José Fonseca from the Institute of Cosmology and Gravitation at the University of Portsmouth gave the AIMS cosmology seminar on Monday 13th February on his work with David Wands (in astro-ph/1101.1254 and ongoing research) on non-gaussianities in curvaton models of inflation.
José began by giving us a brief overview of local non-gaussianities, specifically in the bispectrum. These non-gaussian parts of the primordial spectrum are contributions to the 3pt correlation function with two large momentum modes and one small one. They are of particular interest, because if measured they would be an indication of multifield inflation.
After this, José described the background of the curvaton model and the techniques used to analyse it. Beginning with a summary of slow roll inflation and the the equation of motion for the fluctuations produced during that epoch, José then moved on to explain the delta N formalism. Delta N provides a straightforward mechanism to relate perturbations in fields at horizon exit to the curvature perturbation far outside the horizon. This is important in curvaton models since, the curvature continues to evolve outside the horizon.
The curvaton model, José explained, is a model of inflation with two fields: an inflaton and a curvaton. The former drives the expansion during inflation, while the latter is sub-dominant during the exponentially expanding phase. Then, after inflation ends, the inflaton decays first, leaving most of the energy density in fluctuations in the curvaton. Then, when the curvaton decays some time later, it sources the curvature perturbations that eventually give rise to fluctuations in the CMB and large scale structure..
After completing his overview, José moved on to discussing the work he and his collaborator have done, constraining the parameters of the curvaton model in terms of observable quantities. They do this by noting that fNL (which characterizes the amount of non-gaussianity) and r (the ratio of tensor to scalar power) provide complementary constraints on the space of models.
The different curvaton model parameters are characterized by 3 parameters, and José showed us how numerical simulations can constrain these for a variety of potentials when only the curvaton is responsible for curvature fluctuations. He then moved to on to discuss more complicated mixed scenarios where both the curvaton and the inflaton contribute to the spectrum.
After the discussion of different models, José finished by summarizing his work and reiterating the potential of jointly using fNL and r to constrain curvaton models.