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

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.