| 10-43 Seconds ≤ t ≤ 1 Second During this period the universe expanded and the matter density dropped from about 1092 times the density of water (!) to about 500,000 times the density of water. During the early part of this period, the matter in the universe consisted primarily of very high energy elementary particles in thermal equilibrium. Although we can be reasonably confident that general relativity provides a correct description of the universe in this and all later epochs, we can only speculate on the physics of the high-energy elementary particle interactions needed to give us a detailed description of what occurred during this period. Nevertheless, some of the most interesting recent ideas in cosmological theory concern phenomena which may have occurred at extremely early times (in most models, typically at t~10-35 seconds). One such idea is that the universe may have undergone an "inflationary era" at this time—that is, a period of extremely large expansion—due to the possibility that the energy density of the universe at this time may have been dominated by the potential energy of one of the fields present... Such an inflationary era, if it occurred, might help account for why the present universe is so homogeneous and isotropic and also could explain why the time scale associated with the dynamics of our universe (i.e., the total lifetime of the universe in the case of a closed universe) is so much larger than time scales appearing in the fundamental laws of physics. A second phenomenon which may have occurred in the very early universe is that an excess of baryons (i.e., protons, neutrons, and other heavy particles of a similar nature) over antibaryons may have been manufactured by elementary particle interactions. This could account for why there is an abundance of protons and neutrons in the present universe but apparently an almost total absence of their antiparticles. A third interesting phenomenon may have occurred during this early epoch if one postulates the existence of a field with potential energy of a certain form. When the universe has cooled sufficiently, the potential energy of such a field may become trapped in linelike structures known as cosmic strings. Such cosmic strings would then evolve in a complicated way, with, for example, new "loops of string" frequently formed by the self-intersection of a cosmic string. The loops of string would then decay by emission of gravitational radiation. Cosmic strings (or similar structures) formed in this epoch have been proposed as possible "seed perturbations" for the formation of galaxies and clusters of galaxies observed in the present universe. By t~1 second, the temperature of the matter cooled to about 10 billion degrees centigrade (!) and all the exotic elementary particles decayed. The matter in the universe by the end of this period consisted of a "soup" predominantly composed of photons (quanta of electromagnetic radiation), neutrinos (elementary particles which interact weakly with matter and which—like the photon—have no rest mass and travel at the speed of light), electrons and their antiparticles (positrons), protons, and neutrons. The presence of protons is favored over the presence of neutrons because protons are slightly less massive; at the end of the period there were about five times as many protons as neutrons. 1 Second ≤ t ≤ 1000 Seconds The universe continued to expand; the density of the soup dropped from about 500,000 times the density of water to about half the density of water; the temperature dropped from about 10 billion degrees to about 1 billion degrees. Early in this epoch, the positrons combined with electrons, converting their mass energy to photons. But the most dramatic thing that took place during this period was nucleosynthesis: the protons and neutrons underwent nuclear reactions and formed elements. Prior to this period the temperature of the matter was too high and any elements which might have been formed would have been dissociated immediately. After this period, the temperature and density of the matter were too low to permit these nuclear reactions to take place. Only during this period lasting about 15 minutes were conditions right for nucleosynthesis. Most of the neutrons present at the beginning of this period combined with the protons to form helium nuclei. (The neutrons which did not react decayed into protons and electrons either during this period or very shortly thereafter.) A very small amount of deuterium (that is, "heavy hydrogen," a proton and a neutron bound together) was synthesized and trace amounts of a few other light elements also were produced, but essentially no other element formation occurred. Thus at the end of this process, about 25% by mass of the original protons and neutrons was converted into helium, while essentially all the rest was left in the form of hydrogen (that is, protons). 1,000 Seconds ≤ t ≤ 100,000 Years The universe continued to expand rapidly and to cool, but otherwise this was a relatively dull period. The important constituents of the "soup" of matter and radiation filling the universe during this period were photons, protons, helium nuclei, and free electrons. | — Robert M. Wald, Space, Time, and Gravity: The Theory of the Big Bang and Black Holes, Chapter 5 – The Evolution of Our Universe | Indexes/21 |
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