Cosmology started out as branch of astronomy. Lately it seems to be becoming branch of particle physics, or at least meeting and commingling place of astronomy and particle physics. If dinnertable conversation at the recent Eleventh Texas Symposium on Relativistic Astrophysics (held in Austin) is any indicator, some of the traditional astronomer cosmologists resent the invasion of the particle physicists. But as Alan H. Guth of Massachusetts Institute of Technology pointed out in talk at the same meeting, particle physicists have nowhere else to go. The theorists of particle physics have made much progress toward that would unify most of that science, but in so doing they have left the realm of practical experiment behind. To test the several candidates proposed to be the Grand Unified Theory (GUT), physicists have to study phenomena that occur at fantastically high energies. There is no hope of producing such energies in laboratory. As Guth remarks, it would take a linear accelerator one light-year long-unlikely to be funded during the Reagan administration. The only place to find such energies is in the early stages of the history of the universe, and so numbers of particle physicists are landing in cosmology. They hit the ground running. The application of what Guth calls the simplest of the GUT theories, the one based on the mathematical symmetry group SU(5) and proposed by Howard Georgi and Sheldon Glashow, produces radical changes in cosmology. The standard astronomically derived big-bang has the universe expanding smoothly, causally and adiabatically from the moment of origin to the present time. (Adiabatic cooling is drop in temperature due to expansion alone without loss of heat from the system.) GUT cosmology rejects this, proposing that the universe in its very early stages went through one or more phase transitions (like freeze or onset of boiling), and that these transitions interrupted causality and adiabatics. Another dramatic difference from earlier cosmology, in Guth's words, is that GUT cosmology seems to be of creation truly ex nihilo, and the universe seems to remain perpetually nothing as long as it exists. That is, all of the quantities that are the subjects of conservation laws and so important to physical analysis of the system (such as electric charge, angular momentum, color charge, etc.) seem to be so arranged that negative and positive amounts of them are equal and so always add up to zero, situation you can't distinguish from nothing, Guth says. (Guth also says that he has been trying to persuade his colleagues to start abbreviating theory with Th instead of just T. If they do, it would be tempting to call this the world according to GUTh.) GUT cosmology has three main consequences. It predicts first that there was one or more phase transitions at time when the temperature of the universe was 10's billion electron-volts (101' GeV). In Guth's use of units 1 GeV is about the equivalent of 10's kelvins, so in kelvins that temperature comes out to 1027 compared to the universal mean temperature of about 3 kelvins at present. The second prediction is that magnetic monopoles exist (SN: 11/27/82, p.348; 12/ 4/82, p. 362) and that their mass is about 1016 GeV. In Guth's units this equals about hundred-millionth of gram. The third consequence is that the law of conservation of baryons no longer holds. Baryons are class of particles whose lightest member is the proton. They include the neutron and several dozen heavier, radioactively unstable varieties. The baryon conservation law, the proposition that the net number of baryons and antibaryons never changes (which means that baryons change into other baryons when they do change), was pillar that held up the roof of the older particle physics and the older cosmology. Application of the new particle physics to cosmology deals in particular with three serious problems, the horizon problem, the flatness problem and the magnetic monopole problem. Under the assumptions of the standard big-bang -that is, the old cosmology -the universe in its earliest moments expands too fast to maintain causal relations. The speed of light is too slow for messages to catch up, and different parts of the universe get out of communication with each other. However, at the present time we observe high degree of isotropy in the universe: things are very much the same in all directions. That tells us that all parts of the universe were in communication with each other throughout the expansion or at least through as much of it as serves to de~~~~~~~~~~~~~~ Q
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Jan 1, 1998
Earl R Maccormac 
Open Access
