The Cosmology of the Early and Late Universe

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We live in a Universe in which distant galaxies are accelerating apart from one another, but the cause for this can not be the ordinary matter present in it as the universe should be slowing down due to the attractive pull of gravity on the matter it contains. Whatever is causing this acceleration must be a new substance that we have not come across before in nature, we call it Dark energy (DE). The best candidate for this is the Cosmological Constant (CC), however, there can't be much of it around today because if there was, it would have caused galaxies to blow apart by now. The problem is reconciling the observed amount of DE with the predicted amount that should be present in the form of a CC. The latter is much higher and reconciling this has been a problem for many years. Recently, we proposed a resolution to the problem in which the CC is cancelled, effectively removing it. We intend to develop our understanding of the mechanism involved in resolving this problem and establish whether it can be found to exist in particle theory models of our Universe. This leaves open the question of what is the nature of DE and we will be working on models proposed to account for this in which the force driving the acceleration is screened from view in regions of high density such as here on Earth. Such models are hard to rule out, but we have established a number of features they have that would allow us to test for them in the laboratory and on galactic scales. There is an alternative point of view to DE driving the acceleration of our universe, and that is that on large scales we need to modify Einstein's General Theory of Relativity (GR). This is a big field of enterprise and we are heavily involved in working on models in which GR is modified. We will be testing many of these models, both on vast cosmological scales and by seeing how they effect the formation of massive objects like neutron stars. We will also constrain models of Dark Matter, the key component of the universe which keeps galaxies together. We have yet to find out what DM really is, and one of our aims is to help uncover the mysterious particle that is binding our galaxy together. We will test whether it can be in the form of primordial black holes formed in the early universe or maybe as an extremely light particle called the axion. Many people think the early Universe underwent a period of almost exponential expansion for a brief period of time, known as Inflation. We will work on models of inflation to establish whether or not successful models can be found in string theory, something that has recently been questioned by many. We will also ask how did the universe establish that there was more matter than antimatter in it, something known as Baryogenesis.A new area which has emerged here at Nottingham recently is the possibility of testing theories of gravity in the laboratory by creating analogue experiments. Although at a completely different energy scale to the early universe, they enjoy similar equations, hence the hope that by doing such experiments we can mimc early universe effects and even use the results to make predictions about those earliest moments. Another area we are developing at Nottingham for the first time is to use the power of Machine Learning to create neural networks to analyse data. We are hoping to apply this new technology to search for cosmic strings, dark energy and provide new ways to analyse astronomical data. It is very exciting. In Quantum Gravity, we attempt to establish a connection between General Relativity and Quantum Mechanics. We are developing models of cosmology that allow us to talk about the earliest moments in the universe where the role of Quantum Mechanics is vital and intend to develop methods to constrain and test these models with astronomical data such as the fluctuations seen in the thermal radiation associated with the Big Bang.