The Cosmology of the Early and Late Universe

宇宙学涵盖了广泛的物理学领域，从早期宇宙的极小尺度和高能量，到晚期宇宙中的星系和星系团。宇宙中的特殊特征，如宇宙微波背景(CMB)(来自大爆炸的遗迹辐射)和个别超新星爆炸，使我们能够探索宇宙的膨胀历史和组成。这些观测表明，宇宙中的大部分物质都是由奇异的暗物质组成的。此外，宇宙的膨胀最近开始加速(最近我们指的是几十亿年前，但与宇宙137亿年的年龄相比是最近的)，而不是像预期的那样继续减速。这种后期加速可能是由于一种奇异的暗能量成分，或者是爱因斯坦引力定律的修正。此外，在宇宙晚期，我们还有丰富的星系和星系团结构，据信它们是从物质分布的微小初始波动中生长出来的，而物质分布又是在非常早期的宇宙中产生的。然而，造成这些波动的机制尚不完全清楚。将晚时间宇宙的观测与早期宇宙和基本粒子物理联系起来是宇宙学的一个重大悬而未决的问题，也是我们工作的主要目标之一。粒子物理学和广义相对论都在极端能量下崩溃，在那里量子引力的统一理论有望发挥作用。我们将测试这些理论的观测结果。特别值得一提的是，弦理论允许超弦在宇宙中伸展，从而改变CMB中的波动。我们将研究宇宙弦和超弦网络的演化，并对它们的观测特征做出准确的预测。然后，我们将根据普朗克卫星的数据直接测试这些预测，从而限制这些字符串在我们宇宙中可能采取的形式。这项工作可能会通过宇宙学为弦理论提供第一个证据。弦理论和其他新的粒子物理模型也为我们提供了一个以弱相互作用大质量粒子(WIMP)的形式存在的暗物质候选者。弱者可以在实验室中直接探测到(通过它们与原子的罕见相互作用)，或者通过反粒子和高能伽马射线间接探测到，这些反粒子和高能伽马射线是当它们聚集在一起并湮灭时产生的。它们也可以在大型强子对撞机等粒子对撞机中产生。我们将开发明确探测暗物质并测量其性质所需的工具。我们将采取双管齐下的方法来理解观测到的宇宙后期加速背后的物理基础。我们将开发技术，使我们能够测试引力定律和暗能量模型，在宇宙的早期和晚期。我们还将继续探索可能提供加速机制的基本物理模型。我们将调查的一个令人兴奋的提议涉及像哈利波特一样的隐身斗篷。在我们开发的一类修正的引力模型中，宇宙常数可以任意大，因为它通过标量场的存在而被隐藏在宇宙的背景几何中，标量场的作用类似于斗篷。这一提议有许多有趣的方面，我们将深入研究，尤其是其中，它在观测上是否一致？我们的研究涵盖了广泛的规模和能量。这种多样性伴随着一系列相应的技术来研究各种现象，从量子重力到经典动力学，从分析计算到使用超级计算机的数值模拟。

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.