Joint Israeli Seminar Series on Gravitational Physics

28/11: Robert Wald (Chicago)

Title: Black Holes Decohere Quantum Superpositions

Abstract: We show that if a massive body is put in a quantum superposition of spatially separated states, the mere presence of a black hole in the vicinity of the body will eventually destroy the coherence of the superposition. This occurs because, in effect, the gravitational field of the body radiates soft gravitons into the black hole, allowing the black hole to harvest "which path'' information about the superposition. A similar effect occurs for quantum superpositions of electrically charged bodies. The effect is very closely related to the memory effect and infrared divergences at null infinity.

12/12: Horng Sheng Chia (IAS)

Title: In Pursuit of Love: Black Holes and Gravitational Waves as Multifaceted Probes of the Universe

Abstract: Black holes and gravitational waves are remarkable probes of astrophysics and physics beyond the Standard Model. In this talk, I will start by briefly describing the tidal effects of black holes, including the vanishing Love numbers of Kerr black holes and the interesting dissipative phenomenon known as “superradiance.” I then discuss how black hole superradiance is a powerful probe of ultralight dark matter, as it allows for the spontaneous formation of bosonic bound states around highly-spinning black holes, also commonly known as the “gravitational atoms”. Finally, I will describe a recent search for new signals, specifically compact objects with large tidal Love numbers, in the public LIGO-Virgo data. While our work focuses on astrophysical bodies with large Love numbers in binary systems, this novel search strategy can be adapted to detect many new types of binaries that differ from the standard quasi-circular binary black holes.

16/1: Daniel Kapec (Harvard)

Title: The Kerr Black Hole as a Quantum System

Abstract:  To an outside observer, a black hole appears to be an ordinary quantum mechanical system with finite entropy and highly chaotic internal dynamics. Nevertheless, the low-temperature thermodynamics of the Kerr black hole presents several puzzles. For instance, the leading order semiclassical approximation to the black hole density of states predicts a surprisingly large ground state degeneracy, while poorly understood quantum corrections are known to become increasingly important at low temperatures. I will review the modern picture of black holes as quantum systems and then discuss a recent result on the leading correction to the low-temperature thermodynamics of the Kerr black hole that resolves many of the old puzzles.

30/1: Barak Kol (HUJI)

Title: The flux-based statistical theory of the three-body system

Abstract:  The three-body system in Newtonian gravity is one of the most fruitful and longest-standing open in physics. It was involved in the creation of several branches of science including physical perturbation theory, Poisson brackets, chaos and topology. 

This talk shall describe recent progress regarding the chaotic non-hierarchical case: a symmetric formulation of the problem and an exact reduction of its statistical solution. We shall focus on novel theoretical constructions. Strong evidence from computer simulations will be presented.

Based on: 

- BK, Natural dynamical reduction of the three-body problem, Celest. Mech. Dyn. Astron. 135, 29 (2023) [arXiv:2107.12372]

- BK, Flux-based prediction of three-body outcomes, Celest. Mech. Dyn. Astron. 133, 17 (2021) [arXiv:2002.11496]

- V. Manwadkar, A. Trani and BK, Measurement of three-body chaotic absorptivity predicts chaotic outcome distribution, accepted for publication by Celest. Mech. Dyn. Astron., arXiv:2302.08312

- And more.

13/2: Barak Zackay (Weizmann)

Title: The Current State of Ground-Based GW Astronomy and the Efforts to Study the Origin of Compact Objects in Relativistic Binaries

Abstract: In this talk, I will present the recent compact binary coalescence discoveries (using both the official LIGO pipelines, and ours) and present the current fronts in figuring out the origin of binary black holes (BBH).

I will then present the current efforts in my group to address the main questions: 

1) Is there a mass cutoff to stellar mass black holes? - This calls for searching the highest mass events using higher modes.

2) Do BBH form dynamically or through binary stellar evolution? - This motivates a search for precessing systems. 

3) Are Binary neutron star mergers always accompanied by a GRB? Is there a population of faint GRBs coming from BNS systems in which we are off the jet-axis? - This motivates us to improve GRB detection.

4) Does the spin magnitude of the black holes change with mass? This calls for better measurements of the found system's parameters. This is possible if we could remove noise from the low-freq part of the band, calling for the use of the auxiliary channels to cancel bi-linear noise sources.

27/2: Amos Yarom (Technion)

Title: Holographic turbulence

Abstract: I will discuss recent developments regarding the interplay between the dynamics of asymptotically anti de Sitter black branes and turbulent flow.

12/3: Ramy Brustein (BGU)

Title: Sourcing the Kerr geometry

Abstract: I will start by presenting general arguments as to why we should expect significant, horizon scale, departures from the classical Schwarzschild and Kerr geometries inside black holes. Then, I will present a simple model which realizes this idea: the Frozen Star model, briefly showing that a static frozen star looks exactly like a Schwarzschild black hole to an external observer and that it is sourced by a “string fluid”. The main part of the presentation is about the interior geometry of the rotating frozen star which modifies Kerr such that there is neither an inner ergosphere nor an inner horizon, and the metric and Einstein tensors are regular everywhere except for a mild, removable singularity at the center of the star. The geometry of each radial slice of the interior is a nearly null surface with the same geometry, but different radial size, as that of the would-be horizon on the outermost slice. The integral of the interior energy density of the “string fluid” source is equal to the irreducible mass of a Kerr black hole, and the integral of the angular-momentum density confirms that the ratio of the angular momentum to the mass is equal to the Kerr spin parameter. Including the rotational energy in the standard way, we then obtain the total gravitational mass and angular momentum of the corresponding Kerr black hole.

21/5: Alessandra Buonanno (Max Planck Institute for Gravitational Physics, Potsdam)

Title: Theoretical Advances and Challenges in Modeling Gravitational Waves from Binary Black Holes

Abstract: Enhancements in the sensitivity of current gravitational-wave (GW) detectors, along with the development of next-generation ground-based observatories like the Einstein Telescope and Cosmic Explorer, and future space-based detectors such as LISA, are poised to dramatically increase the number of GW sources. These advancements will enable observations with a signal-to-noise ratio up to two orders of magnitude higher than what is currently achievable, necessitating a corresponding improvement in the accuracy of waveform models.

In the first part of my talk, I will discuss recent theoretical advances in waveform modeling, notably the extension to generic orbits (instead of quasi-circular orbits), which have provided evidence of eccentricity in the LIGO-Virgo-KAGRA population. I will then highlight that state-of-the-art waveform models, designed for quasi-circular, spin-precessing binary systems, exhibit systematic biases when applied to future observational runs and detectors. This bias becomes particularly pronounced in scenarios with high spin rates and significant asymmetries in spins and masses. Such biases could significantly impact our interpretation of GW events, potentially leading to misconceptions regarding the properties of black holes, neutron stars, and supernovae, as well as the universe's expansion rate, the origins of compact-object binaries, and lead to false claims of violations of General Relativity.

25/6: Noa Zilberman (Technion)

Title: Quantum effects inside black holes

Abstract: Astrophysical black holes are known to be rotating. Within classical General Relativity, the simplest spacetime solution (the Kerr solution) describing a rotating black hole reveals a traversable passage through an inner horizon – which in turn may lead to another external universe.  But does this remain the case when taking quantum effects into account?

Answering this question, along others, requires one to understand the manner in which quantum energy fluxes affect the internal geometry of a black hole. It has been widely anticipated, yet inconclusive (till this work), that such effects would diverge at the inner horizon of a spinning black hole. This divergence, if indeed takes place, may drastically affect the internal black hole geometry, potentially preventing the inner horizon traversability. Clarifying this issue requires the computation of the quantum energy fluxes in black hole interiors. However, this has been a serious challenge for decades.

Using a combination of old and new methods, we have managed to compute the quantum energy fluxes at the inner horizon of a spinning black hole, in a vacuum state corresponding to an evaporating black hole. We found that these fluxes are either positive or negative, depending on the black hole spin (and polar angle). The sign of these fluxes may be crucial to the nature of their backreaction on the geometry (as should be dictated by the semiclassical Einstein equation).

In this seminar, we shall describe the basic framework and the renormalization procedure, and then present our novel results for the quantum fluxes at the inner horizon of a rotating black hole, briefly mentioning possible implications for the inner horizon traversability.

9/7: Ido Ben-Dayan (Ariel)

Title: The Quantum focusing Conjecture and the Improved Null Energy Condition

Abstract: By rearranging its terms, the Quantum Focusing Conjecture (QFC) can be viewed as a quantum energy condition, and we can consider various limits. I will review the status of energy conditions in general relativity and the QFC. Of specific interest is a restricted version where the quantum focusing vanishes Θ → 0, which has been proven for Braneworld scenarios. 

As a result, I derive an improved quantum null energy condition (INEC), that can be proven with field theory techniques. I sketch the beginning of a proof, and briefly discuss possible interpretations in the absence of one.

16/7: Gautam Satishchandran (Princeton)

Title: Generalized Entropy is von Neumann Entropy: Black Holes and Cosmology

Abstract: It was recently shown that the von Neumann algebras of observables dressed to the mass of a Schwarzschild-AdS black hole or an observer in de Sitter are Type II, and thus admit well-defined traces. The von Neumann entropies of "semi-classical" states were found to be the generalized entropy. However, these arguments relied on the existence of an equilibrium (KMS) state and thus do not apply to black holes out of equilibrium (e.g., asymptotically flat or de Sitter black holes) or general cosmological universes. In this talk we will show that, quite generally, algebra of observables in exterior of any black hole or cosmological spacetime seeded from inflation has a well-defined entropy equal to the generalized entropy.

23/7: Alexandru Lupsasca (Vanderbilt)

Title: The Black Hole Photon Ring

Abstract:  What does a black hole look like? The first images of the supermassive black hole M87* display a bright ring encircling the event horizon, which appears as a dark patch in its surrounding emission. But Einstein's theory of general relativity predicts that within this image there also lies a thin "photon ring" consisting of multiple mirror images of the main emission. These images arise from photons that orbited around the black hole multiple times, probing the warped space-time geometry just outside its horizon. The photon ring carries an imprint of the strong gravity in this region and encodes fundamental properties of the black hole. A measurement of this predicted (but not yet observed) ring could provide a precise test of general relativity and will be one of the main targets of a NASA mission proposed to fly within the next decade: the Black Hole Explorer (BHEX).