Diagnostics of Cosmic Discordance

在过去的二十年里，宇宙学领域发生了一场革命，这场革命导致了一个标准模型的建立，该模型至少在粗略的意义上解释了我们宇宙的大多数观察到的属性；这个模型经常被称为‘和谐’模型。这一进展是由关于大爆炸早期阶段的理论观点与大规模星系调查和宇宙微波背景观测数据的令人兴奋的综合所带来的。宇宙学未来发展面临的最重要挑战之一是发现、诊断和解释与和谐模型的背离，因为这些背离将为更多地了解我们宇宙的起源提供前景。和谐宇宙学的一个特别重要的元素是，在膨胀期间，亚原子尺度上的量子涨落被拉伸了许多个数量级，在这个过程中变成了声波。这就是“大爆炸”中的“大爆炸”。此外，这些声波既可以在宇宙微波背景中看到的天空温度变化模式中检测到，也可以在星系大规模分布的星系团属性中检测到。在最简单的通货膨胀版本中，这些波动是随机的，并具有最简单的统计形式，即高斯(或“正态”)分布。然而，我们不知道通胀是如何发生的，甚至不知道它是否真的发生了，也不知道它是否简单；显然有一些可行的模型，比如那些源自弦理论的模型，它们对早期宇宙的描述截然不同。探索宇宙起源所涉及的基本物理学的最令人兴奋的方法之一是在我们观察到的宇宙属性中寻找原始非高斯的证据。我们从和谐模型的成功中知道，任何偏离“常态”的情况都必须相对较小，因此，这种方法需要复杂的统计技术，其性质与建立标准模型所需的测量相对简单的特征所需的统计技术不同。数据集的大小和复杂性，被噪声或系统误差污染的可能性，以及描述随机波动场所涉及的数学微妙之处，这些都使这种方法成为一种具有挑战性的方法。即使是在当前的数据集中，例如来自威尔金森微波各向异性探测器(WMAP)的数据集，也已经有迹象表明，可能存在与标准宇宙学不一致的情况。天空中有一个无法解释的冷点，温度模式有奇怪的排列，天空中有大范围的变化，仅举几个例子。这些是由实验系统学引起的，还是它们暗示了和谐模型可能是不完整的？未来，像普朗克这样的卫星将提供更多关于这些被认为的异常的信息。显然，通过开发能够利用新数据的分析技术来为新数据的洪流做好准备是至关重要的。这项建议中描述的研究旨在开发和测试复杂的新方法来分析天文数据以寻找原始不一致的迹象。我们采取的方法突出了宇宙学的耐人寻味的本质，因为它更接近法医学或考古学，而不是基于实验室的学科。我们只有一次大爆炸的机会，所以没有机会用略有不同的初始条件重新运行实验。我们必须从零碎和嘈杂的数据中拼凑出发生了什么，筛选原始爆炸的余波，寻找导致爆炸的线索。我们问的问题越详细，提取相关信息所需的处理就越复杂。这是一项艰苦而精确的工作，但它是宇宙学终于成为一门真正的科学的标志。

The last twenty years have seen a revolution in the field of Cosmology which has led to the establishment of a standard model that accounts, at least in a broad-brush sense, for most of the observed properties of our Universe; this model is frequently dubbed 'the concordance' model. This progress has been brought about by an exciting synthesis of theoretical ideas about the early stages of the Big Bang with data from large-scale galaxy surveys and observations of the cosmic microwave background. One of the most important challenges for the future development of cosmology is to discover, diagnose and explain departures from the concordance model because these would offer the prospect of learning more about the origins of our Universe. One particularly important element of the concordance cosmology is that quantum fluctuations on sub-atomic scales were stretched many orders of magnitude during a period of inflation, becoming sound waves in the process. This is what put the 'bang' in the 'big bang'. Moreover these sound waves are detectable both in the pattern of temperature variations on the sky seen in the cosmic microwave background and in the clustering properties of the large-scale distribution of galaxies. In the simplest versions of inflation, these fluctuations are random and have the simplest possible statistical form, a Gaussian (or 'normal') distribution. We do not, however, know how, or even if, inflation actually happened or whether it was simple or not; there are apparently viable models, such as those deriving from string theory, that offer radically different descriptions of the early Universe. One of the most exciting ways of probing the fundamental physics involved in the origin of the Universe is to look for evidence of primordial non-Gaussianity in the observed properties of our Universe. We know from the success of the concordance model that any departures from 'normality' must be relatively small, so this approach requires sophisticated statistical techniques of a different nature to those required for measuring the relatively simple characteristics needed to establish the standard model. The size and complexity of the data sets, the possibility of contamination with noise or systematic errors, and the mathematical subtleties involved in characterizing random fluctuation fields all combine to make this approach a challenging one. Even in current data sets, such as those derived from the Wilkinson Microwave Anisotropy Probe (WMAP), there are already indications that there might be things going on that are inconsistent with the standard cosmology. There is an unexplained 'cold spot' on the sky, there are peculiar alignments in the temperature pattern, and there is a large-scale variation across the sky, to name just a few. Are these caused by experimental systematics or do they suggest that the concordance model may be incomplete? In future, satellites such as Planck will yield more information about these suggested anomalies. It is obviously essential to prepare for the flood of new data by developing analysis techniques capable of exploiting it. The research described in this proposal is designed to develop and test sophisticated new ways of analysing astronomical data for signs of primordial discordance. The approach we take highlights the intriguing nature of cosmology, in that it is closer to forensic science or archaeology than it is to laboratory-based disciplines. We have acess to only one Big Bang, so there are no opportunities to re-run the experiment with slightly different initial conditions. We must piece together what happened from fragmentary and noisy data, sifting through the aftermath of the primordial explosion for clues to what caused it. The more detailed the questions we ask, the more complicated is the processing required required to extract relevant information. It is painstakingly precise work, but it is a sign that cosmology has at last become a proper science.