Early Universe

About one per cent of the noise in an analogue TV set is due to photons originating from the cosmic era of 'recombination', when the Universe was only around 400,000 years old. Before that time, photons were tightly coupled with the hot soup of interacting particles that made up our Universe. Once the temperature dropped enough to allow the formation of atoms, the Universe became transparent and the photons travel relatively unimpeded through the Universe. These photons form a universal background of black-body radiation. Their wavelengths are stretched by cosmic expansion and today lie in the microwave region of the spectrum, corresponding to a temperature of around 2.7 K. This is the celebrated Cosmic Microwave Background (CMB) radiation. There are tiny (0.01%) anisotropies in the temperature of the CMB. These anisotropies are a direct imprint of the density inhomogeneities at the time of recombination. The detailed properties of the CMB anisotropies contain a vast amount of information about the early and late Universe. For example, the seeds for the anisotropies must be generated in the early Universe by, for instance, a period of Inflation. The CMB photons are also affected by the presence of matter in the early and late Universe, and therefore provide an incredibly powerful tool for understanding the detailed physics of our Universe. We study the CMB in relation to the physics of both the early and late Universe, with particular interest in Inflation, Cosmic strings, Dark matter, Dark energy and Modified gravity.

We are all used to phase transitions. We freeze and boil water, and melt solids in labs. The Early Universe underwent a series of transitions, from one high energy regime to another lower energy phase. One of these transitions could have led to the production of defects, regions of the original high energy phase trapped, or surrounded, by regions of the lower energy phase. A particular class of defects formed in the very early Universe are one-dimensional objects known as cosmic strings. They are exotic objects. Their width is far less than the size of a proton and a kilometer of string has the same mass as the moon! They therefore have potentially significant observational consequences, in particular producing signatures in the cosmic microwave background anisotropies. We are very interested in determining these signatures and then searching for them in the maps of the CMB.

Inflation is a proposed period of accelerated expansion in the very Early Universe, where the size of the Universe increased by a factor of million billion billion over a period of a millionth of a billionth of a billionth of a billionth of a second. This huge expansion is best explained by invoking the energy of a scalar field, known as the inflaton. As the field slowly evolves down its potential it undergoes quantum fluctuations due to Heisenberg's Uncertainty Principle. These fluctuations generate density perturbation which produce anisotropies in the CMB. Therefore by measuring the properties of the CMB anisotropies we can learn about the physics of the inflaton field. The origin of the field in fundamental particle theory is an open question. In particular, does the inflaton field exist in string theory? Making inflation work in string theory is non-trivial, as the potentials there are generally far too steep. One goal of our research is to establish exactly how inflation took place in the Early Universe.

Field theory describes a huge range of phenomena, including the standard model of particle physics; gravitational physics; "low energy" dynamics of string theory; as well as forming the framework for understanding exotic long-lived solitons. Field theory comes in two types: classical and quantum mechanical. Both versions of field theory are important and can be approached using analytic calculations, large-scale numerical computations, or a combination of both. The application of these techniques is allowing us to examine how matter was created at the end of inflation, and why there is more matter than antimatter in the Universe; it enables us to explore new theories of gravity, and test them against observations; we study how cosmic strings and other solitons form, and then evolve in the Universe. This is a rich area of study, and one that we will continue to explore.

Primordial Black Holes (PBHs) are light black holes that can form in the early Universe from, for instance, the collapse of large density perturbations. Their gravitational effects and the consequences of their evaporation products lead to tight limits on their abundance. They therefore provide a powerful probe of models of the early Universe. We are particular interested in the constraints which PBHs place on models of inflation.