A revisited study of the cosmic matter recombination in expanding universes

Abstract

It is generally believed by cosmologists that the universe back in its past was hot, and matter was completely ionized and in thermal equilibrium, while in the present era matter, due to a strong cosmic temperature decrease, has recombined to neutral atoms. As we argue here, contrary to general assumption, a non-equilibrium state predominates at this phase of the recombination and the usually used Saha-Eggert theorem hence is inapplicable. In a preceding paper, we had already derived a specific kinetic transport equation which describes the distribution function of cosmic baryon gas (i.e. hydrogen atoms) just after cosmic matter recombination. We could solve the relevant kinetic transport equation for that period and found the gas distribution function f(v,t) as function of the particle velocity v and of the cosmic time . However here, in this paper, we shall go one important step further back in the cosmic evolution and do study in more detail, how in successive steps the recombination of cosmic electrons and protons did actually occur. We clearly show that matter and radiation in the phase of cosmic recombination is not anymore in a thermodynamic equilibrium state, since matter and radiation do cool off in different forms, and a thermodynamic situation predominates where protons, electrons and photons have different temperatures, and collision-based energy transfer processes operate between them. Hence cosmic recombination, thought to have occured about 400000 years after the Big-Bang, does not take place starting from a thermodynamic equilibrium state as generally presumed, and hence the standard Saha-Eggert assumptions and predictions on the ionization degree of cosmic matter as function of the system temperature can not be used. We follow in detail the processional track how the cosmic radiation and the cosmic matter behave in this critical non-equilibrium phase and show that the recombination of cosmic matter actually occurs, though there is a yet unrespected tendency that freely moving particles in an expanding universe become heated by the action of the differential Hubble drifts which, in a first glance, should impede their recombination.