Thermochemistry of the Dark Age

Abstract

In the conventional view the Universe began in a hot Big Bang some 15 billions years ago, and has been expanding ever since. The dark age of the Universe is pointed out as the period between the hydrogen recombination epoch and the horizon of current astrophysical observations. Atvery early stage in the expansion, when the temperature of the Universe was still 10⁹-10¹⁰ K, collisions between subatomic particles created hydrogen and helium nuclei with very minor traces of deuterium and lithium nuclei. The chemistry of the early Universe is the chemistry of these light elements. This chemistry (Standard Big Bang Chemistry or SBBC) has been source of large studies. One of the most important consequences, of the existence of a significant abundance of molecules, is the crucial role played in the dynamical evolution of the first collapsing structures. The arrow of time in the cosmic history describes the progression from simplicity to complexity, because the present Universe is clumpy and complicated unlike the homogeneous early Universe. Thus it is crucial to know the nature of the constituents, in order to understand the conditions of the formation of the first bound objects. In this paper we analyse the chemical history of this Dark Age and the consequences on the birth of the first astrophysical objects. Thus in section 2 we describe the chemical evolution of the Universe, then in section 3 we analyze the implications on the formation of first stars. In section 4 some possible outlooks are pointed out.