In stars,helium can sometimes be fused into carbon and heavier elements (in their final stages of life).Why didn't the same fusion processes produce carbon and heavier elements in the early universe?
A)By the time stable helium nuclei had formed,the temperature and density had already dropped too low for helium fusion to occur.
B)Helium fusion occurred,but the carbon nuclei that were made were later destroyed by the intense radiation in the early universe.
C)Temperatures in the early universe were never above the roughly 100 million Kelvin required for helium fusion.
D)No one knows-this is one of the major mysteries in astronomy.

Question

Quizplus · Accepted Answer

In the early universe, the temperature and density were much higher than in stars, but as the universe expanded and cooled, the density and temperature dropped too low for helium fusion to occur. By the time stable helium nuclei had formed, the conditions necessary for further fusion had already passed.

B and C are not correct because there was not enough radiation yet in the early universe to destroy the carbon nuclei that were formed, and temperatures were high enough for helium fusion to occur, at least initially. D is not correct because we do have some understanding of why this didn't happen in the early universe.

In Stars,helium Can Sometimes Be Fused into Carbon and Heavier