It is believed that several spontaneous symmetry breaking (SSB) phase transitions occured in the early Universe as it expanded and cooled, including the grand unification transition (GUT) at 10–34 s after the Big Bang in which the strong nuclear force split off from the weak and electromagnetic forces (this also marks an era of inflationary expansion and the origin of matter-antimatter asymmetry through baryon, charge conjugation, and charge + parity violating interactions and nonequilibrium effects); the electroweak (EW) SSB transition at 10–11 s when the weak nuclear force split from the electromagnetic force; and the chiral or quantum chromodynamic (QCD) symmetry breaking transition at 10–5 s during which quarks condensed into hadrons. The most stable hadrons (baryons, or protons and neutrons comprised of three quarks) survived the subsequent period of baryon-antibaryon annihilations, which continued until the Universe cooled to the point at which new baryon-antibaryon pairs could no longer be produced. This resulted in a large number of photons and relatively few surviving baryons. Topological defects, defined as stable configurations of matter in the symmetric (high temperature) phase, may persist after any of the phase transitions described above to influence the subsequent evolution of matter structures. The nature of the defects is determined by the phase transition and the symmetry properties of the matter, and some examples include domain walls, cosmic strings, monopoles, and textures.
A period of primordial nucleosynthesis followed from 10–2 to 102 s during which light element abundances were synthesized to form 24% helium with trace amounts of deuterium, tritium, helium-3, and lithium. Observations of these relative abundances represent the earliest confirmation of the standard model. It is also during this stage that neutrinos (produced from proton-proton and proton-photon interactions, and from the collapse or quantum evaporation/annihilation of topological defects) stopped interacting with other matter, such as neutrons, protons, and photons. Neutrinos that existed at this time separated from these other forms of matter and traveled freely through the Universe at very high velocities, near the speed of light.
By 1011 s, the matter density became equal to the radiation density as the Universe continued to expand, identifying the start of the current matter-dominated era and the beginning of structure formation. Later, at 1013 s (3 × 105 yr), the free ions and electrons combined to form atoms, effectively decoupling the matter from the radiation field as the Universe cooled. This decoupling or post-recombination epoch marks the surface of last scattering and the boundary of the observable (via photons) Universe, and plays an important role in the history of the Cosmic Microwave Background Radiation (CMBR). Assuming a hierarchical Cold Dark Matter (CDM) structure formation scenario, the subsequent development of our Universe is characterized by the growth of structures with increasing size. For example, the first stars are likely to have formed at from molecular gas clouds when the Jeans mass of the background baryonic fluid was approximately , as indicated in Figure 2. This epoch of pop III star generation is followed by the formation of galaxies at and subsequently galaxy clusters. Though somewhat controversial, estimates of the current age of our Universe range from 10 to 20 Gy, with a present-day linear structure scale radius of about , where is the Hubble parameter (compared to 2 – 3 Mpc typical for the virial radius of rich galaxy clusters).
© Max Planck Society and the author(s)