APPENDIX #4: The Evolution of the Universe
The evolution of the universe is effectively the change in distribution of matter through time - moving from a virtual homogeneity in the early universe to a very lumpy universe today, with matter condensed as galaxies, clusters, superclusters, and even larger structures. We can view that evolution as a series of phase transitions, in which matter passes from one state to another under the influence of decreasing temperature (or energy). We are all familiar with the way that steam, on cooling, condenses: This is a phase transition from a gaseous to a liquid state. Reduce the temperature further, and eventually the water freezes, making a phase transition from the liquid to the solid state. In the same way, matter has gone through a series of phase transitions since the first instant of the big bang.
At a ten-millionth of a trillionth of a trillionth of a trillionth (10 - 42) of a second after the big bang - the earliest moment about which we can sensibly talk, and then only with some suspension of disbelief - all the universe we can observe today was the tiniest fraction of the size of a proton. Space and time had only just begun. (Remember, the universe did not expand into existing space after the big bang; its expansion created space-time as it went.) The temperature at this point was a hundred million trillion trillion (1032) degrees, and the three forces of nature - electromagnetism and the strong and weak nuclear forces - were fused as one. Matter was undifferentiated from energy, and particles did not yet exist.
By a ten-billionth of a trillionth of a trillionth of a second (10 - 34 second) inflation had expanded the universe (at an accelerating rate) a million trillion trillion (1030) times, and the temperature had fallen to below a billion billion billion (1027) degrees. The strong nuclear force had separated, and matter underwent its first phase transition, existing now as quarks (the building blocks of protons and neutrons), electrons, and other fundamental particles.
The next phase transition occurred at a ten-thousandth of a second, when quarks began to bind together to form protons and neutrons (and antiprotons and antineutrons). Annihilations of particles of matter and antimatter began, eventually leaving a slight residue of matter. All the forces of nature were now separated.
The temperature had fallen sufficiently after about a minute to allow protons and neutrons to stick together when they collided, forming the nuclei of hydrogen and helium, the stuff of stars. This soup of matter and radiation, which initially was the density of water, continued expanding and cooling for another three hundred thousand years, but it was too energetic for electrons to stick to the hydrogen and helium nuclei to form atoms. The energetic photons existed in a frenzy of interactions with particles in the soup. The photons could travel only a very short distance between interactions. The universe was essentially opaque.
When the temperature fell to about 3,000 degrees, at three hundred thousand years, a crucial further phase transition occurred. The photons were no longer energetic enough to dislodge electrons from around hydrogen and helium nuclei and so atoms of hydrogen and helium formed and stayed together. The photons no longer interacted with the electrons and were free to escape and travel great distances. With this decoupling of matter and radiation, the universe became transparent, and radiation streamed in all directions - to course through time as the cosmic background radiation we experience still. The radiation released at that instant gives us a snapshot of the distribution of matter within the universe at three hundred thousand years of age. Had all matter been distributed evenly, the fabric of space would have been smooth, and the interaction of photons with particles would have been homogeneous, resulting in a completely uniform cosmic background radiation. Discovery of the wrinkles reveals that matter was not uniformly distributed, that it was already structured, thus forming the seeds out of which today's complex universe has grown.
(Smoot, G. and Davidson, K. 1993. Wrinkles in time. Morrow, New York. 331 pp.)