Recent observations arising from a number of sources suggest that the Universe is currently undergoing a period accelerated expansion. The standard approach to modeling this behaviour is to introduce a new component total energy density of the Universe. This so called dark energy drives the accelerated expansion by virtue of having negative pressure. To date, there exists no compelling explanation as to the physical nature of dark energy. Various models exist, however they are typically plagued by fine tuning issues and unnanaturally suppressed couplings to the Standard Model. An alternative explanation of the observed acceleration is to postulate that at low curvature (that is, at large scales), gravity is modified from its standard General Relativistic form. The aim of this approach is to construct a theory in which the late time acceleration of the Universe arises naturally as a consequence of modifying the Einstein equations. In this thesis, we will be concerned with the cosmological evolution of two classes of modified gravity models. We begin with a study of so called F(R) models, where an arbitrary function F(R) introduced into the gravitational action. We compile a set of simple consistency conditions that these models must satisfy in order to possess no dangerous instabilities, and in doing so constrain tje form that the function F(R) may take. We then construct a model which satisfies these conditions. In order for our F(R) function to be observationally viable, we find that it must mimic General Relativity very closely throughout the cosmological history. We discuss the genererality of this conclusion.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:498785 |
| Date | January 2009 |
| Creators | Appleby, Stephen Andrew |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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