The Cosmology of the Early and Late Universe

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Cosmology spans a wide range of physics, from the very small scales and high energies of the early Universe to galaxies and galaxy clusters in the late Universe. Specific features in our Universe such as the Cosmic Microwave Background (CMB) (relic radiation from the big bang) and individual supernovae explosions allow us to probe the expansion history and constituents of the Universe. These observations suggest that most of the matter in the Universe is composed of exotic dark matter. Furthermore the expansion of the Universe recently started accelerating (by recent we mean a few billion years ago actually, but recent compared to the Universe's age of 13.7 billion years), rather than continuing to slow down as expected. This late time acceleration could be due to an exotic dark energy component, or a modification of Einstein's laws of gravity. Moreover, we also have in the late Universe rich structures of galaxies and galaxy clusters which are believed to have grown from small initial fluctuations in the matter distribution, which in turn were generated in the very early Universe. However the mechanism responsible for producing these fluctuations is not yet fully understood. Connecting observations of the late time Universe, with the early Universe and fundamental particle physics is a major outstanding issue for cosmology and one of the main goals of our work. Particle physics and general relativity both break down at extreme energies, where a unified theory of quantum gravity is expected to operate. We will test the observational consequences of such theories. In particular string theory allows superstrings to stretch across the Universe, altering the fluctuations in the CMB. We will study the evolution of networks of cosmic strings and superstrings and make accurate predictions for their observational signatures. We will then test these predictions directly against data from the Planck satellite and thereby constrain the allowed form these strings could take in our Universe. This work could potentially provide the first evidence for string theory through cosmology. String theory, and other new particle physics models, also provide us with a dark matter candidate in the form of Weakly Interacting Massive Particles (WIMPs). WIMPs can be detected directly in the lab (via their rare interactions with atoms) or indirectly via the antiparticles and high energy gamma-rays that are produced when they come together and annihilate. They can also be produced in particle colliders such as the LHC. We will develop the tools required to unambiguously detect dark matter and measure its properties. We will take a two pronged approach to understanding the physics underlying the observed late time acceleration of the Universe. We will develop techniques which will allow us to test the laws of gravity, and models of dark energy, in both the early and late Universe. We will also continue to explore fundamental physics models which may provide a mechanism for the acceleration. One exciting proposal we will investigate involves a Harry Potter like invisibility cloak. In a class of modified gravity models we have developed, the cosmological constant can be arbitrarily large as it is hidden from the background geometry of the Universe by the presence of a scalar field which acts like the cloak. There are many interesting facets to this proposal which we will investigate in depth, not least among them, is it observationally consistent?Our research encompasses a wide range of scales and energies. This diversity is met with a corresponding array of techniques to study various phenomena, ranging from quantum gravity to classical dynamics and analytic calculations to numerical simulations using supercomputers.