Programme Director Kari Enqvist
With the MAP and Planck satellite experiments, cosmology is about to become aprecision science. With subjects such as neutrino properties, the nature of dark matter andthe origin of baryons, it is increasingly seen to complement accelerator-based particle physics. Because of the forthcoming satellite and balloon borne experiments, the main focus of cosmology in the years to come will, however, be in the study of the temperature fluctuations of the cosmic microwave background (CMB).
Finnish science participation in the Planck Surveyor Mission is being coordinated by K. Enqvist, who is one of the two Finnish Co-Investigators in the Low Frequency Instrument Consortium. CMB-related theoretical considerations are expected to play an increasing role in the total effort of the cosmology project, which receives its main funding from the Academy of Finland. The topics to be studied are:
Limits on isocurvature perturbations of the CMB. Simple inflation models predict an adiabatic perturbation spectrum. However, particle physics models such as double inflation, axions or Affleck-Dine baryogenesis, will give rise both to adiabatic and isocurvature perturbations. The measurements by Planck are expected to provide information about CMB polarization which is accurate enough to resolve the degeneracy between isocurvature and adiabatic tensor perturbations
Affleck-Dine baryogenesis and cosmological consequences of Q-balls. The minimally supersymmetric standard model has many directions in the space of scalar fields where the potential vanishes identically. During inflation, fields may fluctuate along the directions, forming scalar (Affleck-Dine) condensates. The condensate is not the state of lowest energy but fragments into non-topological solitons, Q-balls, with a host of interesting cosmological consequences. Q-balls may be the sources of both dark matter and baryons. The fragmentation process is highly non-linear and requires computer simulations (in progress).
Inhomogenous nucleosynthesis. Big bang nucleosynthesis has been the best way to estimate the average density of ordinary matter in the universe. Cosmic microwave background measurements are beginning to provide a completely independent competing estimate. As preliminary results suggest a higher density, it is important to study possible deviations from the standard picture in nucleosynthesis, in particular how inhomogeneities could raise the allowed average density. Nucleosynthesis can also be used to constrain the amount of antimatter in the early universe. On the other hand, the presence of antimatter can relax other constraints derived from nucleosynthesis. We are now looking at the effect on the upper limits to new relativistic particle species in the early universe.
Active-sterile neutrino oscillations in the early universe. These pertain both nucleosynthesis and CMB. Sterile neutrinos may be excited through mixing with ordinary, active neutrinos. In general, neutrino and antineutrino populations evolve in a different way, and there is a possibility of generating large neutrino asymmetries. However, there are chaotic regions in the space of mixing parameters where the sign of the asymmetry is practically unpredictable. Spatial fluctuations in the initial lepton asymmetry (which in the standard model is related to the baryon asymmetry) enhance the effect. As a result, calculations of light elements abundances have an intrinsic uncertainty. More extensive simulations are needed, however, with the neutrino momentum distributions taken into account.
Consequences of Randall-Sundrum cosmologies. The basic idea is to postulate an extra dimension occupied by two four-dimensional branes. The geometrical properties of the solutions of the equations of motion give severe constraints on possible bulk and brane equations of state. An interesting question is possible flow of bulk matter into the branes. Here we collaborate closely with the HIP mathematical physics group.
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