The First Three Minutes by Steven Weinberg

Author:Steven Weinberg [Weinberg, Steven]
Language: eng
Format: epub, pdf
Tags: A modem view of the origin of the universe
Published: 2011-02-18T00:54:54+00:00

• Recipe for a Hot Universe 95

antiparticles. Both seem to be absolutely conserved, but this is not known with great certainty.)

A good example of the working of these rules is furnished by the radioactive decay of a neutron n into a proton p, an electron e-, and an (electron-type) antineutrino. The values of the charge, baryon number, and lepton number of each particle are as follows:

The reader can easily check that the sum of the values of any conserved quantity for the particles in the final state equals the value for the same quantity in the initial neutron. This is what we mean by these quantities being conserved. The conservation laws are far from empty, for they tell us that a great many reactions do not occur, such as the forbidden decay process in which a neutron decays into a proton, an electron, and more than one antineutrino.

To complete our recipe for the contents of the universe at any given time, we must thus specify the charge, baryon number, and lepton number per unit volume as well as the temperature at that time. The conservation laws tell us that in any volume which expands with the universe the values of these quantities remain fixed. Thus, the charge, baryon number, and lepton number per unit volume simply vary with the inverse cube of the size of the universe. But the number of photons per unit volume also varies with the inverse cube of the size of the universe. (We saw in Chapter 3

that the number of photons per unit volume is proportional to the cube of the temperature, while, as remarked at the beginning of this chapter, the temperature varies with the

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