Strong Gravity

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.