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. Specific features in our Universe such as the Cosmic Microwave Background (CMB) (relic radiation from the big bang) and individual supernovae explosions allow us to probe the expansion history and constituents of the Universe. These observations suggest that most of the matter in the Universe is composed of exotic dark matter. Furthermore the expansion of the Universe recently started accelerating (by recent we mean a few billion years ago actually, but recent compared to the Universe's age of 13.7 billion years), 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. Moreover, we also have in the late Universe rich structures of galaxies and galaxy clusters which are believed to have grown from small initial fluctuations in the matter distribution, which in turn were generated in the very early Universe. However the mechanism responsible for producing these fluctuations is not yet fully 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. We will study the evolution of networks of cosmic strings and superstrings and make accurate predictions for their observational signatures. We will then test these predictions directly against data from the Planck satellite and thereby constrain the allowed form these strings could take in our Universe. This work could potentially provide the first evidence for string theory through cosmology.

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 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. One exciting proposal we will investigate involves a Harry Potter like invisibility cloak. In a class of modified gravity models we have developed, the cosmological constant can be arbitrarily large as it is hidden from the background geometry of the Universe by the presence of a scalar field which acts like the cloak. There are many interesting facets to this proposal which we will investigate in depth, not least among them, is it observationally consistent?


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 Quantum Gravity (QG) community working in cosmology are the obvious group of researchers who will benefit directly from the research proposed in this application. Our intention is to predict observational signatures for a number of Early Universe and Late Universe features. For example we will be using the latest data from Planck to constrain models of cosmic superstrings as a function of the string tension and coupling and deriving Likelihood curves to determine the evidence for their existence. This would be of direct interest to the string theory community as well as the CMB community who are searching for polarisation signals in the CMB. Similarly in the late universe we intend to develop and apply the most complete parameterisation to date of modified gravity models, the PPF formalism. Through it cosmologists working on CMB and LSS surveys will have a means to constrain and possibly rule out classes of models by calculating the PPF parameters and comparing them with observations. We are members of DES and EUCLID and the PPF formalism we are developing with be invaluable to those searching for evidence of evolution in the dark energy equation of state.

Our work extending the Fab Four model will allow astronomers to directly test for self tuning scenarios of the cosmological constant. 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 other WIMP indirect detection experiments. Our proposed work on detecting dark energy in the laboratory will be of real interest to the experimental particle and cold atoms communities. Moreover it will provide a wonderful forum for a new PhD student to learn a series of skills linked to both theory and experiment. On the more mathematical end of the subject our work on singularity avoidance mechanisms in LQC, the low energy regime of spin foam models of QG and new chiral descriptions of gravity will be of interest to the cosmological community interested in testing General Relativity on small scales and developing tests of QG.

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. Four of the students who have graduated in the past three years are now working in climate modelling, finance, plasma fusion and modelling wind turbines. There are also clear benefits for the wider public. For example undergraduate and M.Sc 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 introduced, through media activities such as radio and television appearances, as well as the continuing participation of group members in the highly successful Sixty Symbols project, and more recently in the even more successful Numberphiles 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 benefit society.