Cosmic acceleration: connecting theory and observation.

目前的宇宙学数据第一次足够精确，可以对我们的非常早期的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“膨胀”期间设定的，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位势主导的标量场，它提供了一种机制来产生原始涨落，今天在宇宙微波背景辐射(CMB)辐射中可以看到这种起伏，也就是大爆炸留下的热量。膨胀的微观物理目前是未知的，但可以理解，因为不同的膨胀模型对CMB各向异性的统计性质做出了不同的预测。此外，由于膨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，用于测试粒子物理的能量，其能量远远超出任何可想象的地球实验的范围。有趣的是，宇宙膨胀可能会再次加速，而不是像预期的那样在一个充满物质的宇宙中减速。来自超新星、CMB和星系调查的独立数据都支持这一令人惊讶的结果。如果宇宙由一种被称为“暗能量”的负压成分主导，那么这些观测结果就可以得到解释。“暗能量”约占宇宙能量密度的三分之二。暗能量的理论模型包括真空能量(“宇宙学常数”)和由旋转标量场模拟的时变的、空间不均匀的分量(“精华”)。我建议开发和应用新的工具来确定膨胀的精确机制和暗能量的性质。能源规模的巨大差异使得同一种潜在机制不太可能对这两种影响负责。然而，统一的框架是解决这两个问题的最有效的方法，因为它们在手头的物理学和天体物理学方面有许多相似之处。为暴涨和暗能量提出的理论模型的数量非常多，轮流分析每个模型都无利可图。相反，我提议的研究计划的基本哲学是识别和系统地面对具有精确的宇宙学数据和良好动机的理论先验的广泛类别的模型。将对数据进行强有力的分析，以便最终约束不会因不精确的模型或系统误差而产生重大的未知影响，并将使用先进的模型比较技术将相互竞争的理论相互比较，以确定与数据一致的最简单的模型。这项研究肯定会提高我们对膨胀和暗能量的微观物理的理解，如果宇宙符合最“最小”的当前概念，但如果数据包含更奇异的物理学的统计意义上的重要线索，这项研究也足够灵活，可以充分利用数据。宇宙加速在早期和晚期的双重谜团是当今宇宙学家面临的两个最大的问题，这项研究的结果将有助于回答关于这两个暴涨的重要问题：-暴涨物理学是否有不止一个自由度？-暴胀是在大统一理论尺度上发生的，还是在较低的能量下发生？-哪些暴涨初始条件与数据一致？-暴涨潜力是平稳的吗？暗能量：-我们能排除宇宙常数有利于标量场吗？-暗能量会聚集吗？-暗能量会和物质耦合吗？因此，这项工作有可能利用对CMB、大尺度结构和引力波的测量--所有这些都是PPARC资助的重大项目的目标--以增加我们对基础物理的理解。

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.