Thermal Relics from the Big Bang

9.5 Summary

• The matter antimatter asymmetry of the Universe is of order 10⁻¹⁰. Although the ingredients needed to generate such an asymmetry in the Universe exist in particle physics models, a detailed understanding of its origin is still lacking.

• Any stable neutral particle with weak interactions should have been produced in large quantities in the early Universe and would make a non-negligible contribution to the matter density in the Universe today, perhaps explaining the dark matter problem.

• Stable neutrinos in the mass range between 90 eV and 3 GeV are excluded for cosmological reasons.

• Big Bang nucleosynthesis took place during the first few minutes after the Big Bang. The observed abundances of helium, deuterium and lithium agree well with the predictions of the Big Bang model.

• Photons decoupled from thermal equilibrium at a redshift around 1100. Since that time of last scattering they have been redshifted by a corresponding factor and are observed today as the cosmic microwave background, one of the cornerstones of the Big Bang model.