Abstract
More than a century after its formulation, General Relativity stillpasses every single experimental and observational test that
physicists have designed. However, this does not mean that it is a
complete theory of gravity, but rather that observations are not
accurate enough to falsify it yet. In this thesis, several theoretical
extensions of General Relativity are considered and some of their
implications are investigated.
Important developments in the past decades have demonstrated that
General Relativity considered in antide Sitter spacetimes (AdS) can
be used to study strongly interacting conformal field theories (CFT).
We use this fruitful AdS/CFT connection to study outofequilibrium
quantum dynamics, and we develop an understanding of
thermalization and quantum chaos in terms of gravitational black
hole physics. This description allows us to characterize these
quantum phenomena in regimes where conventional computational
methods fail. In gravitational language, thermalization corresponds to
the disappearance of matter behind a black hole’s event horizon, to
which one can associate thermal properties such as temperature and
entropy. On the other hand, chaos emerges from the gravitational
blueshift of particles falling into a black hole, causing them to interact
strongly with other particles traveling near the black hole horizon.
In parallel, we study vacuum quantum fluctuations in cosmology,
considered one of the best observational windows into fundamental
physics. A potentially major role of quantum fluctuations is their
contribution to the cosmological constant, which sources the
accelerated expansion of our Universe. We revisit the tension
between theory estimates and cosmological measurements of
the cosmological constant and propose a lowenergy effective theory
that might potentially resolve this tension. In this model, the
cosmological dynamics of a new scalar field tends to screen most of
the vacuum energy, such that the Hubble expansion rate is relaxed
to the observed small value. Lastly, we also contribute to the
development of the AdS/CFT correspondence in application to
cosmology, which enables us to generalize part of the known
vacuum structure of free quantum fields in de Sitter space. In
particular, we find that analogues of the MottolaAllen vacua also
appear in the context of AdS/CFT.
Date of Award  18 Sep 2019 

Original language  English 
Awarding Institution 

Supervisor  Ben Craps (Promotor), Oleg Evnin (Copromotor), Alexandre Sevrin (Jury), Stijn Buitink (Jury), Ruslan Efremov (Jury), Laura Lopez Honorez (Jury), J Erdmenger (Jury) & Thomas Van riet (Jury) 