Strong Gravity

Recent horizon-scale images of Messier 87* and Sagittarius A* have been used to demonstrate that their spacetimes are well-described by the Kerr metric. The latter is a solution to the vacuum Einstein equations of general relativity, and is used to describe spinning black holes. While of fundamental importance, it has undesirable features such as a spacetime singularity or a Cauchy horizon. To find phenomenological resolutions of such features, using observations, studies of astrophysical processes in non-Kerr spacetimes have recently gained prominence. We will begin by briefly reviewing the current status of observational constraints on such alternatives. We will then demonstrate how future observations of the "photon ring" can grant access to new observables that will refine our physical understanding of strong-gravity. We will end by sketching how, using state-of-the-art numerical simulations, the energetics of relativistic outflows (jets) is universally described by a simple electromagnetic Penrose process (the Blandford-Znajek mechanism).

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Gravitational waves detected by advanced ground-based interferometers have given us an unprecedented look at the universe. Yet, a central mystery remains: How are sources of gravitational waves — in particular, black-hole and neutron-star mergers — formed in nature? The measured masses, spins, and redshifts of these compact objects offer some clues to solving this mystery. In this talk, I will describe state-of-the art techniques for characterizing and simulating pairs of black holes and neutron stars. To conclude, I will argue that a multi-pronged approach combining both model-agnostic data analysis and detailed simulations will help unlock the histories of these exotic objects.

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To set the stage, I discuss different assumptions about physics beyond GR and resulting expectations about where to look for the breakdown of the Kerr paradigm. In particular, I present counterexamples to the prevailing expectation that deviations from GR are necessarily tied to local curvature scales. This provides a motivation to probe for a potential breakdown of black-hole uniqueness, not just for solar-mass but also for supermassive black holes. I will then focus on how to leverage VLBI observations of light emitted in the accretion disk to probe the stationary and axisymmetric background spacetime. I will detail different assumptions about additional hidden symmetries of the spacetime and how to then systematically parameterize deviations. Within such a parametrization, I will demonstrate a systematic lensing-band framework to exclude spacetimes for which an observed VLBI feature cannot arise from geodesics that traversed the equatorial plane more than once. If time permits, I will also address complimentary tests with solar-mass binaries and possible pathways to achieve stable nonlinear evolution.

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Recent developments point to a remarkable resistance of black holes to tidal deformations under the influence of external gravitational fields. Relying on hidden symmetries, compelling progress has been achieved to explain that the Love numbers, characterizing tidal deformations for Kerr black holes, vanish. How does the phenomenon of tidal squeezing manifest in broader dynamical contexts? An examination of the dynamical tidal deformations in rotating black holes will be presented.

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The theory of General Relativity has successfully passed a large number of observational tests. The theory has been extensively tested in the weak-field regime with experiments in the Solar System and observations of binary pulsars. The past 7-8 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and mm Very Long Baseline Interferometry observations. In my talk, I will summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data, discussing its recent progress and future developments.

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The existence of fermionic compact objects, such as neutron stars, is supported by a plethora of observational evidence — an intriguing idea is whether one can construct stars made up of the bosonic counterparts. Such theoretical objects are called boson stars, proposed to make up a fraction of the dark matter in our Universe and claimed to be promising horizonless black hole mimickers. In this talk I will discuss the state-of-the-art of numerical evolutions of boson star models in spherical symmetry. In particular, I will discuss how improper initial data construction can lead to spurious physical effects in the evolution of boson star mergers and introduce methods, necessary to remedy such spurious features. I will present our results on boson star head-on collisions of various mass ratios, highlighting the differences from the black hole mergers. Lastly, I will discuss our recent results on the high-precision numerical relativity simulations of quasi-circular boson star inspirals in pursuit of understanding the implications of boson star signatures on the LIGO-Virgo-KAGRA observations.

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The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has recently reported evidence for the background of low-frequency gravitational waves using an array of rapidly rotating, highly stable radio pulsars distributed across the galaxy. Each pulsar emits a radio beam that passes our line of sight and is observed as regular, pulsed emission. We measure the times of arrival of pulses and compare with a model that includes: the rotational motion of the pulsar, orbital motions, and interstellar propagation delays; random timing noise from pulsars themselves and from the interstellar medium; and finally a correlated gravitational-wave signal recently reported on in a suite of papers. I will highlight our recent results, including implications for the dynamics of supermassive black hole binary at the centers of merging galaxies, searches for individual supermassive black hole binary systems, and searches of gravitational wave signals from new physics. I will end by highlighting future prospects for radio facilities that will advance our look into this newly opened window to the Universe.

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Spinning supermassive black holes (BHs) in active galactic nuclei (AGN) can launch relativistic collimated outflows, or jets, by coiling up the magnetic field lines that cross the event horizon on the scale of BH gravitational radius, Rg. Sustained jet launching and propagation to kpc-scales are thought to require non-zero angular momentum to form an accretion disk, keep the magnetic field lines on the BH, and ensure stable jet propagation. Past 3D general relativistic magnetohydrodynamic (GRMHD) work predicted that absent gas angular momentum, magnetized jets are expected to become unstable to the kink instability already at 800 Rg, i.e., orders of magnitudes smaller distances than the BH sphere of influence, or the Bondi radius, RB (e.g., for Sgr A* and M87, RB~10^[5-6] Rg). This raises the question: Do the jets need gas angular momentum to survive the kink instability and make it out to large distances, outside of Rb? I will present 3D GRMHD simulation results for a rapidly spinning BH immersed into uniform zero angular momentum gas threaded by a weak vertical magnetic field, with the largest simulated Bondi radius to date, RB = 1000 Rg. I will show that the BH accumulates dynamically-important magnetic field, enters the magnetically-arrested disk state, and produces powerful jets that make it out of Rb. I will also show how the jets get twisted into knots and dissipate their energy after the MAD state ends.

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Zoom link https://pitp.zoom.us/j/99925612004?pwd=QkhIUmpKRkVKUURpclhPekJST2NIQT09

In this talk I will present our 3+1 formulation of the Four-Derivative scalar-tensor theory of gravity with a modified puncture gauge that proves to be well-posed. Then I will show the results from Binary Black Hole evolutions that we have obtained from the implementation of these equations with GRFolres, the extension for modified gravity of our numerical relativity code GRChombo.

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Zoom link https://pitp.zoom.us/j/96339156414?pwd=ajhWb1hQNFRYalRpUEd3ajhYdTFldz09

Gravitational waves have uncovered a treasure trove of nearly 90 merging black holes and neutron stars, each with its own unique story to tell. In the first part of the talk, our focus will center on black hole spins, seeking to decipher the secrets hidden within, including their origins, hometowns, and the forces driving their mergers. We will challenge the belief that isolated binary black holes should have spins aligned with orbital angular momentum. We will explore the mechanisms influencing these spins, from their origins to the forces driving their mergers.

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Zoom link https://pitp.zoom.us/j/95680635803?pwd=Yll6NjFPbmpMZ1lTS0JtdUp6dG1RZz09

Superradiant instabilities may create clouds of ultralight bosons around rotating black holes, forming so-called "gravitational atoms". The presence of a binary companion can induce transitions between bound states of the cloud ("resonances"), as well as from bound to unbound states ("ionization"). These processes backreact on the binary dynamics and leave characteristic imprints on the emitted GWs, carrying direct information about the mass of the boson and the state of the cloud. Even though some of the resonances can destroy the cloud before the system becomes observable in GWs, their backreaction forces the binary inclination towards attractors, opening up the possibility to infer the existence of the boson from a statistical analysis of a population of binary black holes. We also discuss implications for eccentric orbits and merger rates

Extreme mass-ratio binary black hole systems, known as EMRIs, are expected to radiate low-frequency gravitational waves detectable by planned space-based Laser Interferometer Space Antenna (LISA). We hope to use these systems to probe black hole spacetimes in exquisite detail and make precision measurements of supermassive black hole properties. Accurate models using general relativistic perturbation theory will allow us to unlock the potential of these unique systems. Such models must include post-geodesic corrections, which account for forces driving the smaller black hole away from a geodesic trajectory. When a spinning body orbits a black hole, its spin couples to the curvature of the background spacetime, introducing post-geodesic correction called the spin-curvature force. In this talk, I will present our calculation of EMRI waveforms that include both spin-curvature forces and the leading backreaction due to gravitational radiation. We use a near-identity transformation to eliminate dependence on the orbital phases, allowing for very fast computation of completely generic worldlines of spinning bodies; such efficiency is crucial for LISA data analysis. Finally, I will discuss what aspects still need to be included in future calculations so that we can use EMRIs for a new era of precision gravitational-wave astronomy, addressing outstanding puzzles in astrophysics, cosmology and fundamental theoretical physics.

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Zoom link https://pitp.zoom.us/j/91917788358?pwd=MWp5OUhxbkRmZDFxWWE4cHR0VlBTUT09