Cosmological consequences of Quantum Gravity proposals

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

In this thesis, we study the implications of Quantum Gravity models for the
dynamics of spacetime and the ensuing departures from classical General
Relativity. The main focus is on cosmological applications, particularly the
impact of quantum gravitational effects on the dynamics of a homogenous and
isotropic cosmological background. Our interest lies in the consequences for
the evolution of the early universe and singularity resolution, as well as in the
possibility of providing an alternative explanation for dark matter and dark
energy in the late universe.
The thesis is divided in two parts, dedicated to alternative (and complementary)
ways of tackling the problem of Quantum Gravity. The first part
is concerned with cosmological applications of background independent approaches
to Quantum Gravity, as well as minisuperspace models in Quantum
Cosmology. Particularly relevant in this work is the Group Field Theory approach,
which we use to study the effective dynamics of the emergent universe
from a full theory of Quantum Gravity (i.e. without symmetry reduction).
We consider both approaches based on loop quantisation and on quantum
geometrodynamics.
In the second part, modified gravity theories are introduced as tools to
provide an effective description of quantum gravitational effects, and show how
these may lead to the introduction of new degrees of freedom and symmetries.
Particularly relevant in this respect is local conformal invariance, which finds a
natural realisation in the framework of Weyl geometry. We construct a modified
theory of gravity based on such symmetry principle, and argue that new fields
in the extended gravitational sector may play the role of dark matter. New
degrees of freedom are also natural in models entailing fundamental ‘constants’
that vary over cosmic history.
Finally, we discuss prospects for future work and point at directions for the
derivation of realistic cosmological models from Quantum Gravity candidates.
Date of Award2017
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorMairi Sakellariadou (Supervisor)

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