Baryon Inhomogeneity in the Early Universe

Abstract

In the early high-density high-temperature universe, matter and antimatter are in equilibrium and exist in almost equal quantities. In a comoving element of volume the ratio of the baryon number $\ensuremath{\Delta}B$ to the number of baryons $B$ is small. We must explain why initially there is a slight excess of matter over antimatter and why finally, after annihilation, matter is concentrated into objects of galactic size. By proposing that the initial conditions consist of spatial variations in the baryon number rather than variations in density (i.e., the number of baryons), we can link together the survival and aggregation of matter or antimatter as closely related subjects. Various topics are discussed, such as: the amplification of baryon compositional inhomogeneity $\frac{\ensuremath{\Delta}B}{B}$ in an expanding universe; the prestellar hadron, lepton, and radiation eras; the effect of inhomogeneity on final composition; the approximate constancy of the total number of particles (including photons) in a comoving element of volume; the muon-neutrino and electron-neutrino temperatures; and the required amount of initial baryon inhomogeneity.