Cosmic acceleration: connecting theory and observation.

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Current cosmological data are, for the first time, precise enough to allow detailed observational tests of our models of the very early universe. The initial conditions of the Big-Bang are thought to have been set during ``inflation'', an era of almost exponential expansion in the primordial universe. Usually modelled as a slowly rolling potential-dominated scalar field, it provides a mechanism to generate the primordial fluctuations which are seen today in the cosmic microwave background (CMB) radiation, the left-over heat from the Big Bang. The microphysics of inflation is presently unknown, but accessible because the different inflationary models make distinctive predictions about the statistical properties of the CMB anisotropies. Moreover, because inflationary models have their basis in theoretical high energy physics, the early universe is effectively a unique laboratory for testing particle physics at energies far beyond the reach of any conceivable Earth-bound experiments. Intriguingly, the cosmological expansion may be accelerating once again, not slowing down as expected in a universe filled with matter only. Independent data from supernovae, the CMB and galaxy surveys all support this surprising result. These observations can be explained if the universe is dominated by a negative-pressure component, coined ``dark energy'', which makes up roughly two-thirds of the cosmological energy density. Theoretical models for the dark energy include vacuum energy (the ``cosmological constant''); and a time-varying, spatially inhomogeneous component modelled by a rolling scalar field (``quintessence''). I propose to develop and apply new tools to pin down the precise mechanism of inflation and the nature of dark energy. The enormous difference in energy scales makes it unlikely that the same underlying mechanism is responsible for both effects. However, a unified framework is the most efficient way to approach these two problems, as they share many similarities in terms of the physics and astrophysics at hand. The number of theoretical models proposed for inflation and dark energy is very large, and it is not profitable to analyse each in turn. Instead, the basic philosophy of my proposed research programme is to identify and systematically confront broad classes of models with precision cosmological data and well-motivated theoretical priors. The data will be analysed robustly so that the final constraints do not have significant unknown contributions from imprecise models or systematic errors, and the competing theories will be compared against each other using advanced model-comparison techniques to identify the simplest models that are consistent with the data. The research is certain to improve our understanding of the microphysics of inflation and dark energy if the universe corresponds to the most ``minimal'' current ideas, but also flexible enough to exploit the data fully if it contains statistically significant hints of more exotic physics. The dual mysteries of cosmic acceleration at early and late times are two of the biggest questions confronting cosmologists today, and the results of this research will help to answer important questions about both Inflation: - Does inflationary physics have more than one degree of freedom? - Did inflation take place at the Grand Unified Theory-scale or at lower energies? - What inflationary initial conditions are consistent with the data? - Was the inflationary potential smooth? and Dark Energy: - Can we rule out a cosmological constant in favour of scalar fields? - Does dark energy cluster? - Is dark energy coupled to matter? This work thus has the potential to exploit measurements of the CMB, large scale structure and gravitational waves -- all of which are the target of major projects funded by PPARC -- in order to increase our understanding of fundamental physics.