Research

 

My research focuses on the interplay between theoretical and observational cosmology.  Throughout my research career I have worked on a wide variety of topics: the origin of structure due to cosmological defects, the origin and evolution of primordial magnetic fields, the large scale structure of space-time (such as topology and geometry), theories of dark energy and dark matter, techniques for analysis of cosmological data sets (primarily the cosmic microwave background but also galaxy surveys and large scale flows), the development of pipelines for cosmic microwave background (CMB) experiments, statistical methods in cosmology and astrophysics. As a result I have been (and still am) a key member on a number of international collaborations: MAXIMA, BOOMERanG, CBI, QUIET and Clover.

Over the past couple of years I have focused on particularly topical projects. They range from the experimental to the highly theoretical.

Polarization of the CMB:  The next major frontier in experimental CMB research is the measurement of the polarization of relic radiation. It will contain valuable information about the history Universe but most important of all, it will be able to measure the bath of gravity waves left over from the beginning, the Big Bang. If successful, these experiments could signal a new step change in cosmology: we would finally be able to peer at the Big Bang directly, at energies of 1017 GeV, in the same way that looking at CMB tells us about the Universe at energies of 1 eV and neutrinos tell us what was going on at energies of 1 MeV.


I am a member of the QUIET collaboration (lead from U. of Chicago) and the Planck Satellite expression and over the next few years I will be  devoting much of my time to extracting the gravity wave signal.


Measuring the State of the Universe: One of the great successes of contemporary cosmology is the ability to measure the properties of the Universe. A stunning example of this is the measurement of the geometry of the Universe by the BOOMERanG experiment. With the planned battery of experiments, it should be possible to pin down the state of the Universe with exquisite precision. There is, however, a substantial amount of work to be done on two fronts.


First of all, much of the analysis undertaken until now is heavily dependent on what assumptions one makes for the cosmological model and initial conditions. For example it is possible to posit initial conditions, or modify the geometry of the Universe that can, for example, mimic the effect of a cosmological constant on the CMB.


Second, we are at a time when powerful cosmological experiments are being proposed, either ground based or space based (the SKA, a square kilometre array of radio telescopes is one with particularly exciting possibilities and in which Oxford plays a leading role). I am particularly interested in figuring out which methods will maximize scientific returns and have devoted part of my time to the comprehensive study of different strategies.


The Problem of the Dark Sector: From the analysis of cosmological data there seems to be compelling evidence that only 5% of the total energy budget us made up of atoms. There are strong indications that 25-30% of the total energy is in a form of matter that clumps  known as dark matter and 65-70% is in a form of very smooth, gravitationally repulsive energy known as dark energy. The main question in cosmology at the moment is to understand the nature of these two forms of energy that make up the dark sector. In the past I have worked on a broad family of candidates for dark energy. More recently I have started to focus on different aspects of the dark sector and, in particular, focus on the assumptions that go into its existence.


For a start, a key assumption is that the Universe is homogeneous and isotropic, permeated by a continuous bath of relativistic and non-relativistic particles. Albeit a powerful assumption it makes sense to study how dropping it may affect our measurements of dark energy. With my collaborators, I have looked at the possibility that we live in a giant void, or in a network of voids or even in an empty universe populated by discrete island of matter (galaxies).


Secondly, it is possible that the observational evidence for dark energy and dark matter may be coming from a misunderstanding of gravity. It is possible that our current theory of gravity (the General Theory of Relativity) is flawed and we must find more accurate theories.  We have shown that it is possible to mimic many of the effects of the dark sector through gravitational degrees of freedom, related it to proposals which might be tested through fundamental theory but more importantly we have isolated the generic properties which make these theories work.