Question

What elements were created in the big bang? During what time period in the early universe did nucleosynthesis occur? Why just during this period? Describe qualitatively why the hot big bang model predicts that the universe should contain ∼26% by mass helium. Why is the present deuterium abundance sensitive to the value of Ω?

Answer 1

Some light elements like hydrogen, deuterium, helium, lithium and some amounts of beryllium were formed in the big bang theory. Basically, these were produced due to the fusion reactions that took place due to very high temperatures.

Nucleosynthesis occured 13.8 billion years ago just after the big bang theory. It happened at that period only as at that time only the universe was getting expanded which was then cooled down and resulted into the formation of subatomic particles which later turned out to be atoms.  

The hot big bang theory model predicts that the universe should contain around 26% by mass helium as it is the lightest gas produced by the big bang explosion and that too in very large quantity.

Since, the temperature is weakly dependent on the omega and at the time of the big bang, the temperature was very high so deuterium could not show much sensitivity but now the temperature is very low as compare to the big bang temperature hence deuterium is more sensitive to omega.