Modified Actions for Gravity: Theory and Phenomenology

Abstract

This thesis is devoted to the study of gravitational theories which can be seen as modifications or generalisations of General Relativity. The motivation for considering such theories, stemming from Cosmology, High Energy Physics and Astrophysics is thoroughly discussed (cosmological problems, dark energy and dark matter problems, the lack of success so far in obtaining a successful formulation for Quantum Gravity). The basic principles which a gravitational theory should follow, and their geometrical interpretation, are analysed in a broad perspective which highlights the basic assumptions of General Relativity and suggests possible modifications which might be made. A number of such possible modifications are presented, focusing on certain specific classes of theories: scalar-tensor theories, metric f(R) theories, Palatini f(R) theories, metric-affine f(R) theories and Gauss--Bonnet theories. The characteristics of these theories are fully explored and attention is payed to issues of dynamical equivalence between them. Also, cosmological phenomenology within the realm of each of the theories is discussed and it is shown that they can potentially address the well-known cosmological problems. A number of viability criteria are presented: cosmological observations, Solar System tests, stability criteria, existence of exact solutions for common vacuum or matter configurations etc. Finally, future perspectives in the field of modified gravity are discussed and the possibility for going beyond a trial-and-error approach to modified gravity is explored.

PhD thesis (Advisors: Stefano Liberati and John C. Miller), 230 pages

Cited by in corpus (18)

• Dark energy, inflation and dark matter from modified F(R) gravity
• The viability of theories with matter coupled to the Ricci scalar
• Black hole entropy in scalar-tensor and f(R) gravity: an overview
• f(R) gravity: successes and challenges
• The role of nonmetricity in metric-affine theories of gravity
• Unveiling the Dynamics of the Universe
• Palatini f(R) gravity as a fixed point
• Clifton's spherical solution in f(R) vacuo harbours a naked singularity
• Beyond Einstein's General Relativity
• Horizons and singularity in Clifton's spherical solution of f(R) vacuum
• Perturbations of quasi-Newtonian universes in scalar-tensor gravity
• Extension of the EGS theorem to metric and Palatini f(R) gravity
• Superradiant instability and asymptotically AdS hairy black holes in $F(R)$-charged scalar field theory
• Vainshtein mechanism in general purely disformal gravity theory
• Notes on f(R) Theories of Gravity
• Weyl-invariant extension of the Metric-Affine Gravity
• Cosmic hierarchy in $f(R)$ gravity
• Cosmological Asymptotics in Higher-Order Gravity Theories