By around a microsecond after the Big Bang, the cosmos had cooled

sufficiently so that quarks and gluons could coalesce into protons and

neutrons.  During the next few minutes, deuterium, helium, and a small

amount of lithium nuclei were formed.  One can use nuclear reaction cross

sections obtained in the laboratory to predict the abundances of these

elements which can be compared with astronomical observations.  However,

the most basic cross section, that for the formation of deuterium, has not

yet been well-measured.