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Project

Quantum structures of black holes

Black holes are the inevitable endpoints of gravitational collapse of matter with sufficient mass. They are ubiquitous in our own Universe.
Hawking famously showed that black holes carry entropy proportional to their horizon area, meaning that they hold a large number of microscopic states. To do so, he relied on a semi-classical approximation, where he assumed that quantum fields fluctuate around a fixed classical black hole background. In this thesis, we use the methods of string theory to go beyond the semi-classical approximation in two distinct ways.

First, we assume the fuzzball viewpoint, which considers the microscopic states as compact and horizonless bound states of branes. In the supergravity regime some of these microstates manifest themselves as microstate geometries: smooth solitonic solutions to Einstein's equations. We show qualitatively that, while a single microstate geometry returns information in finite proper time as an echo, a quantum superposition of microstate geometries has the tendency to smear and spread these echoes and drastically reduce their amplitude. Our results imply that observing any deviation from the classical black hole behavior is exponentially suppressed and, thus, very difficult to detect using low energy observables, such as gravitational waves.

Second, we consider a certain supersymmetric asymptotically Anti-de Sitter black hole in 5 dimensions, composed of a backreacted set of $ N$ M5-branes. We re-derive known results for its semi-classical features. We proceed and include higher derivative corrections to the 5-dimensional supergravity theory that hosts the black hole and we fix the coefficients of these corrections, via our knowledge of its M-theory embedding. In the corrected effective theory of gravity we recover the usual semi-classical entropy and, additionally, we find subleading corrections. As the black hole is in asymptotically Anti-de Sitter space, the holographic conjecture postulates that it should be dual to an ensemble of supersymmetric states in a lower dimensional superconformal field theory. Using quantum field theory methods, one can show that the degeneracy of states in this ensemble scale as $ N^3$ at leading order and as $ N$ at subleading order. We show that the coefficients of the gravitational subleading term and the quantum field theoretic subleading term precisely match. As such, we perform a highly non-trivial test of the holographic conjecture beyond leading order.

Date:6 Sep 2018 →  31 Oct 2023
Keywords:Black holes, String Theory, Holography
Disciplines:High energy physics, Field theory and string theory, Quantum information, computation and communication
Project type:PhD project