# Gravity Seminars

## "Observational signatures of tidal deformability in compact objects"

**Carl-Johan Haster , MIT Kavli Institute for Astrophysics and Space Research**

[Host: Alexander Saffer]

After having observed gravitational waves from a population of nearly 100 compact-object 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 gravitational-wave observations, and discuss some of the potential complications and limitations the observations over the coming years might bring as the sensitivity of the gravitational-wave detectors are improved. I will also showcase a study about what possibilities the next generation of gravitational-wave 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 “non-Neutron 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 uniform-density 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 ultracompact-object binaries observed so far.

**Gravity**

Monday, April 18, 2022

11:00 AM

Physics Building, Room 313

Note special time.

## "Gravitational-wave multi-messenger observations: from the production of gold to the age of the Universe"

**Hsin-Yu Chen , MIT - Massachusetts Institute of Technology**

[Host: Alexander Saffer]

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 gravitational-wave events? (ii) What can we learn about neutron star nuclear physics from gravitational-wave observations? I will close with my outlook on the immense scientific opportunities that the next-generation gravitational-wave detectors will provide.

## "Binary-black-hole systems in general relativity"

**Arwa Elhashash , University of Virginia - Department of Physics**

[Host: Prof. David Nichols]

Studying binary-black-hole 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.

We can study such systems by considering them to be isolated in an empty space that approaches flatness at infinity. Any changes in the spacetime can be quantified using the changes in charges conjugate to the asymptotic symmetries of the spacetime. In this talk, I will first discuss some of these charges, focusing on the angular momentum and the discrepancies in its definition in several different formalisms. Then, I will discuss a lasting effect called the gravitational-wave memory and an ongoing effort of developing waveform models of this effect for nonspinning binary-black-hole mergers.

**Join Zoom Meeting:**https://virginia.zoom.us/j/98187156266**Meeting ID: **981 8715 6266 **Passcode: **569653

Monday, March 28, 2022

1:30 PM

Online, Room via Zoom

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## "Black Holes and Compact Objects in Gravitational-Wave Astronomy"

**Horng Sheng Chia , Institute for Advanced Study**

[Host: Alexander Grant]

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 so-called "finite-size effects" of the binary constituents, which leave measurable imprints on waveforms. In this talk, I will first discuss the finite-size 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 finite-size effects to detect new types of compact objects in the data.

## "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]

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 matter-quark matter phase transition that can potentially resonate during the inspiral stage, producing effects in the GW signal measurable by the ground-based detectors. In the second project, we study the effect of dynamical tide on eccentric binary white dwarf systems. We show that the non-resonant buildup of the oscillation modes can produce an orbit precession resolvable by the upcoming space-based GW detector. For orbits with high eccentricities and small pericenter separations, it can even cause the waveform to become chaotic.

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 tensor-like 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.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/97128942029

**Meeting ID: **971 2894 2029 **Passcode: **200012

Monday, February 28, 2022

1:30 PM

Online, Room via Zoom

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## "Probing subatomic physics with gravitational waves from neutron star binary inspirals"

**Dr. Tanja Hinderer , Utrecht University**

[Host: Alexander Saffer]

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 supra-nuclear 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 strong-field gravity leads to characteristic signatures in the gravitational waves. I will also highlight new insights gained from recent gravitational-wave discoveries, and conclude with an outlook onto the remaining challenges and exciting prospects for the next years, as gravitational-wave science continues to move towards an era of precision physics.

TBA

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/97270745862

**Meeting ID: **972 7074 5862 **Passcode: **567149

Monday, January 31, 2022

1:30 PM

Online, Room via Zoom

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## "Gravitational waveforms for compact binaries from second-order self-force theory "

**Barry Wardell , University College Dublin**

[Host: Alexander Grant]

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 second-order gravitational-wave energy flux and for the gravitational waveform.

**Join Zoom Meeting:**##### https://us02web.zoom.us/j/84365378162

**Meeting ID:** 843 6537 8162 **Passcode:** 881928

Monday, January 24, 2022

1:30 PM

Online, Room via Zoom

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## "Test of black hole firewall hypothesis by electromagnetic emissions"

**Sudipta Hensh , Silesian University**

[Host: Prof. Kent Yagi]

A recent "firewall" proposal claims to have settled the black hole information paradox. It suggests that an infalling observer is expected to encounter high-energy 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 emission-line profile of a radiating Keplerian disk. The optical appearance of the black hole-accretion disk system will also be discussed in this context.

**Join Zoom Meeting:**https://virginia.zoom.us/j/95625108281**Meeting ID:** 956 2510 8281 **Passcode:** 864174

Monday, December 6, 2021

1:00 PM

Online, Room via Zoom

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## "Probing neutron star matter with gravitational waves"

**Philippe Landry , Canadian Institute for Theoretical Astrophysics**

[Host: Alexander Grant]

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 gravitational-wave observations, complemented by radio and x-ray 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.

**Join Zoom Meeting:**https://virginia.zoom.us/j/92573320085**Meeting ID:** 925 7332 0085 **Passcode:** 208444

Monday, November 15, 2021

1:00 PM

Online, Room via Zoom

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## "Symmetries and Charges of Gravitational Subsystems"

**Venkatesa Chandrasekaran , Institute for Advanced Study**

[Host: Alexander Grant]

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.

**Join Zoom Meeting:**https://us02web.zoom.us/j/89487769062**Meeting ID: **894 8776 9062 **Passcode:** 287799

Monday, November 8, 2021

1:00 PM

Online, Room via Zoom

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## "Aspects of Rotating Black Holes in Dynamical Chern-Simons Gravity"

**Leah Jenks , Brown University**

[Host: Kent Yagi]

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.

**Join Zoom Meeting**##### https://virginia.zoom.us/j/94853461679

**Meeting ID:** 948 5346 1679 **Passcode:** 780696

Monday, November 1, 2021

1:00 PM

Online, Room via Zoom

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## "Black hole hair: from no-hair theorems to scalarization"

**Professor Thomas Sotiriou , The University of Nottingham**

[Host: Alexander Saffer]

In general relativity black holes are fully characterised by their mass, spin, and electromagnetic charge. No-hair theorems indicate that scalar fields cannot affect black hole spacetimes. However, the devil is in the details and, in practice, no-hair 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.

**Join Zoom Meeting**##### https://virginia.zoom.us/j/98368369604

**Meeting ID:** 983 6836 9604 **Passcode:** 130144

Monday, October 18, 2021

1:00 PM

Online, Room via Zoom

Note special time.

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## "Gravitational radiation from a binary black hole coalescence in Einstein-scalar-Gauss-Bonnet gravity"

**Felix-Louis Julie , Johns Hopkins University**

[Host: Alexander Saffer]

I will show how to derive analytic gravitational waveforms associated to the coalescence of a “hairy” black hole binary in Einstein-scalar-Gauss-Bonnet gravity (ESGB). I will present the ESGB post-Newtonian (PN) Lagrangian and gravitational wave fluxes. The PN framework relies on reducing the black holes to point particles with scalar-field-dependent 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 Effective-One-Body (EOB) formalism to ESGB gravity.

**Join Zoom Meeting**##### https://virginia.zoom.us/j/92281876173

**Meeting ID:** 922 8187 6173 **Passcode:** 606888

Monday, October 4, 2021

1:00 PM

Online, Room via Zoom

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## "Resonances in black hole spacetimes"

**Dr. Béatrice Bonga , Radboud University**

[Host: Alexander Grant]

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.

**Join Zoom Meeting**##### https://virginia.zoom.us/j/98166651049

**Meeting ID: **981 6665 1049 **Passcode: **726235

Monday, April 26, 2021

1:00 PM

Online, Room Zoom

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## "High-performance Gravitational Wave Extraction in SpECTRE"

**Jordan Moxon , California Institute of Technology**

[Host: Alexander Grant]

Numerical relativity stands as the primary method of generating precise waveform predictions for comparable-mass 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 well-behaved 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+.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/98166651049

**Meeting ID:** 981 6665 1049

**Password:** 726235

**Meeting ID:**

**Password:**Monday, April 5, 2021

1:00 PM

Online, Room via Zoom

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## "Formation and dynamics of extreme mass ratio inspirals with environmental effects"

**Huan Yang , Perimeter Institute and University of Guelph**

[Host: Alexander Grant]

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 disk-assisted 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.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/97049511556

**Meeting ID: **970 4951 1556

**Password:** 707548

**Meeting ID:**

**Password:**Monday, March 22, 2021

6:00 PM

Online, Room via Zoom

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## "Adventures in Gravitational-wave Astronomy: testing for hair, memory, and eccentricity"

**Paul Lasky , Monash University**

[Host: Alexander Saffer]

Since the first gravitational-wave 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 ultra-strong regime of gravity, as well as the formation history of these systems. I will discuss ongoing efforts to test general relativity in the ultra strong-field regime, including tests of the no-hair theorem and searches for gravitational-wave 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 second-generation black hole merger.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/98166651049

**Meeting ID: **981 6665 1049

**Password:** 726235

**Meeting ID:**

**Password:**Monday, March 8, 2021

1:00 PM

Online, Room Zoom

Note special time.

Note special room.

## "Recent developments in the post-Minkowskian approach to the spinning black hole binary problem"

**Justin Vines , Max Planck Institute for Gravitational Physics (Albert Einstein Institute)**

[Host: Alexander Grant]

The detection and analysis of gravitational wave signals from coalescing binary systems crucially relies on analytic perturbative approaches to the two-body problem in general relativity (as well as on numerical approaches). While the post-Newtonian (weak-field and slow-motion) approximation is most directly relevant to observations by LIGO et al., recent developments have revived interest in the more inclusive post-Minkowskian (weak-field but arbitrary-speed) 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 high-energy quantum physics and gravitational-wave science has led to several new results and useful insights, particularly regarding relationships between complimentary approximation schemes; this importantly also includes the "self-force" or "post-test-body" approach, treating small mass ratios but arbitrary field strengths and speeds. We will review some of these developments, focusing on the post-Minkowskian treatment of the spinning black hole binary problem.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/98692700509

**Meeting ID:** 986 9270 0509

**Password: **525364

**Meeting ID:**

**Password:**Monday, February 22, 2021

1:00 PM

Online, Room Zoom

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## "Testing the Schwarzschild/Kerr black hole hypothesis with gravitational and electromagnetic waves"

**Alejandro Cardenas-Avendano , University of Illinois Urbana-Champaign**

[Host: Alexander Saffer]

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 comparable-mass black holes collide, and when a small compact object falls into a supermassive one in an extreme mass-ratio 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 near-future electromagnetic observations.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/99269571512

**Meeting ID:** 992 6957 1512

**Password:** 474477

**Meeting ID:**

**Password:**Monday, February 8, 2021

1:00 PM

Online, Room Zoom

Note special time.

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## "The classical evolution of binary black hole systems in scalar-tensor theories"

**Justin Ripley , University of Cambridge**

[Host: Alexander Saffer]

In this talk I will discuss recent work on numerically solving for the binary black hole dynamics of black holes in Einstein scalar Gauss-Bonnet (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 relativity--in 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 scalar-tensor 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.

To add a speaker, send an email to dn2ep@Virginia.EDU Include the seminar type (e.g. Gravity Seminars), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-speakers@Virginia.EDU.]