How does the predicted abundance of H and He in the universe support the big bang theory?

Answer 1

In the early seconds after the end of Inflation, protons and neutrons were very close to each other. Because we know the strength of the strong nuclear force, we know the probability for these nucleons at this density to form deuterium, tritium, and helium. Since the latter is far more stable than the first two, it is more likely that any such formation would remain as such. We also know the amount of time between the formation of hadrons and the density falling to the point where no more formation of helium could occur. From these two numbers, we can calculate the percentage of hydrogen that should have been converted into helium during that early time, and the result is that about 10% of the hydrogen should have done so. This corresponds well with the observed ratio of hydrogen to helium in all parts of our Universe. When a hypothesis makes predictions that are confirmed by experiment, that hypothesis is elevated to a theory. This happened with planetary gravity, and also happened with the Big Bang -- both are on the same level of scientific support.

Answer 2

The predicted abundance of hydrogen and helium in the universe matches the observations, providing evidence that supports the Big Bang theory. The idea is that these light elements were formed during the initial stages of the Big Bang when the universe was hot and dense, allowing for nuclear fusion to occur to create these elements in large quantities. The consistent presence of these elements across the universe corroborates the predictions of the Big Bang theory.