gravitational and electromagnetic waves"Alejandro CardenasAvendano , University of Illinois UrbanaChampaign [Host: Alexander Saffer]
ABSTRACT:
Over the past years, electromagnetic and gravitational observations have been used to understand the nature of black holes and the material around them. Our ability to learn about the underlying physics, however, depends heavily on our understanding of the gravity theory that describes the geometry around these compact objects, and for the electromagnetic observations, also on the complex astrophysics that produces the observed radiation. In this talk, I will discuss our current ability to constrain and detect deviations from general relativity using (i) the electromagnetic radiation emitted by an accretion disk around a black hole, and (ii) the gravitational waves produced when comparablemass black holes collide, and when a small compact object falls into a supermassive one in an extreme massratio inspiral. I will also compare the constraining capabilities of these two types of observations to show how current gravitational wave observations have already placed constraints on possible modifications to general relativity, that are more stringent than what can be achieved with current and nearfuture electromagnetic observations. VIDEO:

Gravity Seminar Monday, February 22, 2021 1:00 PM Online, Room Zoom Join Zoom Meeting:https://virginia.zoom.us/j/98692700509Meeting ID: 986 9270 0509

"The classical evolution of binary black hole systems in scalartensor theories"Justin Ripley , University of Cambridge [Host: Alexander Saffer]
ABSTRACT:
In this talk I will discuss recent work on numerically solving for the binary black hole dynamics of black holes in Einstein scalar GaussBonnet (ESGB) gravity. This modified gravity theory can be motivated by effective field theory reasoning, and admits scalarized black hole solutions. These two facts make it a promising theory to constrain using binary black hole, gravitational wave observations. I will discuss how recent advances in mathematical relativityin particular, the development of the "modified harmonic formulation"have opened up the possibility of constructing fully nonlinear solutions to the equations of motion of ESGB gravity (in addition to a class of scalartensor modified theories known as "Horndeski" theories). I will discuss numerical simulations of single and binary black hole systems in these theories, and the gravitational and scalar radiation they emit. 
Gravity Seminar Monday, February 8, 2021 1:00 PM Online, Room Zoom Join Zoom Meeting:https://virginia.zoom.us/j/99269571512Meeting ID: 992 6957 1512

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