Black hole soft hair and celestial holography

Black holes are at the root of the most striking puzzles that arise when attempting to combine the principles of quantum mechanics and Einsteins theory of gravitation; they are therefore thought to be key to a formulation of a theory of quantum gravity. In recent years, progress in our understanding of the elusive quantum nature of black holes has been made thanks to the so-called holographic principle. The latter establishes that the behavior of gravity in a given region of space is actually entirely encoded in terms of a different system, which lives only along the edge of that region. As such, it provides physicists with a dictionary that translates complicated problems into accessible ones. However, the holographic principle has not been developed yet for realistic kinds of spacetimes, such as the ones describing the universe we live in. The first goal of my research project is to fill this gap by developing a holographic correspondence for spacetimes which are of relevance for most astrophysical purposes (celestial holography). The second goal of my project is to address some of the unresolved key issues in black hole physics, especially the mysterious origin of their vast entropy. My approach will be based on a recently discovered intriguing set of infinite symmetries (soft hair) that appear close to black hole horizons. These symmetries were previously overlooked, and are expected to give important novel insights into the physics of black holes as they strongly constraint physical processes that occur in their vicinity. These two programs (black hole soft hair and celestial holography) both provide on their own a new way to address outstanding open questions in quantum gravity. My research project aims not only at pushing the respective frontiers of these two programs but also proposes for the first time to combine them. Thanks to this unique interplay of powerful new approaches, my research project will unravel the fundamental holographic nature of spacetimes that include realistic black holes, such as the ones we observe in the sky. My project will be implemented at TU Wien, the ideal host in Austria for research on black holes and holography, since theoretical developments and applications of holography have become key research areas at the Institute for Theoretical Physics at TU Wien in the past decade.