Introduction

The interstellar medium has a structure and composition which have been modified by Galactic and stellar evolution over the lifetime of the Universe. While the gas remains rich in primordial H and He, it also contains small but significant amounts of C, N, O and other heavy elements, notably of the Fe-group, which have been produced by nucleosynthesis and then returned, in more or less violent events, to the interstellar medium. In addition to the gas, there is dust in this medium, which contributes only about 1% to the mass but which has important effects on the chemistry and the thermal balance (cf. Chapter 1). The primordial gas, on the other hand, contained no dust and was composed only of those elements that were produced in the primeval fireball — essentially hydrogen and helium, with trace amounts of deuterium, lithium and beryllium. Under these conditions, it is perhaps surprising that molecules existed and even more surprising that they should have played an important role in the evolution of the Universe. Nonetheless, this is believed to have been the case and, in this chapter, we explain why.

The governing equations

The cosmic background radiation field that we observe today has a black-body temperature of 2.73 K; it is a remnant of the ‘big bang’, which is believed to have occurred at the origin of the Universe.