What cosmic abundance percentages did Big Bang Nucleosynthesis establish for Hydrogen and Helium by mass?
Answer
Roughly 75% hydrogen and 25% helium by mass
Big Bang Nucleosynthesis (BBN) occurred in the universe's initial moments and was highly efficient at creating the lightest nuclei. This early, foundational process resulted in the established cosmic baseline abundance where the universe was composed of approximately 75% hydrogen and 25% helium when measured by mass. Trace amounts of lithium and beryllium were also produced during this time, but the overwhelming majority of the observable matter inventory dating back to the universe's infancy is comprised of these two lightest elements.
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