Inflationary cosmology

Understanding the universe we live in has been a preoccupation of human societies for as long as they have existed. Since the time of Einstein, however, a clear picture of how the universe evolved from the far distant past until today has emerged.

The universe began a hot dense state and then expanded and cooled for 14 billion years into the form we currently see around us. Looking at the universe on the largest scales it is on average very smooth. On smaller scales, however, there is structure — galaxies and clusters of galaxies. A particular phase of evolution called inflation, thought to occur in the very earliest moments of the universe, has been postulated to explain these properties. My research is concerned with exploring this phase of evolution, both understanding¬†it’s consequences for how the universe looks today, and using it to understand more about the most fundamental laws of physics.

During inflation the universe grew very quickly. This behaviour allows a small smooth patch of space-time to become sufficiently large to evolve into our entire observable universe, explaining the smoothness we see today. At the same time, tiny quantum fluctuations were also rapidly expanded to become classical over- and under-densities in the distribution of matter after inflation. Ultimately over-densities collapse under their self-gravitation, forming structures.

The over- and under-densities can be observed in their near original state in the Cosmic Microwave Background — relic radiation from a time only 300,000 years after the beginning. The original state of the density fluctuations can also be inferred from the distribution of galaxies. These observations are tests of the initial density fluctuations and hence of the inflationary regime.

A key goal of my research is to make precise predictions for the statistical properties of these fluctuations. These properties are sensitive to the exact nature of the physical fields which drove inflation, and hence depend on the particle physics in operation at the very high energy scales at which inflation took place. In recent years considerable effort has gone into building models of inflation using fundamental theories of particle physics. The hope is to test these theories at energy scales beyond anything that could be reached in particle accelerators, complementing results from the LHC. In truth, however, there are too many models, they are often very complicated, and there is a lack of theoretical tools and numerical codes to test them against observations.

Yet from an observational point of view this is a exciting time, with the quality of data increasing apace. The typical magnitude of a fluctuation, its variance, is now extremely well constrained, and new data has put unprecedented constraints on higher order statistics, the skewness and kurtosis. These measure how likely deviations from the typical value are. Realistic models are known to produce interesting results for these measures, particularly when many fields are present, or complicated dynamics occur at the end of inflation. Currently, however, we cannot test many models against these observations, hindering progress in testing the physics of the early universe and fundamental theories of physics.

My research is timely and exciting because it will address this pressing issue. It will develop the precision techniques and numerical packages needed to compare what a model predicts with what we observe. This will allow realistic models of inflation, and the underlying theories of particle physics in which they are constructed, to be tested for the first time.

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