A universe unexpected: Dark matter, dark energy, and the accelerating cosmos

The problems of dark matter and dark energy in the Universe are somewhat astonishing. After centuries of scientific endeavor, after breathtaking advances in technology, and after the innumerable predictive successes of physical science, we find ourselves in an odd position: we do not know what constitutes 96% of the Universe. The recent measurements that indicate an accelerating Universe imply that 73% of the mass of the Universe consists of an unknown, unseen and uniformly distributed dark energy. Only 4% of the mass resides in familiar baryonic objects such as stars, gas, and planets. The remaining 23% is composed of an unknown dark matter, possibly consisting of thermal relics known as WIMPS (weakly interacting massive particles). The resolution to these problems of dark matter and dark energy may involve an overlooked aspect of contemporary physics, or may entail the development of entirely new physical theories. This dissertation grapples with aspects of both of these issues, and may be broadly divided into two parts.; In the first part, we introduce a series of modifications to the Friedmann equation that can explain the observed acceleration of the Universe without recourse to conventional dark energy and that can be motivated in the context of: (1)  brane-world scenarios, in which the Universe is assumed to be a four-dimensional hyperplane embedded in a higher dimensional space; or, (2) generic fluid models, in which the effective modifications to the Friedmann equations arise from the presence of an exotic fluid in the Universe, possibly due to the long range interaction of dark matter particles. We investigate the phenomenology of each of these models, focusing on the observational consequences of each.; In the second part, we confront the problem of understanding the dark galactic halo. In particular, we would like to use WIMP direct detection experiments to search for dark halo substructure. Such experiments seek to measure the energy deposited when a WIMP interacts with a nuclei in a detector. We describe a Bayesian multiscale method for investigating the structure of the dark halo using non-directional WIMP detection experiments. The WIMP interaction rate, the energy deposited per collision, and the time dependence of these effects all depend sensitively on the velocity distribution of the dark halo, so that direct detection experiments can not only detect the presence of WIMPs, but also probe the structure of the dark halo itself. The methods developed in the dissertation estimate the dark halo properties from the raw Poisson distributed detector counts without requiring assumptions about the underlying halo structure.