The universe contains the record of its past the way that sedimentary layers of rock contain the geological record of the Earth's past..
Heinz Pagels

The discovery that the universe is expanding leads to the question:  what was the universe like in the past? If one runs the history of the universe backward, it appears that the universe must have once been very dense as all the matter in all the visible galaxies was concentrated into a very small volume of space. Perhaps this initial dense state is itself responsible for the creation of the elements that make up the matter we see. This was the concept that motivated George Gamow and his collaborators to develop the theory of nucleosynthesis in the big bang during the late 1940's and early 1950's. Indeed, their work led to the first prediction of the cosmic background radiation (CBR) and its temperature. Unfortunately, their prediction of microwave photons from the big bang was not taken seriously until it was rediscovered by others in the mid 1960's. The discovery of the CBR led scientists to realize that they had overlooked clues to the existence of the cosmic background for over 15 years. The discovery of the CBR also showed how theortical physics could be applied to understand even so remote and remarkable an event as the beginning of the universe.

The history of the universe can be divided into many eras, depending on which constituent was most important. Today the matter density of the universe dominates its gravity completely; thus we say that we live in the matter era. However, conditions were not always as they are today. The relationship between temperature and redshift of the CBR demonstrates that as we look backward in time toward t=0, the photons in the CBR become increasingly hot. Density also changes with time, becoming ever higher as we look toward earlier times; but the variation with scale factor is different for matter density and for radiation energy density, with matter varying as the cube of the scale factor, while radiation energy density varies as the fourth power of R. Hence there was a time in the past when radiation was more important than ordinary matter in determining the evolution of the universe. This high-temperature epoch defines the early universe. Temperature is a measure of energy, and Einstein's equation E=mc2 tells us that energy and matter are equivalent. At sufficiently high temperatures, particles with large mass can be created, along with their antiparticles, from pure energy. The temperature also influenced how the fundamental forces of nature behaved during the earliest intervals of the universe's history.

The interval of domination by radiation is called the radiation era. As we approach t=0 in this era, we encounter increasingly unfamiliar epochs, dominated by different physics and different particles. The earliest was the Planck epoch, during which all four fundamental forces were unified and "particles" as we known them could not have existed. Next followed the GUT epoch, when gravity had decoupled but the other three forces remained unified. The small excess of matter that makes up the universe today must have been created during this epoch, by a process still not completely understood. As the temperature dropped, the universe traversed the quark epoch, the hadron epoch, the lepton epoch, and the epoch of nucleosynthesis.

The first atomic nuclei formed during the nucleosynthesis epoch. Most of the helium in the universe was created from the primordial neutrons and protons by the time the nucleosynthesis epoch ended scarcely three minutes after the big bang. A few other trace isotopes, specifically deuterium (heavy hydrogen) and lithium 7, were also created, and their density depends sensitively upon the density of the universe during this time. If the universe were too dense, then most of the deuterium would have fused into helium. Only in a low-density universe can the deuterium survive. The major factor controlling the ultimate densities of helium and deuterium is the abundance of neutrons. The more neutrons that decay before combining with protons, the smaller the abundances of heavier elements. The availability of neutrons depends on the expansion rate as well as the cosmic matter density. Comparing the observed densities of the primordial isotopes to those computed from models and translating the results into Omega, the density parameter, gives Omega = 0.015/h2 where h is the Hubble parameter divided by 100 km/sec/Mpc. The smaller Ho, the larger Omega; if Ho=50, Omega is approximately 0.06, whereas Ho=100 gives Omega of only 0.015. This range is still much less than Omega=1, but nucleosynthesis limits can indicate only the density of baryons, because only baryons participate in nuclear reactions. Hence we must conclude that the universe contains less than the critical density of baryons.

After nucleosynthesis, nothing much happened for roughly a million years as the universe continued to cool. The ordinary matter consisted of a hot plasma of nuclei and electrons. The free electrons made the plasma opaque; a photon of radiation could not have traveled far before being scattered. However, once the universe cooled to approximately 3000 K, the electrons no longer moved fast enough to escape the attraction of the nuclei, and atoms formed. Although there had been no previous combination, this event is still known as recombination. The last moment at which the universe was opaque forms the surface of last scattering; it represents the effective edge of the universe that is even theoretically visible with an optical telescope, since no optical telescope could ever penetrate the dense, opaque plasma that existed prior to recombination. Once the radiation was able to stream freely through the universe, matter and radiation lost the tight coupling that had bound them since the beginning. Henceforth matter and radiation evolved almost entirely independently. The photons that filled the universe at the surface of last scattering make up the CBR today, but now their energy is mostly in the microwave band. At some point before or near recombination, the matter density and the energy density were equally important. This is the epoch in which structure formation began to occur. The seeds of structure formation may have been planted much earlier, during the GUT epoch, but the tight coupling between radiation and matter prevented the density perturbations from doing much. Once matter and radiation went their separate ways, density pertubations could evolve on their own. The most overdense areas collapsed gravitationally, forming galaxies and clusters of galaxies. Less dense areas probably led to voids, the large underdense areas we see on the sky today. The process by which structure formed in the early universe is still very poorly understood; better data from instruments such as the planned successor to COBE will help to elucidate the mystery of the galaxies.

For more information see Questions and Answers related to Chapter 12.

Original content © 2005 John F. Hawley