Timothy Clifton

Modified Gravity and Cosmology

Abstract In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, scalar–tensor, Einstein–aether, and Bimetric theories, as well as TeVeS, f ( R ) , general higher-order theories, Hořava–Lifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including Kaluza–Klein, Randall–Sundrum, DGP, and higher co-dimension braneworlds. We also review attempts to construct a Parameterised Post-Friedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a self-contained, comprehensive and up-to-date introduction to the subject as a whole.

The power of general relativity

We study the cosmological and weak-field properties of theories of gravity derived by extending general relativity by means of a Lagrangian proportional to

Living in a void: testing the Copernican principle with distant supernovae.

{R}^{1+\ensuremath{\delta}}

The existence of Gödel, Einstein, and de Sitter universes

. This scale-free extension reduces to general relativity when

Exact cosmological solutions of scale-invariant gravity theories

\ensuremath{\delta}\ensuremath{\rightarrow}0

Perturbation theory in Lemaȋtre-Tolman-Bondi cosmology

. In order to constrain generalizations of general relativity of this power class, we analyze the behavior of the perfect-fluid Friedmann universes and isolate the physically relevant models of zero curvature. A stable matter-dominated period of evolution requires

Further Exact Cosmological Solutions to Higher-Order Gravity Theories

\ensuremath{\delta}g0

Spherically Symmetric Solutions to Fourth-Order Theories of Gravity

or

What the small angle CMB really tells us about the curvature of the Universe

\ensuremath{\delta}l\ensuremath{-}1/4

Ups and downs of cyclic universes

. The stable attractors of the evolution are found. By considering the synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the bound