Strong Gravity

Conventional equations of state suggest that in complete gravitational collapse a singular state of matter with infinite density could be reached finally to a black hole, the characteristic feature of which is its apparent horizon, where light rays are first trapped. The loss of information to the outside world this implies gives rise to serious difficulties with well-established principles of quantum mechanics and statistical physics.

The formation of a gravitational vacuum condensate star with a p=−ρ interior solves these problems and remarkably, actually follows from Schwarzschild's second paper over a century ago. The surface tension of the condensate star surface is the difference of equal and opposite surface gravities between the exterior and interior Schwarzschild solutions. The First Law is then recognized as a purely mechanical classical relation at zero temperature and zero entropy. The Schwarzschild time of such a non-singular gravitational condensate star is a global time, fully consistent with unitary time evolution in quantum theory.

The advent of BH imaging by the EHT and Gravitational Wave Astronomy with LIGO should allow for observational tests of the gravastar hypothesis, particularly in the discrete surface modes of oscillation and GW resonances or “echoes,” which may be observable by advanced LIGO and successor GW detectors.

This talk will explore the applications of the computing power of numerical relativity to gravitational theories beyond general relativity. Specifically, I will consider dynamical Chern-Simons gravity, which has roots in string theory and loop quantum gravity. I will discuss our  formalism and efforts to simulate binary black holes in this theory to generate waveforms LIGO and LISA. Additionally, I will discuss the generation of numerical black hole solutions in this theory, and applications to probing black hole shadows with the Event Horizon Telescope. 

Black-hole recoils are arguably the strong-gravity phenomena with the most striking astrophysical consequences. In the late inspiral and final coalescence of black-hole binaries, anisotropic emission of gravitational waves causes significant linear momentum loss. The remnant black hole, therefore, recoils in the opposite direction. These final kicks can reach magnitudes up to 5000 km/s (“superkicks”), larger than the escape speed of even the most massive galaxies, thus opening the possibility of black hole ejections. In this talk, I summarize recent advances in modeling black-hole recoils and their astrophysical environments. I will address the relevance of disk-assisted spin alignment, new approaches to model black-hole recoils with waveform approximants, and prospects to directly detect superkicks with gravitational-wave detectors.

The recent detections of gravitational waves from compact object binary
lead to detailed investigations of the origin of these objects. In my talk
I will discuss the questions: What is the astrophysical origin of these objects?
What do these detections tell us about the formation of black holes and neutron stars?
What are the main problems that they pose?
What to expect in the coming gravitational observations?

Detections of compact binary coalescences with Advanced LIGO and Advanced Virgo are now starting to become routine. However, thereis still considerably more information that can be gleaned from these observations, particularly as detector sensitivity and waveform modelsboth improve. We start by describing the methods currently used in LIGO/Virgo data analysis to determine the mass and spin of the remnant black hole of the binary black hole coalescences. These black holes have the most well-measured masses and spins of any stellar-mass black holes observed and comparable or better mass accuracies to Sgr A*. We also describe the method used to obtain a lower bound on the radiated energy of the binary neutron star coalescence GW170817, and discuss further information one can extract from these observations by postprocessing parameter estimation results. We also describe a method for placing constraints on properties of black hole mimickers, such as boson stars or gravastars, if binaries of these objects are to produce the signals identified as coming from binary black holes. We present initial results of the method applied to injections in simulated noise, and as a proof of principle show how it is possible to rule out or constrain the properties of a specific model of boson stars using a given detection.

First I will introduce the concept of the shadow of a black hole and what it means for the shadows of two observers to be degenerate. I will then present
our results showing that no continuous
degenerations exist between the shadows of observers at any point in the exterior region of any Kerr-Newman black hole spacetime of unit mass. Therefore an observer can, by measuring the black holes shadow, in principle determine the angular momentum and the charge of the black hole under observation, as well as his radial distance from the black hole and his angle of elevation above the equatorial plane.

Starting from the well known Laplace-Runge-Lenz vector of the Kepler problem, I will introduce a notion of dynamical (hidden)
symmetries. These are genuine phase space symmetries that stand in contrast to the more familiar symmetries of the configuration space
discussed in truncated versions of Noether's theorem. Proceeding to a relativistic description, I will demonstrate that such symmetries -- encoded
in the so called Killing-Yano tensors -- play a crucial role in the study of rotating black holes described by the Kerr geometry. Even more remarkably, I will show that one such special symmetry is enough to guarantee complete integrability of particle and light motion in general rotating black hole spacetimes in an arbitrary
number of spacetime dimensions. Recent developments on the separability of test fields in these spacetimes will also be discussed.

In this talk I will consider the black hole formed as a result of a binary black hole coalescence.  As expected, the final black hole eventually approaches a Kerr solution.  We show numerically, in full general relativity and in the highly non-linear merger regime, how the final black hole approaches equilibrium. In particular we show how the infalling radiation wipes out the deviations from Kerr and the rate at which the multipole moments decay to their asymptotic values.

In general relativity, the effective-one-body (EOB) approach, which consists in reducing the two-body dynamics to the motion of a test particle in an effective static, spherically symmetric metric, has proven to be a very powerful framework to describe analytically the coalescence of compact binary systems.

In this seminar, we address its extension to modified gravities, considering first the example of massless scalar-tensor theories (ST). We reduce the ST two-body dynamics, which is known at second post-Keplerian order, to a simple parametrized deformation of the general relativistic EOB Hamiltonian, and estimate the ST corrections to the strong-field regime; in particular, the ISCO location and orbital frequency.

We then discuss the class of Einstein-Maxwell-dilaton (EMD) theories, which provide simple examples of "hairy" black holes. We compute the EMD post-Keplerian two-body Lagrangian, and show that it can, as well, be incorporated within the EOB framework. Finally, we highlight that, depending on their scalar environment, EMD black holes can transition to a regime where they strongly couple to the scalar and vector fields, inducing large deviations from the general relativistic two-body dynamics.

Gravitational waves from the mergers of five binary black holes and one binary neutron star were detected in the past two years by the advanced LIGO and Virgo detectors. These detections allowed our Universe to be observed in gravitational waves for the first time, and they have tested the predictions of general relativity for dynamical and strongly gravitating systems. I will discuss these results and also highlight a few additional examples of ways in which gravitational waves can shed light on open questions in theoretical physics and astrophysics. One involves the gravitational-wave memory effect, which is a constant change in the gravitational-wave strain produced by the energy that gravitational waves and matter carry away from an isolated system. I will describe the challenges in detecting the memory with LIGO and Virgo, and how the memory is related to the symmetries and conserved quantities of isolated gravitating systems. A second involves using precision astrometry to detect a stochastic background of gravitational waves from astrophysical, and potentially even cosmological, sources. With the upcoming data release from the Gaia mission, it will likely be possible to place improved constraints on the stochastic background at frequencies higher than those coming from the cosmic microwave background, but below those of pulsar timing searches. I will show how these constraints can be determined. Finally, I will discuss gravitational waves from a compact object orbiting an intermediate mass black hole surrounded by a dark matter spike. I will describe how details about the dark matter distribution can be imprinted in the emitted gravitational waves.

The observations of gravitational waves from coalescing compact binary systems allow us to test gravity in its strong field regime. In order to better constrain alternative theories of gravity, one has to build template waveforms for these theories. In this talk, I will present a post-Newtonian Lagrangian approach adapted to the specificities of scalar-tensor theories. I will derive the equations of motion of a compact binary system at 3PN order in harmonic coordinates. This result is primordial in order to compute the scalar and gravitational waveforms at 2PN order. I will conclude by a short discussion on tidal effects.

When two black holes merge, they 'ringdown' as they settle into a final Kerr black hole. The ringdown part of the gravitational wave signal probes the strong field gravity, enabling us to test the general theory of relativity (GR) in that regime. In this talk, I will focus on one particular challenge associated with the ringdown - "When does a ringdown start during a binary black hole merger?". Then I will end the talk with a brief summary of our prospects to perform GR tests with the ringdown signals using the current and future ground-based gravitational wave observatory.