2.7. The history of the Universe

Presently the Universe is dominated by non-relativistic matter, but because radiation reduces more quickly with the expansion, this implies that at earlier times the Universe was radiation dominated. During the radiation era temperature and time are related by

(19)

The highest energies accessible to terrestrial experiment, generated in particle accelerators, correspond to a temperature of about 10¹⁵ K, which was attained when the Universe was about 10^-10 sec old. Before that, we have no direct evidence of the applicable physical laws and must use extrapolation based on current particle physics model building. After that time there is a fairly clear picture of how the Universe evolved to reach the present, with the key events being as follows:

• 10^-4 seconds: Quarks condense to form protons and neutrons.
• 1 second: The Universe has cooled sufficiently that light nuclei are able to form, via a process known as nucleosynthesis.
• 10⁴ years: The radiation density drops to the level of the matter density, the epoch being known as matter-radiation equality. Subsequently the Universe is matter dominated.
• 10⁵ years: Decoupling of radiation from matter leads to the formation of the microwave background. This is more or less coincident with recombination, when the up-to-now free electrons combine with the nuclei to form atoms.
• 10¹⁰ years: The present.