Late-time acceleration

: interacting dark energy and modified gravity

Abstract

In 1998 astronomical observations of distant stars exploding at the ends of
their lives led to the discovery that the expansion of the Universe is accelerating.
This is likely to be caused by an intrinsic part of Einstein’s General
Theory of Relativity known as the cosmological constant, but naturalness
issues and the need to improve observational tests have motivated the study
of alternative models of the Universe. The research in this thesis is part
of ongoing efforts to pin down the cause of late-time acceleration by better
understanding these alternatives and their signatures in cosmological observations.

One such alternative is known as interacting dark energy and would be
caused by additional matter in the Universe, as yet unknown to particle
physics. This would interact with another unknown particle called dark
matter that has been part of the standard model of cosmology since the
1970’s. The first part of this thesis contains a review of works on interacting
dark energy and investigates a particular version of the model which had not
been studied in detail before, placing recent observational constraints on its
parameters.

Another alternative to the cosmological constant is known as modified
gravity, where General Relativity is extended by the addition of new degrees
of freedom. Theories of modified gravity are mathematically related to some
models of interacting dark energy and can appear very similar in cosmological
observations. The second part of this thesis investigates the extent to which
the two can be distinguished using current observational data.

Date of Award	23 Sept 2013
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Kazuya Koyama (Supervisor), Roy Maartens (Supervisor) & Marco Bruni (Supervisor)