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Abhay Ashtekar

Penn State University

Dynamical horizons in Classical and Quantum Gravity

Abstract:

While event horizons are generally used to distinguish black holes from other compact astrophysical objects in classical general relativity, this notion has some severe limitations. In the classical theory, they are teleological: An event horizon may well be forming in the room you are sitting in, in anticipation of a gravitational collapse in the center of our galaxy a billion years from now. For evaporating black holes in the quantum theory, there are no event horizons at all. Fortunately one can replace them with quasi-local horizons which are free of these limitations. In this description, a black hole in equilibrium is described by an isolated horizon and and an evolving black hole by a dynamical horizon. What forms in a gravitational collapse and evaporates due to Hawking radiation is a dynamical horizon. I will discuss properties of dynamical horizons in classical and quantum gravity.

Maximilian Attems

University of Barcelona

Fast Hydrodynamization of Non-conformal Holographic Shockwaves

Abstract:

Ever since fast hydrodynamization has been observed in heavy ion collisions the understanding of the early out-of-equilibrium stage of such collisions has been a topic of intense research. We use the gauge/gravity duality to model the creation of a strongly coupled Quark-Gluon plasma in a non-conformal gauge theory. This numerical relativity study is the first non-conformal holographic simulation of a heavy ion collision and reveals the existence of new relaxation channels due to the presence of non-vanishing bulk viscosity. We study collisions at different energies in gauge theories with different degrees of non-conformality and compare three relaxation times which can occur in different orderings: the hydrodynamization time (when hydrodynamics becomes applicable), the EoSization time (when the average pressure approaches its equilibrium value) and the condensate relaxation time (when the expectation value of a scalar operator approaches its equilibrium value). Finally, I will discuss a new example of the applicability of hydrodynamics to systems with large gradients. We show that the time evolution and saturation of the spinodal instability (corresponding to black branes afflicted by the Gregory-Laflamme instability in the gravity dual) are accurately described by second-order hydrodynamics, where a set of locally unstable states with a first-order thermal phase transition settle down to a static, inhomogeneous configuration.