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"Ringdown beyond Kerr"


Aaron Zimmerman , University of Texas
[Host: Alexander Grant]
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 "no-hair" 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

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"Infrared Finite Scattering Theory in QFT and Quantum Gravity"


Gautam Satishchandran , Princeton University
[Host: Alexander Grant]
ABSTRACT:

A long-standing problem in QFT and quantum gravity is the construction of an "IR-finite" S-matrix. 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 IR-finite S-matrix 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, Yang-Mills theories, linearized quantum gravity with massless/massive sources, and in full quantum gravity. In the case of quantum gravity, we prove that the only "Faddeev-Kulish" state is the vacuum state. We also show that "non-Faddeev-Kulish" 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 IR-finite scattering theory.

Gravity Seminar
Monday, October 24, 2022
1:30 PM
Physics, Room 313

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Available
ABSTRACT:

Available

Gravity Seminar
Monday, October 31, 2022
1:30 PM
Physics, Room https://virginia.zoom.us/j/92309274285?pwd=WU9IS3NGa3hKRUdnanN6N
Note special room.

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ABSTRACT:

Recently there has been significant interest in building data-driven gravitational-wave models directly from numerically generated data. These surrogate (or reduced-order) 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 one-body or numerical relativity (NR) waveform models, thereby reducing the overall runtime of a multi-query 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 extreme-mass ratio systems. For surrogates to be useful, it is necessary that they be publicly available, easy-to-use, and decoupled from the building codes which produce them. In this talk, I will also describe a lightweight open-source code, GWSurrogate, which addresses this issue and enables surrogate models to be used in gravitational-wave data analysis studies. Some preliminary results for re-analyzing GWTC-3 will also be shown.

Gravity Seminar
Monday, November 7, 2022
1:30 PM
Physics, Room 313

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"Ringdown overtones in black hole binary mergers"


Roberto Cotesta , John Hopkins University
[Host: Kent Yagi]
ABSTRACT:

The gravitational-wave signal emitted by the black-hole 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 so-called “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 post-merger 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 log-Bayes factor does not indicate the presence of an overtone, while the support for a non-zero 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 noise-dominated. We perform GW150914-like 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

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"Machine Learning for Gravitational Wave Astronomy "


Stephen Green , University of Nottingham
[Host: Alexander Grant]
ABSTRACT:

Since 2015, the LIGO-Virgo-KAGRA 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 simulation-based inference to enable improved accuracy in the face of non-stationary or non-Gaussian noise.

Gravity Seminar
Monday, December 5, 2022
1:30 PM
Physics, Room via Zoom (Zoom link and meeting ID provided below)
Note special room.

Meeting link: https://virginia.zoom.us/j/94422619812?pwd=L3RCS1FoZXliZzJ0Q3FQNDl3c0RUdz09


Meeting ID: 944 2261 9812

Passcode: 507640

 


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