General Relativity & Quantum Cosmology

We consider monochromatic and isotropic photon emission from circular equatorial Kerr orbiters. We calculate the critical curve delineating the region of photon escape from that of photon capture in each emitter’s sky, allowing us to derive analytic expressions for the photon escape probability and the redshift-dependent total flux collected on the celestial sphere as a function of emission radius and black hole parameters. This critical curve generalizes to finite orbital radius the usual Kerr critical curve and displays interesting features in the limit of high spin. These results confirm that the near-horizon geometry of a high-spin black hole is in principle observable.

In this seminar, I will consider a deformed kinematics that goes beyond special relativity as a way to account for possible low-energy effects of a quantum gravity theory that could lead to some experimental evidences. This can be done while keeping a relativity principle, an approach which is usually known as doubly (or deformed) special relativity. In this context, I will give a simple geometric interpretation of the deformed kinematics and explain how it can be related to a metric in maximally symmetric curved momentum space. Moreover, this metric can be extended to the whole phase space, leading to a notion of spacetime. Also, this geometrical formalism can be generalized in order to take into account a space-time curvature, leading to a momentum deformation of general relativity. I will explain theoretical aspects and possible phenomenological consequences of such deformation.

We investigate the precession of the spin of the smaller black hole in binary black hole simulations. By considering a sequence of binaries at higher mass ratios, we approach the limit of geodetic precession of a test spin. This precession is corrected by the ``self-torque'' due to the smaller black hole's own spacetime curvature. We find that the spins undergo spin nutations which are not described in conventional descriptions of spin precession, an effect that has been noticed previously in simulations. These nutations arise because the spins are not measured in a frame where the smaller hole is stationary. We develop a simple model for these frame nutations, extract the instantaneous spin precession rate, and compare our results to PN and extreme-mass-ratio approximations for the self-torque.
 

The quantum states of matter in the immediate vicinity of a black hole can be studied using no other information than Standard Model physics combined with perturbative gravity. The point is that the relevant energy scale of the most important fields involved is low compared to the Planck scale, provided the black hole is big compared to the Planck scale. Usually this problem is investigated by using the metric that includes the effects of matter that formed the black hole in the distant past and sometimes also matter that is radiated away in the distant future.  Arguments are presented however to justify that one should ignore those effects. The metric then becomes invariant under  time translation, and this is what we need to get the energy eigen states. It is this scheme that forces us to impose the antipodal identification as a new boundary condition, giving us a beautiful picture of black hole quantum evolution. The logic of choosing this compulsory topological twist in space-time is explained. There are still many very hard questions and I hope to be able to inspire people to look into these.

In compact astrophysical objects, such as neutron star magnetospheres, black-hole accretion disk coronae and jets, the main energy reservoir is the magnetic field. The plasma processes such as magnetic reconnection and turbulence govern the extraction of that energy, which is then deposited into heat and accelerated particles and, ultimately, the observed emission. To understand what we observe, we first need to describe from first principles how these processes operate in violent regimes applicable to certain classes of compact objects, where radiative drag and pair production/annihilation play a significant role. As a specific example, I will briefly cover our state-of-the-art understanding of one of these processes — magnetic reconnection — and present the first self-consistent simulations of QED-mediated reconnection in application to neutron star magnetospheres and explain how it helps us understand the observed gamma-ray emission from these objects. I will also talk about the future prospects of this area of research; QED-mediated plasma processes also take place in a variety of other astrophysical objects, such as the accretion disk coronae in X-ray binaries, coalescing neutron stars shortly before their merger, and short X-ray bursts in magnetars.

I will discuss challenges of quantum gravity, highlighting conceptual, methodological as well as phenomenological aspects. Focusing on asymptotically safe quantum gravity, I will review recent progress in addressing key theoretical challenges using continuum and lattice methods. Furthermore, I will explain how the high predictive power of the asymptotically safe fixed point for quantum gravity and matter might allow us to explain fundamental properties of our universe, for example its dimensionality. Finally, I will point out possible connections between different approaches to quantum gravity, and how these connections might help us formulating a fundamental description of nature.

I will show that quasinormal modes of black holes could be used to investigate quantum gravity or modified gravity in specific situations. Some general comments about isospectrality will also be made. Finally a few other quantum gravity phenomena associated with black holes will be underlined.

Abstract: Gamma-ray bursts (GRBs) associated with gravitational wave events are, and will likely continue to be, viewed at a much larger inclination than GRBs without gravitational wave detections.  Viewing GRBs and their afterglows at large inclination can massively affect the observed electromagnetic emission, as dramatically demonstrated by the binary neutron star merger event GW170817.  Analyzing this event and future ones requires an extension of the common GRB afterglow models which typically assume emission from a structureless (top-hat) jet viewed on-axis. I will review the evidence for inclination effects in GRB afterglows, and present a characterization of afterglows viewed at all angles from jets with and without structure. We find new closure relations for off-axis structured jets: the slope of the light curve is found in many cases to be a simple function of the inclination angle. With our theoretical tools in hand I will discuss new candidate kilonova events found in the GRB archive, showing features very similar to GW170817. Finally I will discuss our numerical model to calculate synthetic light curves and spectra, publicly available as the open source Python package afterglowpy, and the steps needed to improve our capabilities for the next LIGO observing run.

Compact binaries may be formed dynamically in globular clusters, with large (close to unity) orbital eccentricity and emitting gravitational waves within the detection band of ground based detectors. The gravitational waves from such sources resemble more a discrete set of bursts than the continuous signal of their quasi-circular counterparts. I here discuss the construction of new analytic waveforms for such systems, which rely on treating the problem as a perturbation of a parabolic fly-by. I will discuss the results of parameter estimation with these waveforms for binary black holes, and the inclusion of tidal effects for binary neutron stars.

We have tentative evidence of massive stars that disappear without a bright transient. It is commonly argued that this massive stars have low angular momentum and can collapse into a black hole without significant feedback. In this talk I will make use of general-relativistic hydrodynamical simulations to understand the flow around a newly-formed black hole. I will discuss the angular momentum needed in order for the infalling material to be accreted into the black hole without forming a centrifugally supported structure, thus generating no effective feedback. If the feedback from the black hole is significant, the collapse can be halted and, as a result, it is likely followed by a bright transient. With the results from the simulation, I will constrain the maximum rotation rate for the disappearing massive progenitors know, and set a limit on the rate of expected disappearing stars.

I will present and motivate a program establishing, in full generality, the symmetries and charge analysis for gravitational theories near a generic null hypersurface without specifying any boundary condition.  I will illustrate the first steps of this program on three dimensional Einstein gravity. In this case, there are three charges which are generic functions over the codimension one null surface. The integrability of the charges and the charge algebra depend on the state-dependence of symmetry generators which is a priori not specified. I will establish the existence of infinitely many choices that render the surface charges integrable. Then, I will argue that one expects this result to be valid for d > 3 when there is no Bondi news by developing some aspects of the four dimensional case. Finally, I will put our results in the context of earlier constructions of near horizon symmetries.  

Based on: arxiv:2002.08346, arxiv:2005.06936 and arxiv:2007.12759