2.2 Successes of the Standard 2 BACKGROUND2 BACKGROUND

2.1 A Brief Chronology 

With current observational constraints, the physical state of our Universe, as understood in the context of the standard, or Friedmann-Robertson-Lemaître-Walker (FLRW) model, can be crudely extrapolated back to the Planck epoch image seconds after the Big Bang, beyond which the classical theory of general relativity is invalid due to quantum corrections. At the earliest times, the Universe was a plasma of relativistic particles consisting of quarks, leptons, gauge bosons, and Higgs bosons represented by scalar fields with interaction and symmetry regulating potentials. It is believed that several spontaneous symmetry breaking (SSB) phase transitions transpired in the early Universe as it expanded and cooled, including: the grand unification transition (GUT) at image seconds after the Big Bang (Here, 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 image secs (when the weak nuclear force split from the electromagnetic force); and the chiral (QCD) symmetry breaking transition at image secs 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. A period of primordial nucleosynthesis followed from image to image secs during which light element abundances were synthesized to form 24% helium with trace amounts of deuterium, tritium, helium-3, and lithium.

By image secs, the matter density became equal to the radiation density, identifying the start of the current matter-dominated era and the beginning of structure formation. Later, at image secs (image years), the free ions and electrons combine to form atoms, decoupling the matter from the radiation field as the Universe continued to expand and cool. This decoupling or post-recombination epoch marks the surface of last scattering and the boundary of the observable (via photons) Universe. Assuming a hierarchical (CDM-like) 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 form at image years from molecular gas clouds when the Jeans mass of the background baryonic fluid is approximately image, as indicated in Figure 1 . This epoch of pop III star generation is followed by the formation of galaxies at image years and then galaxy clusters.


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Figure 1: 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 image to the current time. A residual ionization fraction of image following recombination allows for Compton interactions with photons to image, during which the Jeans mass remains constant at image . 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 image . Star formation activity can then reheat the Universe and raise the mean Jeans mass to above image . This reheating could affect the subsequent development of structures such as galaxies and the observed Lyman-alpha clouds.

2.2 Successes of the Standard 2 BACKGROUND2 BACKGROUND

image Computational Cosmology: from the Early Universe to the Large Scale Structure
Peter Anninos
© Max-Planck-Gesellschaft. ISSN 1433-8351
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