2.1 A brief chronology

UpdateJump To The Next Update Information With current observational constraints, the physical state of our Universe, as understood in the context of the standard or Friedman–Lemaître–Robertson–Walker (FLRW) model, can be crudely extrapolated back to ∼ 10–43 seconds after the Big Bang, before which the classical description of general relativity is expected to give way to a quantum theory of gravity. As the time-line in Figure 1View Image shows, the Universe was a plasma of relativistic particles at the earliest times consisting of quarks, leptons, gauge bosons, and Higgs bosons represented by scalar fields with interaction and symmetry regulating potentials.
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Figure 1: A historical time-line showing the major evolutionary stages of our Universe according to the standard model, from the earliest moments of the Planck era to the present. The horizontal axis represents logarithmic time in seconds (or equivalently energy in electron-Volts or temperature in Kelvin), and the solid red line roughly models the radius of the Universe, showing the different rates of expansion at different times: exponential during inflation, shallow power law during the radiation dominated era, and a somewhat steeper power law during the current matter dominated phase.
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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 8 t ∼ 10 y from molecular gas clouds when the Jeans mass of the background baryonic fluid was approximately 4 10 M ⊙, as indicated in Figure 2View Image. This epoch of pop III star generation is followed by the formation of galaxies at t ∼ 109 yr 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 8h −1 Mpc, where h is the Hubble parameter (compared to 2 – 3 Mpc typical for the virial radius of rich galaxy clusters).

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Figure 2: Schematic depicting the general sequence of events in the post-recombination Universe. The solid and dotted lines potentially track the Jeans mass of the average baryonic gas component from the recombination epoch at z ∼ 103 to the current time. A residual ionization fraction of nH+ ∕nH ∼ 10− 4 following recombination allows for Compton interactions with photons to z ∼ 200, during which the Jeans mass remains constant at 5 10 M ⊙. The Jeans mass then decreases as the Universe expands adiabatically until the first collapsed structures form sufficient amounts of hydrogen molecules to trigger a cooling instability and produce pop III stars at z ∼ 20. Star formation activity can then reheat the Universe and raise the mean Jeans mass to above 108M ⊙. This reheating could affect the subsequent development of structures such as galaxies and the observed Lyα clouds.

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