Fall semester 2024/25
12/11: Christoph Kehle (MIT)
Title: Gravitational collapse to extremal Reissner-Nordström and the third law of black hole thermodynamics
Abstract: In this talk, I will present a proof that extremal Reissner-Nordström black holes can form in finite time in gravitational collapse of charged matter. In particular, this construction provides a definitive disproof of the “third law” of black hole thermodynamics. I will also discuss recent works showing that extremal black holes take on a central role in gravitational collapse, giving rise to a new conjectural picture of “extremal critical collapse.” This is joint work with Ryan Unger (Stanford).
26/11: Pau Figueras (Queen Mary)
Title: Classical Effective Field Theories of Gravity in the Era of Gravitational Waves
Abstract: In the era of gravitational wave astronomy it is necessary to contrast the detected waveforms produced in mergers of compact objects with the predictions from general relativity and also from alternative theories of gravity. However, to be able to do so, such alternative theories of gravity ought to have a well-posed initial value problem. In this talk I will explain how one can formulate the initial value problem for a wide class of alternative theories of gravity that have attracted the attention of the community by thinking about them as classical effective field theories. Furthermore, I will show how one can consistently compute, from first principles, the waveforms produced in binary black hole mergers in these theories. These results open the way to test these theories in present and future gravitational wave observations.
10/12: Barbora Bezdekova (Haifa)
Title: Light propagation around compact relativistic sources in dispersive media
Abstract: The bending of light rays due to the presence of a black hole or a neutron star has been one of the current foremost research topics, mostly thanks to the recent observations of the Event Horizon Telescope (EHT). An analytical description of such a system is highly valuable mostly because it allows one to directly distinguish particular effects which may affect the light trajectories. Especially, it is relevant when a compact object is assumed to be surrounded by a medium with dispersive and refractive character - plasma. I will show how this problem can be straightforwardly described in terms of the Hamiltonian formalism and discuss its several applications. I will also describe how the plasma presence can change the light trajectories.
17/12: Samuel Gralla (Arizona)
Title: Geometry of Strong-Field Electrodynamics
Abstract: I will discuss two different strong-field limits of electrodynamics that possess emergent geometrical properties: (1) In strong magnetic fields, particles obey the Lorentz force law on two-dimensional submanifolds representing worldsheets of magnetic field lines; (2) In strong electric fields, particles move along the principal null directions of the Maxwell field with a (large) Lorentz factor determined by the field line curvature. For many particles coupled self-consistently to Maxwell’s equations, these limits provide a simplified description of the strong-field plasma that arises outside pulsars and active black holes. In addition to providing theoretical insight, the geometrical formulation has the potential to vastly reduce the computational resources needed to model these compact object magnetospheres.
Spring semester 2024/25
20/3: Vitor Cardoso (Niels Bohr Institute & Instituto Superior Técnico)
Title: The sound of black holes
Abstract: One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes. Is the strong field dynamical regime of gravity well described by General Relativity? I will summarize the status of black hole spectroscopy and attempts at probing near horizon physics.
27/3: Scott Hughes (MIT)
Title: Where do we go now? The road ahead for theorists in gravitational wave science
Abstract: The first directly measured gravitational-wave event was announced just over nine years ago. Many events found since then, augmented by very strong evidence for very-low-frequency waves measured using precision pulsar timing, have rapidly made the long promised field of gravitational-wave astronomy a reality. Where does the field go from here? The road ahead will be driven on the facility side by improved instrumentation: greater sensitivity, wider bandwidth, and expansion into other frequency bands. On the theory side, taking advantage of instrumental improvements will require a focus on precision modeling. We expect greater complexity of measured signals as detectors probe more deeply. Precision modeling will be particularly important to ensure we are not confused by systematic errors, particularly as various effects we hope to measure have the potential to “mock up” effects similar to what we might see in non-GR-based gravity analyses. Although the need for precision is true across all gravitational-wave sources, I will use the extreme-mass-ratio inspirals as an example of where we believe we have a good idea of our accuracy needs and some idea of the challenges that theorists must overcome.
10/4: Roberto Emparan (Barcelona)
Title: Quantum Cross-section of Near-extremal Black Holes
Abstract: Is it possible to observe large, controllable quantum fluctuations of the spacetime geometry? We will explore how to probe the quantum throat of a near-extremal black hole, where the dynamics are governed by the Schwarzian theory. To this end, we study the scattering of a low-frequency wave of a massless scalar field off the black hole and calculate the absorption cross-section. In the semiclassical regime, we recover the universal result that the cross-section equals the horizon area. However, in the regime where quantum fluctuations dominate, we find that the absorption cross-section exceeds the semiclassical prediction, since quantum fluctuations greatly enhance absorption transitions between black hole states. We conclude that a measurement showing an enhanced absorption cross-section serves as a clear signature of the large quantum fluctuations in the geometry.
8/5: Ofri Telem (HUJI)
Title: Bound-Unbound Universality and the All-Order Semi-Classical Wave Function in Schwarzschild
Abstract: We present a systematic method for analytically computing time-dependent observables for a relativistic probe particle in Coulomb and Schwarzschild backgrounds. The method generates expressions valid both in the bound and unbound regimes, namely bound-unbound universal expressions. To demonstrate our method we compute the time-dependent radius and azimuthal angle for relativistic motion in a Coulomb background (relativistic Keplerian motion), as well as the electromagnetic field radiated by a relativistic Keplerian source. All of our calculations exhibit bound-unbound universality. Finally, we present an exact expression for the semi-classical wave function in Schwarzschild. The latter is crucial in applying our method to any time-dependent observable for probe-limit motion in Schwarzschild, to any desired order in velocity and the gravitational constant G.
19/6: Davide Gerosa (University of Milano-Bicocca)
Title: Gravitational wave populations: deeper statistics for deeper astrophysics
Abstract: The mergers observed by LIGO and Virgo are ultimately driven by the emission of energy and angular momentum via gravitational waves. Yet, general relativity alone is not enough to explain the existence of merging compact-object binaries. For instance, black-hole binaries of ~10Msun orbiting at separations ~10Rsun would take longer than a Hubble time to merge under gravitational radiation reaction alone. Additional astrophysical processes are therefore required to bring these binaries into the gravitational-wave regime. Understanding the origin and evolution of merging compact binaries remains one of the central challenges in gravitational-wave astronomy and has motivated the development of a variety of formation scenarios. In this talk, I will present a status update on the formation-channel problem, with a specific focus on hierarchical black-hole mergers. Along the way, I will highlight the statistical challenge we face: inferring the properties of a population of sources from noisy measurements subject to strong selection effects. Recent advances in hierarchical Bayesian inference --and, why not, a touch of machine learning--- are enabling us to extract increasingly detailed information from gravitational-wave data. These methods are laying the foundation for the time when our field will fully transition into a genuine big-data science.
26/6: Ilan Strusberg (HUJI)
Title: Universal Waveforms For EMRIs
Abstract: We engage with the challenge of calculating the waveforms of gravitational
waves emitted by spinless binary black hole merger in extreme mass-ratio
limit. We model the stellar-mass black hole as a test-particle, initially on a
circular orbit, that undergoes adiabatic inspiral until it reaches the innermost
stable circular orbit (ISCO), after which it follows a geodesic trajectory. We
compute the gravitational waveforms emitted during both phases—before and
after the ISCO crossing—and demonstrate how to accurately connect them.
While the waveforms are calculated adiabatically up to the ISCO, the
associated phase error near the ISCO scales as and remains below one radian
for sufficiently small mass-ratios . Our complete waveform is universal in the
sense that all computationally expensive calculations are performed once, and
its application to any binary merger can be obtained by appropriately re-
scaling time, phase, and amplitude. We compare our results with existing
models in the literature and show that our complete waveforms are accurate
enough all the way from separations that are an order of one gravitational radii
outside the ISCO, to the merger.