Deuterium Bottleneck

One particularly sensitive test involves the abundance of deuterium. Because deuterium has a nucleus that is very weakly bound compared with most nuclei, it is very sensitive to the conditions in which it is formed (as we have just seen): if the temperatures are too high, deuterium breaks apart, and it can only be formed when there are free neutrons to combine with protons. Detailed analysis of the deuterium abundance gives very strong support to the hot big bang picture.