Figure 1. The Thermal History of the Universe The figure (adapted courtesy of Mike Big Bang Cosmology Turner) shows important epochs in the history of the universe according to stan- dard Big Bang cosmology. It also shows and the Microwave the particles of matter and radiation con- stituting the universe during each epoch. The history is laid out on logarithmic Background scales to emphasize the early part. The scales are temperature, thermal energy kT, universal scale factor a(t) (note that a(t) ϭ 1/(z ϩ 1) where z is the redshift), Salman Habib and Raymond J. Laflamme density (t), and time. Temperature, ther- mal energy, redshift, and energy density (or the equivalent mass density) all de- crease with time, whereas the scale of
the universe a(t) ϵ R(t)/R(t0) increases. ig Bang cosmology implies a cer- (that is, massless particles). The The temperature plotted is the tempera- Btain predictable course for the ther- physics during this time is highly spec- ture of the radiation, which until recombi- mal history of the universe. Here we ulative. It is assumed that thermal en- nation is the temperature of the baryonic review that history, emphasizing the ergies were initially at the Planck scale matter (neutrons and protons) as well. two classic pieces of evidence that sup- (1028 eV), where the exotic and poorly The formulae relating these variables are port it: the relative abundances of the understood phenomenon of quantum shown beneath the figure. They follow light elements produced by primordial gravity could have played a crucial from conservation of energy and from the nucleosynthesis and the existence, spec- role. Many particle physicists and as- assumptions that matter and radiation trum, and fantastic isotropy of the cos- trophysicists believe that a bit later, as were initially in thermal equilibrium and mic microwave background. We will the temperature cooled to 1026 kelvins, that the universe has expanded adiabati- also discuss the modifications that there was a brief period of inflation. cally since the hot Big Bang. The rela- might be caused by the presence of During the inflationary phase, the size tionship between (t) and a(t) changes cold dark matter. of the universe grew exponentially by a depending on whether the universe is ra- Figure 1 outlines the calendar of factor of at least 1028. Less speculative diation-dominated or matter-dominated, events, or major epochs, in the history. is the prediction that at a temperature that is, on whether most of the energy in It can be divided into two main periods: around 1012 kelvins, a quark-hadron the universe is in the form of radiation the radiation-dominated period, which transition occurred, when all the quarks (photons, neutrinos, and other particles lasted for approximately ten thousand and gluons combined to form protons moving at speeds near c) or most of the years (1011 seconds), and the matter- and neutrons. The particles of cold energy is in the form of matter. The tem- dominated period from ten thousand dark matter, if they exist, would also perature and scale-factor axes are lined years after the Big Bang to the present. have been created sometime after the up with respect to each other by the ob-
During the first fraction of a second inflationary phase. servation that at present (t ϭ t0) the tem- after the Big Bang, the constituents of perature of the cosmic background radia- the universe undoubtedly included the The radiation-dominated era. tion is approximately 3 kelvins and that Ϫ2 particles of the standard model of parti- From about 10 second to the present, by definition a(t0) ϵ 1. The time and den- cle physics: leptons, quarks, and the standard Big Bang cosmology presents sity axes are lined up with the size axis gauge bosons that mediate interactions an almost universally accepted picture, by using a specific value for the Hubble among them, all moving at velocities so which is borne out by the COBE data constant and the assumption that the di- close to the velocity of light that from and other recent observations. By that mensionless density parameter ⍀ is equal the point of view of thermodynamics, time the universe consisted of particles to 1. Then one can place any event on they behaved as particles of radiation that were stable (or long-lived com- all three axes if one knows its position on
82 Los Alamos Science Number 22 1994
Big Bang Cosmology and the Microwave Background
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Cold dark matter Recombination: Quark- Primordial begins to neutral atoms form; baryon