3.2 Brief thermal history 73
of the light elements resulting fromprimordial nucleosynthesisare in very good agree-
ment with available observation data and this strongly supports our understanding of the
universe’s evolution back to the first second after the big bang.
∼ 1 s(T∼ 0 .5 MeV) The typical energy at this time is of order the electron mass. The
numerous electron–positron pairs present in the very early universe begin to annihilate
when the temperature drops below their rest mass and only a small excess of electrons
over positrons, roughly one per billion photons, survives after annihilation. The photons
produced are in thermal equilibrium and the radiation temperature increases compared to
the temperature of neutrinos, which decoupled earlier.
∼ 0. 2 s(T∼1–2 MeV) Two important events take place during this period as certain
weak interaction processes fall out of equilibrium. First, the primordial neutrinos decouple
from the other particles and propagate without further scatterings. Second, the ratio of
neutrons to protons “freezes out” because the interactions that keep neutrons and protons
in chemical equilibrium become inefficient. Subsequently, the number of the surviving
neutrons determines the abundances of the primordial elements.∼ 10 − (^5) s(T∼200 MeV) The quark–gluon transition takes place: free quarks and gluons
become confined within baryons and mesons. The physics of the quark–gluon transition
is not yet completely understood, though it is unlikely that this transition leaves any
significant cosmological imprints.
∼ 10 − (^10) – 10 − (^14) s(T∼100 GeV−10 TeV) This range of energy scales can still be probed
by accelerators. The Standard Model of electroweak and strong interactions appears to
be applicable here. We expect that at temperatures above∼100 GeV the electroweak
symmetry is restored and the gauge bosons are massless. Fermion and baryon numbers
are strongly violated in topological transitions above the symmetry restoration scale.
∼ 10 −^14 – 10 −^43 s (10 TeV–10 (^19) GeV) This energy range will probably not be reached by
accelerators in the near future. Instead, the very early universe becomes, in Zel’dovich’s
words, “an accelerator for poor people” that can give us some rough information about
fundamental physics. There is no reason to expect that nonperturbative quantum gravity
plays any significant role below 10^19 GeV. Therefore, we can still use General Relativity to
describe the dynamics of the universe. The main uncertainty here is the matter composition
of the universe. It might be that there are many more particle species than are evident
today. For example, according to supersymmetry, the number of particles species must be
doubled at least. Supersymmetry also provides us with good weakly interacting massive
particle candidates for dark matter.
The origin ofbaryon asymmetryin the universe is also related to physics beyond
the Standard Model. There are good reasons to expect that aGrand Unificationof the
electroweak and strong interactions takes place at energies about 10^16 GeV. Topologi-
cal defects, such as cosmic strings, monopoles, that occur naturally in unified theories
might play some role in the early universe, though, according to the current microwave
background anisotropy data, it is unlikely that they have any significance for large scale
structure.
Perhaps the most interesting phenomenon in the above energy range is the accelerated
expansion of the universe−inflation−which probably occurs somewhere near Grand