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All ideas concerning the very early universe (cosmogony) are speculative. No accelerator experiments have yet probed energies of sufficient magnitude to provide any experimental insight into the behavior of matter at the energy levels that prevailed during this period. Proposed scenarios differ radically. Some examples are the Hartle–Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these are mutually compatible, while others are not. Planck epoch Up to 10–43 second after the Big Bang Main article: Planck epoch The Planck epoch is an era in traditional (non-inflationary) big bang cosmology wherein the temperature was so high that the four fundamental forces—electromagnetism, gravitation, weak nuclear interaction, and strong nuclear interaction—were one fundamental force. Little is understood about physics at this temperature; different hypotheses propose different scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on general relativity and is expected to break down due to quantum effects. Physicists hope that such proposed theories of quantum gravitation as string theory, loop quantum gravity, and causal sets, will eventually provide a better understanding of this epoch.[citation needed] In inflationary cosmology, times before the end of inflation (roughly 10−32 second after the Big Bang) do not follow the traditional big bang timeline. The universe before the end of inflation is a very cold near-vacuum and persists for much longer than 10−32 second. Times from the end of inflation are based on the big bang time of the non-inflationary big bang model, not on the actual age of the universe at that time, which cannot be determined in inflationary cosmology. Therefore, inflationary cosmology lacks a traditional Planck epoch—though similar conditions may have prevailed in a pre-inflationary era of the universe. Grand unification epoch Between 10–43 second and 10–36 second after the Big Bang[3] Main article: Grand unification epoch As the universe expands and cools, it crosses transition temperatures at which forces separate from each other. These are phase transitions much like condensation and freezing. The grand unification epoch begins when gravitation separates from the other forces of nature, which are collectively known as gauge forces. The non-gravitational physics in this epoch would be described by a so-called grand unified theory (GUT). The grand unification epoch ends when the GUT forces further separate into the strong and electroweak forces. This transition should produce magnetic monopoles in large quantities, which are not observed. The lack of magnetic monopoles was one problem solved by the introduction of inflation. In modern inflationary cosmology, the traditional grand unification epoch, like the Planck epoch, does not exist, though similar conditions likely would have existed in the universe prior to inflation.[citation needed][further explanation needed] Electroweak epoch Between 10–36 second (or the end of inflation) and 10–12 second after the Big Bang[3] Main article: Electroweak epoch In traditional big bang cosmology, the Electroweak epoch begins 10−36 second after the Big Bang, when the temperature of the universe is low enough (1028 K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction). In inflationary cosmology, the electroweak epoch begins when the inflationary epoch ends, at roughly 10−32 second. Inflationary epoch Unknown duration, ending 10–32(?) second after the Big Bang Main article: Inflationary epoch Cosmic inflation is an era of accelerating expansion produced by a hypothesized field called the inflaton, which would have properties similar to the Higgs field and dark energy. While decelerating expansion magnifies deviations from homogeneity, making the universe more chaotic, accelerating expansion makes the universe more homogeneous. A sufficiently long period of inflationary expansion in our past could explain the high degree of homogeneity that is observed in the universe today at large scales, even if the state of the universe before inflation was highly disordered. Inflation ends when the inflaton field decays into ordinary particles in a process called "reheating", at which point ordinary Big Bang expansion begins. The time of reheating is usually quoted as a time "after the Big Bang". This refers to the time that would have passed in traditional (non-inflationary) cosmology between the Big Bang singularity and the universe dropping to the same temperature that was produced by reheating, even though, in inflationary cosmology, the traditional Big Bang did not occur. According to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10−32 second after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10−32 second. In fact, in order to explain the observed homogeneity of the universe, the duration must be longer than 10−32 second. In inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation. Baryogenesis Main article: Baryogenesis There is currently insufficient observational evidence to explain why the universe contains far more baryons than antibaryons. A candidate explanation for this phenomenon must allow the Sakharov conditions to be satisfied at some time after the end of cosmological inflation. While particle physics suggests asymmetries under which these conditions are met, these asymmetries are too small empirically to account for the observed baryon-antibaryon asymmetry of the universe. |
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