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The early universe

The linear recession of galaxies, the cosmic microwave background (CMB) and the cosmic abundances of the light nuclides ¹H, ²H, ³He, ⁴He, and ⁷Li are the main observations supporting the idea that the present universe emerged from a hot and dense state by adiabatic expansion. Physicists are eager to unravel all the "relics" left over from these early epochs [1]. Big Bang Nucleosynthesis (BBN), occurring at T$\approx$0.1 MeV is presently the earliest epoch with such observable "relics" from a well understood process. When assuming that all microphysics is understood, the final BBN abundances of the light elements are determined by only one parameter, the entropy, expressible in terms of the baryon to photon ratio $\eta$= $n_B/n_\gamma$. Uncertainties, affecting especially the primordial ⁴He abundance, were related to $N_{\nu}$, the number of neutrino species and $\tau_n$, the neutron lifetime. However, in the late 1980's measurements of the properties of the Z⁰ boson with the large electron-positron collider (LEP) at CERN led to $N_{\nu}$=3, and the neutron lifetime is now also known with sufficient accuracy due to experiments with trapped ultracold neutrons: $\tau_n$=887$\pm$2 s. The cross sections of the relevant thermonuclear reactions linking nuclei from ¹H to ⁷Li have been measured at the appropriate energies to an accuracy better than 10$\%$. The major uncertainties are related to the determination of the primordial abundance of ²H, ^3,4He and ⁷Li with astronomical means. The abundances of the light elements constrain $\eta$ to 2-5 $\times 10^{-10}$, and $\Omega_bh^2$ to 0.01-0.025, where $\Omega_b$ is the baryon fraction of the critical density of the universe and h the Hubble expansion parameter in units of 100 km/s/Mpc (H_o=50-80 km/s/Mpc) [2]. Many variants of the BBN model have been investigated to test the robustness of its nucleosynthetic predictions to ``exotic'' physics. The most extensively studied case concerns the possibility of an inhomogeneous early Universe, due to a first order QCD phase transition taking place at T$\sim$200 MeV. Comparison of light element primordial abundances to the results of recent models of inhomogeneous nucleosynthesis shows, however, the need for baryon densities very similar to the ones resulting from the standard BBN.
 

    Two dark matter problems arise from the above standard BBN constraints: (1) Baryons observed in form of starlight amount to 0.002< $\Omega_{lum} h$<0.006, i.e. there must be baryonic dark matter, (2) galaxies are constrained to $\Omega=0.05\pm 0.03$, clusters of galaxies indicate $\Omega>0.15$, and large scale flows of galaxies in the universe $\Omega$>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) [3]. Early epochs bear other open questions. The standard model of particle physics predicts the electroweak phase transition at a temperature T_c$\sim$200 GeV and for T>T_c 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 (2.727$\pm$0.01)K [4]. It decoupled from matter at about 3000 K ($\sim 0.3$ 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.