Scalar-Tensor Theories for Dark Energy and their Cosmological Consequences

One of the major outstanding questions in cosmology today is the nature of dark energy, the cause of the observed recent accelerated expansion of the universe. This thesis considers scalar-tensor theories as a possible candidate for dark energy and explores their observational consequences. The evolution equations for perturbation equations of a fluid, either relativistic or non-relativistic, disformally coupled to the scalar field are derived for the first time. A new observational probe for such theories, CMB μ-distortion, is then investigated. The effects of screened models of modified gravity on the CMB angular power spectrum are considered, looking at the potential for these to provide constraints on the models, even after imposing constraints coming from local tests and BBN. Finally, in the context of coupled quintessence, the initial conditions for general perturbation modes are derived with a view to determining whether the constraint on the coupling strength may be relaxed when the assumption of adiabatic initial conditions is lifted

Early Modified Gravity: Implications for Cosmology

We study the effects of modifications of gravity after Big Bang Nucleosynthesis (BBN) which would manifest themselves mainly before recombination. We consider their effects on the Cosmic Microwave Background (CMB) radiation and on the formation of large scale structure. The models that we introduce here represent all screened modifications of General Relativity (GR) which evade the local tests of gravity such as the violation of the strong equivalence principle as constrained by the Lunar Ranging experiment. We use the tomographic description of modified gravity which defines models with screening mechanisms of the chameleon or Damour-Polyakov types and allows one to relate the temporal evolution of the mass and the coupling to matter of a scalar field to its Lagrangian and also to cosmological perturbations. The models with early modifications of gravity all involve a coupling to matter which is stronger in the past leading to effects on perturbations before recombination while minimising deviations from Lambda-CDM structure formation at late times. We find that a new family of early transition models lead to discrepancies in the CMB spectrum which could reach a few percent and appear as both enhancements and reductions of power for different scales.Comment: 22 pages, 7 figure

Modified Gravity and the Radiation Dominated Epoch

In this paper we consider scalar-tensor theories, allowing for both conformal and disformal couplings to a fluid with a general equation of state. We derive the effective coupling for both background cosmology and for perturbations in that fluid. As an application we consider the scalar degree of freedom to be coupled to baryons and study the dynamics of the tightly coupled photon-baryon fluid in the early Universe. We derive an expression for the effective speed of sound, which differs from its value in General Relativity. We apply our findings to the μ-distortion of the cosmic microwave background radiation, which depends on the effective sound-speed of the photon-baryon fluid, and show that the predictions differ from General Relativity. Thus, the μ-distortion provides further information about gravity in the very early Universe well before decoupling