An investigation of relativistic gravitational effects of use for understanding and interpreting dark energy

普通物质，如构成我们熟悉的日常事物的电子、质子和中子，具有通过引力相互吸引的特性。如果一个宇宙只充满了这种类型的物质，以及辐射、中微子和其他一些成分，那么随着时间的推移，它的膨胀速度要么会减慢，要么会稳定在一个恒定的膨胀速度上。相反，天文学家看到的是一个似乎正在加速膨胀的宇宙。这是一个非常出乎意料的结果，需要引入一种非常不寻常的物质，这种物质必须渗透整个宇宙，并负责将一切推开。这种物质被命名为暗能量，为了解释观测结果，它必须占宇宙总能量预算的大约2/3。关于暗能量可能是什么，有很多想法，但目前没有人确切知道。我提议进行的研究是对其中一些想法的调查。特别是，我计划重点关注三个具体领域，我将在下面概述。第一个领域是对广义相对论的修正。广义相对论是一种支配时空行为的理论，一些人认为，对暗能量的明显需求可能表明，我们还没有理解引力以及广义相对论在最大尺度上的表现。人们提出了对广义相对论的一些修正，但并不总是清楚这些修正的全部含义是什么，以及它们是否与我们现有的所有天体物理学和宇宙学观测相一致。我打算研究这些修正的理论，并建立一个框架，通过这个框架，它们可以直接与观测数据进行比较，以确定它们的有效性。第二个领域是宇宙的非均匀模型。通常的宇宙模型是基于与背景的微小偏差，在每个点上，在每个方向上看起来都是一样的。这对于理解和解释对宇宙的观测来说是非常成功的，但有人认为这可能还不够，暗能量可能是在一个不正确的模型中解释数据的结果。我的目标是批判性地调查这一建议，要么是为了支持通常模型的有效性，要么是为了提供一个可信的替代方案。第三个领域是弦理论中的暗能量。许多物理学家现在都同意弦理论并没有一个唯一的真空，而是有很多不同的可能真空。每一种都有不同的性质，包括不同类型的暗能量。如果宇宙在早期迅速膨胀，那么这些不同类型的真空将在宇宙的不同部分实现。如果有足够的真空，并且宇宙足够大，那么应该至少有一部分具有像我们这样的暗能量。虽然这是一个很有希望的想法，但在理解我们这种暗能量实现的可能性有多大方面，存在一些根本性的困难。我打算调查这一领域，以提高我们对这些问题的理解，以及如何解决这些问题。理解暗能量对于我们理解宇宙至关重要，而我的研究将为实现这一目标做出重大贡献。暗能量的存在为提高我们对宇宙模型基础的理解提供了极好的动力，而且，随着许多新的和令人兴奋的观测任务在不久的将来计划，现在是执行我上面概述的研究计划的最佳时机。

Normal matter, such as the electrons, protons and neutrons that make up the everyday things we are familiar with, have the property of attracting each other through gravity. A universe that were filled with only matter of this type, together with radiation, neutrinos and a few other ingredients, would either slow down in its expansion as time goes by, or settle into a steady rate of constant expansion. Instead, astronomers see a Universe that appears to be accelerating in its expansion. This is a highly unexpected result, and one that requires the introduction of a highly unusual substance that must permeate the whole of the Universe and be responsible for pushing everything apart. This substance has been given the name Dark Energy, and to explain the observations it must make up approximately 2/3 of the total energy budget of the Universe. There are lots of ideas as to what Dark Energy might be, but at the moment no-one knows for sure. The research I am proposing to perform is an investigation of some of these ideas. In particular, I am planning to focus on three specific areas, which I will outline below. The first area is modifications to general relativity. General relativity is the theory that governs the behaviour of space-time, and it has been suggested by some that the apparent need for Dark Energy may be a sign that we have not understood how gravity, and hence general relativity, behave on the largest scales. A number of modifications to general relativity have been proposed, but it is not always clear what the full implications of these modifications are, and if they are compatible with all of the astrophysical and cosmological observations that we have available to us. I intend to study these modified theories, and to produce a frame-work through which they can be straightforwardly compared to observational data, in order to determine their validity. The second area is inhomogeneous models of the Universe. The usual model for the Universe is based on small deviations from a background that, at every point, looks the same in every direction. This has been very successful for understanding and explaining observations of the Universe, but it has been suggested that it may not be entirely sufficient, and that Dark Energy may be the result of interpreting data within an incorrect model. I aim to critically investigate this suggestion, either to bolster the validity of the usual model, or to provide a credible alternative. The third area is Dark Energy from string theory. Many physicists now agree that string theory does not have a single unique vacuum, but very many different possible vacua. Each of these will have different properties, including different types of Dark Energy. If the Universe expanded very rapidly early on, then these different types of vacuum will be realised in different parts of the Universe. If there are enough vacua, and the Universe is big enough, there should be at least one part with a Dark Energy that looks like ours. While this is a promising idea, there are some fundamental difficulties with understanding how likely it is for our type of Dark Energy to be realised. I intend to investigate this area to improve our understanding of these problems, and how they can be resolved. Understanding Dark Energy is of critical importance in our understanding of the Universe, and the research I aim to perform will contribute substantially towards achieving this goal. The existence of Dark Energy provides excellent motivation for improving our understanding of the foundations of our models of the Universe, and, with many new and exciting observational missions planned in the near future, now is the perfect time to perform the research programme I have outlined above.