Late-time acceleration : interacting dark energy and modified gravity

• Main Author: Clemson, Timothy
• Other Authors: Koyama, Kazuya ; Maartens, Roy ; Bruni, Marco
• Published: University of Portsmouth 2013
• Subjects: 523.1126; Cosmology and Gravitation
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.588653

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.