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Title: Disformal gravity
Author: Noller, Johannes Joachimov
ISNI:       0000 0004 2743 6672
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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An intriguing feature of scalar-tensor theories is the emergence of different metrics, e.g. when matter is minimally coupled to a metric non-trivially related to the Einstein metric g[mu,nu] used to construct the Ricci scalar. Strong equivalence principle constraints then typically force permissible “many-metric” scenarios to reduce to a bimetric picture. In this thesis we first aim to construct the most general bimetric relation, where the two metrics are related by a single scalar degree of freedom [phi] and its derivatives. This results in the disformal metric relation and a natural extension which we present. In the context of primordial structure formation, disformal bimetric theories give rise to “general single field inflation” models of the P(X, [phi]) type. We investigate the perturbative properties of such disformally motivated models. The focus is on non-Gaussian phenomenology and we establish non-Gaussian fingerprints for inflationary single field models and non-inflationary bimetric setups, also going beyond the slow-roll approximation. Furthermore we show that various dualities exist between disformally motivated P(X, [phi]) theories and higher-form models. As an explicit example we use the dual picture to compute non-Gaussian signals for three-form theories. In the context of dark energy/modified gravity, we show that the conformal subgroup of the general disformal relation can be used to construct a generalized “derivative” Chameleon setup. We present and investigate this setup and study its phenomenology. Finally we show that a natural extension of the disformal relation can generate Galileon solutions from a single geometrical invariant - the first Lovelock term - in four dimensions. As such the over-arching theme of this thesis is to show that the disformal bimetric picture and its extensions present us with a geometrical understanding of scalar-tensor/single field models. That they provide a unified description of large classes of scenarios linked to accelerated space-time expansion and also point us towards new physically motivated setups.
Supervisor: Magueijo, Joao Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral