Cosmic acceleration: connecting theory and observation.

目前的宇宙学数据第一次足够精确，可以对我们的早期宇宙模型进行详细的观测测试。大爆炸的初始条件被认为是在“暴胀”时期设定的，这是一个在原始宇宙中几乎呈指数级膨胀的时代。它通常被建模为一个缓慢滚动的势能主导标量场，它提供了一种机制来产生今天在宇宙微波背景（CMB）辐射中看到的原始波动，这是大爆炸留下的热量。暴胀的微观物理学目前尚不为人所知，但由于不同的暴胀模型对宇宙微波背景各向异性的统计特性做出了不同的预测，因此可以接近。此外，由于暴胀模型有其理论高能物理的基础，早期宇宙实际上是一个独特的实验室，可以在能量上测试粒子物理，远远超出任何地球上的实验所能达到的范围。有趣的是，宇宙膨胀可能会再次加速，而不是像在一个只有物质的宇宙中预期的那样减速。来自超新星、宇宙微波背景和星系调查的独立数据都支持这一令人惊讶的结果。如果宇宙是由一种负压成分主导的，这种成分被称为“暗能量”，大约占宇宙能量密度的三分之二，那么这些观测结果就可以得到解释。暗能量的理论模型包括真空能量（“宇宙常数”）；以及一个时变的、空间非均匀的由滚动标量场模拟的分量（“quintessence”）。我建议开发和应用新的工具来确定暴胀的精确机制和暗能量的本质。能量尺度的巨大差异使得同一种潜在机制不太可能对这两种效应负责。然而，一个统一的框架是解决这两个问题的最有效的方法，因为它们在物理学和天体物理学方面有许多相似之处。为暴胀和暗能量提出的理论模型数量非常多，逐个分析是没有好处的。相反，我提出的研究计划的基本哲学是识别和系统地面对具有精确宇宙数据和良好动机的理论先验的广泛类型的模型。数据将被稳健地分析，以便最终的约束不受不精确模型或系统误差的重大未知贡献的影响，并且将使用先进的模型比较技术对相互竞争的理论进行比较，以确定与数据一致的最简单模型。如果宇宙符合目前最“最小”的想法，这项研究肯定会提高我们对暴胀和暗能量的微观物理学的理解，但如果它包含统计上显著的更奇异的物理暗示，也足够灵活，可以充分利用这些数据。宇宙早期和晚期加速的双重谜团是当今宇宙学家面临的两个最大问题，而这项研究的结果将有助于回答关于暴胀的两个重要问题：暴胀物理学是否有不止一个自由度？暴胀发生在大统一理论的尺度上还是在更低的能量上？-什么通货膨胀初始条件与数据一致？-通胀潜力是否平稳？和暗能量：-我们能排除宇宙常数而支持标量场吗？-暗能量会聚集吗？-暗能量与物质耦合吗？因此，这项工作有可能利用CMB，大尺度结构和引力波的测量-所有这些都是PPARC资助的主要项目的目标-以增加我们对基础物理学的理解。

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.