My main research interest is cosmology, with an emphasis on what we can learn about fundamental physics like neutrino masses, models of inflation, and dark energy from cosmological observations. Below you can see a figure illustrating constraints on ratio of the density of massive neutrinos in the Universe to the total mass density.
These constraints were derived by comparing cosmological models with various amounts of massive neutrinos to the 2dF Galaxy Redshift Survey data on the clustering of galaxies in our local Universe. The idea is simple: structures in the Universe form by gravitational collapse of small initial perturbations in the mass distribution. Relativistic particles, like eV-mass neutrinos, have high thermal velocities and therefore make it difficult for structure to form on small scales. Therefore one expects to see less structure on small scales the higher the neutrino mass fraction is. From analyses like this one, one finds upper limits on the combined mass of the three neutrino flavours of around 1 eV, which is much more stringent than current limits from laboratory experiments. Currently, however, most of my time is spent on trying to contribute to the huge international effort of finding out what causes the Universe to expand at an increasing rate. Gravity is the dominating force on cosmological scales, and all known sources of gravitational fields would make the Universe expand at a decreasing rate. Therefore, cosmologists have had to add a new component, dubbed "dark energy", which has negative pressure and can act as a source of gravitational repulsion. In Oslo, cosmologists at the Institute of theoretical astrophysics and in the theory group at the Department of Physics have started a joint project, Shedding Light on Dark Energy, funded by the Research Council of Norway for the period 2004-2007. In this project, we work on theoretical aspects of dark energy, like models where the Universe has extra dimensions, and observational methods for learning about the exact nature of this mysterious component of our Universe.