Two dark matter problems arise from the above
standard BBN constraints: (1) Baryons observed in form of starlight amount
to 0.002< <0.006,
i.e. there must be baryonic dark matter, (2) galaxies are constrained to ,
clusters of galaxies indicate ,
and large scale flows of galaxies in the universe >0.3.
Thus, most of the matter in the Universe has to be non-baryonic. Popular
candidates are massive neutrinos and axions. The solution of the dark matter
problem will require joint forces from astronomy and particle physics,
investigating neutrino oscillations, accelerator searches for super-symmetry
or direct dark matter searches (see also the working group on neutrino
physics and fundamental interactions) .
Early epochs bear other open questions. The standard model of particle
physics predicts the electroweak phase transition at a temperature Tc200
GeV and for T>Tc baryon number violating processes
(sphaleron transitions) are unsuppressed. The early universe might have
left relics of this phase transition in the form of the observed baryon
asymmetry, if the electroweak phase transition is of first order. Accelerator
searches for the Higgs particle which induces this phase transition and
better observational limits on the antimatter content of the universe will
be very valuable to improve our understanding of the universal baryon asymmetry.
The present cosmic microwave background (CMB) radiation follows a perfect blackbody spectrum of (2.7270.01)K . It decoupled from matter at about 3000 K ( eV) when nuclei and electrons combined to neutral atoms. The COBE satellite found also that the CMB is extremely isotropic with l>1 multipole amplitudes less than 10-4. The present matter distribution with anisotropies on scales up to about 50 Mpc must have grown out of small initial fluctuations by gravitational instabilities which caused structure and galaxy formation and should be visible as small fluctuations in the CMB. Two classes of models can predict such a spectrum of primordial fluctuations: (1) quantum fluctuations which expand to super horizon scales during a period of inflationary expansion and (2) a phase transition at a temperature of about 1016 GeV leading to topological defects. The CMB anisotropies may thus provide information about the physics at extremely high energy scales (1014-1016 GeV). A further understanding within a few years is expected by improved CMB observations (PLANCK, MAP), as both models lead to different CMB patterns in a multipole expansion.