The Whole Truth by P. J. E. Peebles

Author:P. J. E. Peebles
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2022-03-14T00:00:00+00:00

Smirnov found what are now known to be reasonable pre-stellar abundances of the isotopes of hydrogen and helium when he used the astronomers’ estimate of the mass density. Smirnov’s computation is essentially the same as the one I was doing, independently, at about the same time, as will be discussed. But he had the wrong information about the abundance of helium. I had the right information at one time, but had forgotten it, and in early 1965 felt open to the possibility of large or small pre-stellar helium abundance. (I consulted two local experts, Martin Schwarzschild and Bengt Strömgren, about the helium abundance. They were polite but reserved. I now suspect they felt I should make my own decision by looking into the literature.)

The apparent failure of Gamow’s hot big bang theory led Zel’dovich (1962) to consider the alternative, a cold big bang. He saw the problem. Suppose the early universe was as cold as possible, temperature T = 0, with matter at its ground level of energy. That would have been an elegant early condition, except that electrons satisfy the exclusion principle, so pushing them together would have raised their minimum energy, what is known as their degeneracy energy. At really high density, in very early stages of expansion of the universe, the electrons would have been energetic enough to force themselves onto protons to turn them into neutrons. The problem with this is that, as the universe expands and the density decreases, the degeneracy energy of the electrons decreases. This allows neutrons to decay to protons, releasing electrons, because room has opened for them. The new protons readily combine with other neutrons to produce deuterons that fuse to heaver elements. This universe ends up with little hydrogen, which is unacceptable because hydrogen, with the proton as its atomic nucleus, is the most abundant element. Zel’dovich’s solution was to postulate that, in the very early stages of expansion of a cold universe, there were equal number densities of protons, electrons, and neutrinos. As before, the high densities in the early universe would have forced the electrons to large degeneracy energies. But it would have forced the neutrinos to have even larger degeneracy energies, because electrons share the energy in two spin states, while neutrinos have only one. This prevents the electrons from combining with protons to form neutrons, because that process creates neutrinos, and their large degeneracy energy left no room for more of them. This neat arrangement would result in a universe of pure hydrogen before stars started forming the elements. But astronomers were starting to recognize the evidence that the helium abundance was large before the stars formed, as expected in Gamow’s universe. Zel’dovich had an elegant theory but the wrong information.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.