Strong Gravity

The advent of precise measurements of neutron star properties has led to an explosion in "nuclear astrophysics": studying the properties of high-density matter using astrophysical phenomena.  Remarkably, constraints provided by nuclear theory and experiment and high-energy astrophysical observations are now competitive (and often complementary) in constraining the equation of state (EoS) of matter at supernuclear densities.  On the astrophysical side, data have provided a clearer picture how these constraints are affected by the choice of modeling the EoS.  Specifically, the nuclear EoS in astrophysical analyses is usually modeled phenomenologically, and often using ad hoc assumptions.  I will discuss why these ad hoc assumptions will likely cause problems, considering the deluge of coming neutron-star measurements, by comparing these approaches to a data-driven, "nonparametric", model.

Zoom link:  https://pitp.zoom.us/j/94435348102?pwd=OHF2MkNMWStNTlhmdkRQaElNL1M1Zz09

With over a hundred detections to date, the discoveries of compact binary mergers by gravitational wave observatories LIGO and Virgo have allowed us to start characterizing the astrophysical population of binary black holes. This task requires measuring the fifteen parameters (masses, spins, location, orientation, etc...) that characterize each merger event. However, these high dimensional distributions are challenging to describe due to the presence of nonlinear correlations and multiple modes. In this seminar I will describe a series of coordinate changes that, by identifying parameter combinations that control specific observable signatures in the data, remove these degeneracies and multimodality, making parameter estimation amenable. Among the new coordinates is a spin azimuth that can be measured surprisingly well in several cases, hinting that some black hole spins are misaligned with the orbit. This is very interesting because the degree of spin-orbit alignment is a robust discriminator between isolated and dynamical formation channels, which predict spins preferentially aligned with the orbit or randomly oriented, respectively. At the same time, I will show that the observed proportion of events with spins aligned versus anti-aligned with the orbit disfavors the hypothesis that the spin distribution is isotropic.

Zoom link:  https://pitp.zoom.us/j/91820222881?pwd=YW9vR0xwTlBCVXg4UlRBNWxuUFhCQT09

In this talk, I present a novel framework to do black-hole spectroscopy. This approach is based on a new technique so-called “quasinormal-mode filter”, which can be classified into two subsets: rational filter and full filter. On the theoretical level, I explain how to use the rational filter to understand the ringdown of numerical-relativity waveforms. On the observational level, I introduce a way to incorporate the filter into Bayesian analysis. The new Bayesian framework not only allows us to analyze the ringdown of a real gravitational-wave event without Markov chain Monte Carlo, but also yields a natural estimate of the ringdown start time. By applying our method to GW150914, we find strong and self-consistent evidence for the first overtone from multiple perspectives. On the other hand, the relationship between the full filter and the metric reconstruction is discussed. Its connection to a numerical-relativity technique “Cauchy-characteristic Matching” is provided. In the final part of the talk, I also briefly present our recent progress on fully relativistic 3D Cauchy-characteristic Matching.

Zoom link: https://pitp.zoom.us/j/95593408817?pwd=U2RXZDV2WUtwNDlaRDVBVTJjTEdBQT09

The Lense-Thirring spacetime describes a 4-dimensional slowly rotating approximate solution of vacuum Einstein equations valid to a linear order in rotation parameter. It is fully characterized by a single metric function of the corresponding static (Schwarzschild) solution. We shall discuss a generalization of the Lense-Thirring spacetimes to the case that is not necessarily fully characterized by a single (static) metric function. This generalization lets us study slowly rotating spacetimes in various higher curvature gravities as well as in the presence of non-trivial matter such as non-linear electrodynamics.  In particular, we construct slowly multiply-spinning solutions in Lovelock gravity and notably show that in four dimensions Einstein gravity is the only non-trivial theory amongst all up to quartic curvature gravities that
admits a Lense-Thirring solution characterized by a single metric function. We will also discuss a `magic square' version of our ansatz and show that it can be cast in the Painlevé-Gullstrand form (and thence is manifestly regular on the horizon) and admits a tower of exact rank-2 and higher rank Killing tensors that rapidly grows with the number of dimensions.

Zoom Link: TBD

The "infrared problem" is the generic emission of an infinite number of low-frequency quanta in any scattering process with massless fields. The "out" state contains an infinite number of such quanta which 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 the existence of a low-frequency tail of the radiation field i.e. the memory effect. In massive QED, the Faddeev-Kulish representations have been argued to yield an I.R. finite S-matrix. We clarify the "preferred " status of such representations as eigenstates of the conserved "large gauge charge'' at spatial infinity. We prove a "No-Go" theorem for the existence of a suitable Hilbert space analogously constructed in massless QED, QCD, linearized quantum gravity with massive/massless sources, and in full quantum gravity. We then suggest an "infrared-finite" formulation of scattering theory in terms of correlation functions without any a priori choice of "in/out" Hilbert spaces.

Population studies of gravitational-wave events allow us to probe stellar evolution and formation mechanisms of compact binaries. Recent work has painted a conflicting portrait of the distribution of black hole spins in merging binaries. In this talk, I describe the emerging picture of black hole spin using observations from LIGO-Virgo–KAGRA with a new spin model. We find evidence for two subpopulations of binary black holes: 1) merging binaries containing black holes with negligible spin, 2) binaries preferentially aligned with the orbital angular momentum and rapidly spinning. These results are consistent with predictions from studies of angular momentum transport in stars and suggest that the subpopulation of spinning black holes may be spun up via tidal interactions. I will also share insights of the binary black hole population using the latest catalogue of gravitational-wave events.

Zoom Link: https://pitp.zoom.us/j/93713298741?pwd=eFo2Q1lSTVBESWcyZXJlcFVKZ2hRQT09

What is the nature of neutron stars? Where are r-process elements formed in the Universe? Multi-messenger observations of neutron star mergers might provide us with the key to address these and other important open questions in astrophysics. However, multi-messenger astronomy also poses new challenges to theorists and observers attempting to turn complex data into answers. In this talk, I will review our current theoretical understanding of neutron star mergers, I will discuss how their dynamic is imprinted in their multi-messenger emissions, and I will present recent simulation results and their implications. Finally, I will discuss future challenges and prospective for this field.

Zoom Link: https://pitp.zoom.us/j/93367275850?pwd=YngycnN4ZXFDQUhQb2lVaHY3V2k0UT09

Among the discoveries in LIGO/Virgo/KAGRA's third observing run are a handful of compact binary coelescences (CBCs) that stand out for one reason or another -- exceptional either because they were the first entry in a previously undetected class of CBCs, or because of the mass, mass ratio, or spins of their component compact objects. After briefly discussing the implications of these observations and their merger rates, I will focus on the tension that has arisen from the discovery of the most massive binaries in LVK's catalog. These binaries have component black holes encroaching on the pair-instability mass gap, where black holes are not expected to be formed directly from stars. I'll discuss an alternate formation channel, where massive black holes are assembled dynamically from repeated binary black hole mergers, and an analysis that constructs a binary black hole population that allows for hierarchical formation in both globular cluster-like and nuclear star cluster-like environments.

Zoom Link: https://pitp.zoom.us/j/94867401133?pwd=bTJmVy8vUk9rb1RvTDlrMzl1ZHdEZz09

In this talk, I would like to discuss gravitational waves in gravitational effective field theories. In particular, I will focus on gravitational waves propagating on a black hole background, and will show that the coefficients of the higher-dimension operators in the EFT can be constrained by causality considerations. I will also comment on observability of these higher-dimension operators in gravitational wave detections.

Zoom Link: https://pitp.zoom.us/j/94226476006?pwd=MEdHWG9oQ2dwQ2hXMUd2ZEFFQnRjZz09

The gravitational radiation from the ringdown of a binary black hole merger is described by the solution of the Teukolsky equation, which predicts both the temporal and angular dependence of the emission. Many studies have explored the temporal feature of the ringdown wave through black hole spectroscopy. In this work, we further study the spatial distribution, by introducing a global fitting procedure over both temporal and spatial dependences, to propose a more complete test of General Relativity. We show that spin-weighted spheroidal harmonics are the better representation of the ringdown angular emission patterns compared to spin-weighted spherical harmonics. The differences are distinguishable in numerical relativity waveforms. We also study the correlation between progenitor binary properties and the excitation of quasinormal modes, including higher-order angular modes, overtones, prograde and retrograde modes. Specifically, we show that the excitation of retrograde modes is dominant when the remnant spin is anti-aligned with the binary orbital angular momentum. This study seeks to provide an analytical strategy and inspire the future development of ringdown test using real gravitational wave events.

In this talk I will outline recent attempts to probe black holes in the strong gravity regime. The access to gravitational wave emission from binary black hole mergers and images of supermassive black holes allow for new tests of general relativity. After reviewing recent activities, I will outline how quasi-normal modes and shadow images can be used to study possible deviations from general relativity. Finally, I will discuss open problems that need to be addressed in the future.