The Cosmology of the Early and Late Universe

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The Particle Cosmology and Gravity groups at Nottingham aim to understand the fundamental particles and forces in our universe. We do this by working on the fundamental theoretical underpinning of our understanding of physics, and also by modelling the behaviour of the physics we know, and the physics proposed to solve fundamental cosmological mysteries, so that this can be compared with experimental data and cosmological observations.We live in a Universe in which distant galaxies are accelerating apart from one another, but the cause for this cannot 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. We call whatever is causing this acceleration dark energy (DE) and the leading candidate is the Cosmological Constant (CC). There can't be much of it around today because if there was, it would have caused galaxies to blow apart by now, yet most quantum theories predict far too much of it. We are trying to resolve this imbalance and have proposed a solution to the problem in which the CC is cancelled, effectively removing it. We intend to develop our understanding of the model and ask both whether it can be found to exist in particle theory models of our Universe and whether we can see direct evidence of it in compact objects. We will also be working on other DE models. In one of them known as Quintessence we will establish whether it exists in fundamental theory, and in another class where the force driving the acceleration is screened from view in regions of high density such as here on Earth, we have established a number of features they have that would allow us to test for them in the laboratory and on galactic scales. An alternative point of view to DE driving the acceleration of our universe, is to modify Einstein's General Theory of Relativity (GR) on large scales, and we work on such models. We will be testing them, both on cosmological scales and by seeing how they affect 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 be working on the possibility that there may be primordial black holes (PBH) formed in the early universe, maybe an extremely light particle called the axion, or possibly another type of particle known as a domain wall formed in the early universe. Many people think the early Universe underwent a phase 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. 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 mimic early universe effects and even use the experiments to make predictions about those earliest moments. Gravity is at its strongest around Black holes and neutron stars. We will make use of this feature both to test for modified gravity theories and to look for new signatures of gravity - such as scalar field hair growing from the BHs! We will use numerical simulations involving quantum theory to demonstrate how black holes actually evaporate by emitting Hawking radiation once they have formed. We are developing models of cosmology that allow us to talk about the earliest moments in the universe, where the role of Gravity and Quantum Mechanics are 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.