Disformal couplings & cosmology

Disformally coupled fields are predicted to occur in nature, and cosmology in particular, by fundamental theories of strings and branes. They also arise independently from considerations of the most general relation permissible between two metric tensors of a given theory of gravitation. This work explores the cosmological consequences that arise when such couplings are added to the standard model of cosmology and the disformally coupled field is asked to play the role of dark energy. Among other things, it is shown that disformal interactions modify the angles of light cones and can induce motion damping of the field, similar to the well known Hubble friction, in the cosmological background. In addition, an extension is considered to the theoretical framework whereby the disformal interaction strengths can vary from species to species. Some models based on this generalisation are found to be well constrained by both astronomical and ground based particle experiments (discussed in chapter 3), whereas others (discussed in chapter 4) are actually able to avoid these constraints, while simultaneously offering insight into potential dark energy-dark matter interactions in the cosmos. Finally, a particularly well behaved form of disformal coupling is invoked to address to the cosmological constant problem (chapter 5).