The Cosmology of the Early and Late Universe

Cosmology spans a wide range of physics, from the very small scales and high energies of the early Universe to galaxies and galaxy clusters in the late Universe. The Cosmic Microwave Background (CMB) (relic radiation from the big bang) and supernovae explosions allow us to probe the expansion history and constituents of the Universe. These observations suggest that most of matter in the Universe is composed of exotic dark matter. Furthermore the expansion of the Universe recently started accelerating, rather than continuing to slow down as expected. This late time acceleration could be due to an exotic dark energy component, or a modification of Einstein's laws of gravity. The late Universe has a rich structure of galaxies and galaxy clusters. These structures are believed to form from small initial fluctuations in the matter distribution, generated in the early Universe. However the mechanism responsible for producing these fluctuations is not yet understood. Connecting observations of the late time Universe, with the early Universe and fundamental particle physics is a major outstanding issue for cosmology and one of the main goals of our work. Particle physics and general relativity both break down at extreme energies, where a unified theory of quantum gravity is expected to operate. We will test the observational consequences of such theories. In particular string theory allows superstrings to stretch across the Universe, altering the fluctuations in the CMB, producing gravitational waves, emitting high energy particles and lensing galaxies. We will study the evolution of networks of cosmic superstrings and make accurate predictions for their observational signatures. This work could potentially provide the first evidence for string theory through cosmology. We will also study the cosmology of models which arise from string theory, including the very early Universe and the generation of the primordial fluctuations from which structure form. String theory, and other new particle physics models, also provide us with a dark matter candidate in the form of Weakly Interacting Massive Particles (WIMPs). WIMPs can be detected directly in the lab (via their rare interactions with atoms) or indirectly via the antiparticles and high energy gamma-rays that are produced when they come together and annihilate. They can also be produced in particle colliders such as the LHC. We will develop the tools required to unambiguously detect dark matter and measure its properties. We will also study the astrophysical and cosmological consequences of specific string theory dark matter candidates. We will take a two pronged approach to understanding the physics underlying the observed late time acceleration of the Universe. We will develop techniques which will allow us to test the laws of gravity, and models of dark energy, in both the early and late Universe. We will also continue to explore fundamental physics models which may provide a mechanism for the acceleration. Our research encompasses a wide range of scales and energies. This diversity is met with a corresponding array of techniques to study various phenomena, ranging from quantum gravity to classical dynamics and analytic calculations to numerical simulations using supercomputers.

The particle physics and astronomy communities as well as the QG community working in cosmology are the obvious group of researchers who will benefit directly from the research proposed in this application. They will benefit in that our intention is to predict observational signatures for a number of Early Universe and Late Universe features. For example we will be determining the polarisation signal for a network of cosmic superstrings as a function of the string tension and coupling. This would be of direct interest to the string theory community as well as the CMB community who are searching for evidence of polarisation signals in the CMB. Similarly in the late universe we intend to develop the most complete parameterisation to date of modified gravity models, the PPF formalism. Through it cosmologists working on CMB and large scale structure surveys will have a means to constrain and possibly rule out classes of models by obtaining the required parameters and comparing them with General Relativity. Through our work on dark matter, we will be providing results of interest both to the experimental particle physics community and the astrophysics community working on dark matter detection. Our work on developing a framework for modeling the dark matter distribution to facilitate the unambiguous detection of dark matter and measure its properties, will be of significance for example for those working with the Cherenkov Telescope Array, as well as other upcoming WIMP indirect detection experiments, which are sensitive to WIMP annihilation in the remnants of WIMP microhalos. Our work on modified gravity emerging from string theory and braneworlds will open up a new avenue of using late universe observations for testing Early Universe physics. It will be of interest and could potentially have a significant impact on string theorists. As well as those directly benefitting from our research, we believe many will benefit indirectly. The graduate students and PDRAs that are trained through these projects often go on to work in industry and finance, taking with them the skill set developed in this research and applying it to new projects. We already have three students who have graduated in the past year and who are now working in climate modelling, finance and modelling wind turbines. There are also clear benefits for the wider public. For example undergraduate students will benefit through the opportunity to do projects with members of the group which will often involve learning about the physics involved in the grant. School and college students will benefit through masterclasses run by members of the group and talks given at schools. Similarly members of the public will benefit from public lectures given by group members in which their work will be discussed, through media activities such as radio and television appearances, as well as the continuing particpation of group members in the highly successful Sixty Symbols project. As mentioned above, the Knowledge Transfer will allow graduate students and PDRAs who have been trained by members of the group to enter the workplace and use their skills to beneift society.