Question

What would happen to the abundance of ⁴He if we lived in a Universe in which there was a large energy-density in relic gravitational radiation (i.e. gravitational waves produced at very early times)?

Answer 1

The earlier universe was very hot and dense. The whole universe was filled with radiation and elementary particles, and a hot plasma in which energy was distributed evenly. During the subsequent expansion, this plasma released the heat and has progressively cooled down. During cooling, Big Bang Nucleosynthesis strongly favours the formation of very light elements including like hydrogen and helium . The standard hydrogen has only one proton and helium-4 has two neutrons and two protons.

If the amount of He-4 is abudance, then due to nuceosynthesis, our universe is again expanding.