An Etymological Dictionary of Astronomy and Astrophysics

The constraint on the → baryon number density at T ~ 10⁹ K in the early → expanding Universe. Gamow recognized that a key to the element buildup is the reaction n + p ↔ d + γ. Deuterium needs to be produced in sufficient abundance for higher elements to form, but if all → neutrons are immediately locked up into → deuterium, no higher elements can form either. The Gamow condition is expressed by n_b<σv>t ~ 1, where n_b is the baryon number density, σ is the cross section for the reaction at relative → velocity v, and t the expansion time-scale for the → Universe. This means that the time-scale for the above reaction is comparable to the expansion time. From this condition the baryon number density at the start of element buildup is found to be n_b ~ (σvt)^-1 ~ 10¹⁸ cm^-3 at T = 10⁹ K (P. J. E. Peebles, 2013, Discovery of the Hot Big Bang: What happened in 1948, arXiv.1310.2146).

See also: → Gamow barrier; → condition.