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Theoretical Cosmology

Early Universe

The early universe was much smaller and hotter than it is today – so hot that matter was ionized and hence opaque to radiation. After about 380,000 years the universe cooled enough for protons and electrons to combine. This process is known as recombination, and after this the universe was transparent so light could travel freely. This light can be observed today as the Cosmic Microwave Background or CMB.

The CMB is one of the key tools for verifying the currently favored Concordance cosmological model. However there are a number of alternative models for the universe. These model can be used to generate simulations and allow us to develop metrics that can be used to test observations.

Cosmic Microwave Background (CMB)

First light Planck survey superposed on an all-sky map. Copyright: ESA, LFI & HFI Consortia (Planck), Background image: Axel Mellinger

The light observed in the CMB shows fluctuations which are thought to have been generated by quantum fluctuations in the early universe. The Concordance model predicts that these fluctuations should be Gaussianly distributed i.e. that we have a universe that is both homogeneous and isotropic on large scales. We can test this hypothesis by studying these fluctuations and comparing them to models and observations.

The Astronomy Instrumentation group also works on Cosmology – please see their web page for further information.

Bianchi Models

The group has recently developed, and been studying, simulations of the CMB generated by anisotropic cosmological models known as Bianchi models. Part of this work is aimed at characterizing the resulting temperature patterns in CMB maps, so as to develop optimal methods to isolate the effect of anisotropy from that of non-Gaussianity. For further information please see here.

Galaxy Clustering

Credit: M. Coless et al., 2df Galaxy Redshift Survey.

It is the fluctuations that we observe in the CMB that are thought to be responsible for seeding structure formation in the universe. Structure is found at all scales – we consider structure on the largest of scales where clusters and super clusters of galaxies form filaments. This filamentary structure, separated by vast voids, is known as the cosmic web. By studying the distribution of structure we can infer information about the universe. Again there are different theories about structures formation and their different predictions can be tested against observation results such as the 2df Galaxy Redshift survey (pictured).

One such model that we are currently looking at the Halo Model, which associates all mass with virialized dark matter halos. Using models of the number and spatial distribution of the halos, as well as the distribution of matter within each halo, predictions about the statistical properties of galaxy clustering can be made.

The Galaxies group also does work in this area – please see their web page for more information.

See Also