Chemical Abundances in the Universe

I stated (early and often) that the Universe is primarily hydrogen and helium. What is this statement based upon? It, fortunately, is based on data. The Universe is thought to be roughly 90 % hydrogen and 10 % helium based on observation.

• Sun, Young stars, planetary nebulae, extra-galactic H II regions have been used to determine the helium abundance in the Univese. However, because stars also produce helium (simply not enough to account for the 10 % by number), this exercise is a little difficult because we must show how much helium was produced by the Big Bang and not by stars. A clever way to do this is to observe objects which are very old (and therefore relatively uncontaminted by stars). Some workers have shown that extra-galactic H II regions may be useful. An interesting result is that the helium abundance is correlated with the abundances of elements such as carbon, nitrogen, and oxygen (CNO elements). The correlation is in the sense that the abundance of helium increases for regions which have high abundances of these CNO elements. This is very suggestive because the CNO elements are produced by stars. Using this data one can infer the primordial abundance of helium guessing the helium abundance in the cloud by imagining that the cloud did not contain any CNO. Performing this exercise, we find that

  • Helium ~ 12 % with an uncertainty of ~ 2.6 %

consistent with the plausible range of abundnces predicted by the the Big Bang,

12 % < Helium < 15 %.

===> the Big Bang is consistent with the helium abundance.

This was an early prediction of the Big Bang Theory (circa 1940's) and reamins strong evidence that the Big Bang Theory is a reasonable model for the early Universe.