Andrea Puhm CNRS researcher, CPHT, Ecole Polytechnique, Black Hole Initiative Visiting Scholar

Title: On the distinguishability of black hole microstates

Abstract: A challenging question in the context of the information paradox is how to distinguish black hole microstates without having access to the entire spacetime. In the context of holography, one can ask how one can distinguish microstates of a black hole in anti-de Sitter space by measurements performed in the dual conformal field theory. I will discuss one...

Abstract: I will discuss some recent work on the problem of motion in general relativity, done in collaboration with Bob Geroch (Chicago). I will introduce a sense in which a collection of bodies -- including the solutions to some hyperbolic system -- could be said to "track" a curve, and then discuss some of the consequences of this idea. See arXiv:1707.04222 [gr-qc] for details.

Abstract: Within the next few years we can anticipate that the LIGO/Virgo detectors will have observed many tens or even hundreds of binary compact object merger events. One avenue to extract more information from this catalog is to stack the signals from a subset of events that are expected to share a common feature, enhancing the effective signal-to-noise ratio that the feature can be measured with. Thanks to the uniqueness properties of black holes in Einstein gravity, binary black hole mergers are ideal targets for stacking, allowing for stringent tests of dynamical, strong-field gravity, or detecting deviations from the predictions of general relativity.

I will describe an initial study exploring the utility of stacking to detect higher-order quasi-normal ringdown modes post-merger.

Though binary neutron star systems do not share such a uniqueness property, there may nevertheless be aspects of merger signals that could be enhanced using stacking. I will discuss one such example that would seek to detect a post-merger signal from the subclass of events where a hypermassive remnant forms.

Aleksi Vuorinen Helsinki Institute of Physics

Abstract: Outside the interiors of black holes, neutron stars contain the densest forms of matter in our present-day Universe. This makes them a unique laboratory for strong interaction physics, as novel phases of QCD matter may be present in their extremely dense cores, or produced at the high temperatures reached in stellar mergers. In my talk, I will concentrate on the quantitative constraints that various types of neutron star observations, including the gravitational wave signatures of their mergers, have recently set for the properties of dense nuclear and quark matter. In particular, I will demonstrate that the Equation of State of cold and dense QCD matter is significantly constrained by the known existence of two-solar-mass stars and by the recent LIGO constraint on the tidal deformabilities of the two stars involved in the gravitational wave observation GW170817.

Abstract: Over the last decade or so, dozens of x-ray or optical transient events have been detected towards the centers of (typically) low mass galaxies. They have been interpreted as the disruption of solar mass stars by the central supermassive black holes believed to lie in the centers of most galaxies. Such events were predicted in the 1970s. Early work indicated that the emission would be thermal, with temperatures of order 50 eV, arising from an accretion disk with an inner edge at or near the innermost stable circular orbit of the black hole, and would last a month or so. Loeb and Ulmer suggested that this disk radiation would be reprocessed into the near infrared, and that the event would last a decade or so. Early detections by ROSAT had spectral energy distributions (SEDs) dominated by soft x-rays. More recent events have been detected by optical transient surveys, and have SEDs dominated by UV emission. I will revisit the reprocessing arguments, and show that they actually predict that the emission should be in the UV rather than the near IR, and they further explain the observed constant effective temperature maintained while the luminosity first increases by a factor of ten, then decreases by a factor of 100.

Yichen Shi Black Hole Initiative

Abstract: We consider a rapidly spinning black hole surrounded by an equatorial, geometrically thin, slowly accreting disk that is stationary and axisymmetric. We analytically compute the broadening of electromagnetic line emissions from the innermost part of the disk, which resides in the near-horizon region. The result is independent of the details of the disk model and therefore universal. This is an example of critical behavior in astronomy that is potentially observable by current or future telescopes.

Abstract: I will present a model for stellar mass black hole binary (BHB) mergers accelerated by an active galactic nucleus (AGN) accretion disk. This model predicted the existence of 'overweight' stellar mass BHB mergers, detectable by LIGO (McKernan, Ford, et al. 2014). In more recent work, we find the rate of BHB merger by this channel can span the range 1e-4-1e4 Gpc^-3 yr^-1, depending on a variety of poorly constrained astrophysical parameters. Thus, with LIGO's measured rates (12-213 Gpc^-3 yr^-1), we can already constrain some aspects of AGN physics. I will also present the predicted mass and spin spectrum of BH produced via this channel. Notably, retrograde spin BH, evolving in a gas disk play a key role in the shape of the spin distribution among AGN-produced BHB mergers. Finally, I will discuss how this channel can be constrained by LIGO observations and other future theoretical and observational work.

Erin Kara University of Maryland

Abstract: Accreting supermassive black holes can produce more electromagnetic and kinetic luminosities than the combined stellar luminosity of an entire galaxy. Most of the power output from an Active Galactic Nucleus is released close to the black hole, and therefore studying the inner accretion flow--at the intersection of inflow and outflow--is essential for understanding how black holes grow and how they affect their surrounding environments. In this talk, I will present a new way of probing these environments, through X-ray reverberation mapping, which allows us to map the gas falling on to black holes and measure the effects of strongly curved spacetime close to the event horizon.

"Nonlinear Evolution of the AdS_4 Black Hole Bomb" Paul Chesler Black Hole Initiative

Abstract: Energy may be extracted from rotating black holes via scattering involving superradiant modes. It was suggested some time ago that if such modes could be confined using a mirror, then an amplification process can occur, converting a significant fraction of the black hole mass into radiation, leading to a so-called "black hole bomb." Anti-de Sitter spacetime contain a natural mirror - the timelike boundary of the geometry - and provides a tractable arena to study the nonlinear evolution of the black hole bomb. Via numerically solving the full 3+1 dimensional Einstein equations, I will present evidence that the AdS black hole bomb is a multistage process. Specifically, via superradiant gravitational modes, Kerr-AdS black holes transition to hairy black holes with a single Killing vector, which then experiences secondary weaker superradiant instabilities.

Peter Hintz University of California, Berkeley

Abstract: I will explain the point of view adopted in a number of recent works, joint with Andras Vasy, in which we prove the global non-linear stability of Minkowski space and of Kerr-(Newman-)de Sitter black holes. Instead of constructing and controlling the non-linear solution incrementally in time, we use a Newton-type iteration scheme: at each iteration step we solve a linearized equation globally. I will in particular address how this informs the choice of gauges, and highlight the crucial role played by constraint damping, first introduced as a tool in numerical relativity by Gundlach et al.

The Black Hole Initiative hosts weekly colloquia every Monday from 11:00am to noon. Recordings of all our colloquia can be found in our YouTube channel: https://www.youtube.com/c/BlackHoleInitiative