# Gravity Seminars

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**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

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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.

## "Measurements with gravitational-wave observations: from the heavy metals production to the Universe expansion "

**(CANCELLED) Hsin-Yu Chen , MIT - Massachusetts Institute of Technology**

[Host: Alexander Saffer]

Over last six years, the observation of gravitational-wave signals from compact binary mergers have significantly expanded our knowledge in physics and astronomy. In this talk, I will discuss what we learned in cosmology, nuclear physics, and heavy-elements production from LIGO-Virgo observations. I will then discuss the future aspects and a few challenges in these areas.

**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 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 room.

## "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.]