When I try to find figures for the amount of helium today and just after the Big Bang, I get similar figures. I was expecting that all of the stars since then would have created more helium. Is the amount below our ability to estimate? Is helium being consumed at a similar or greater rate to produce heavier elements?

Alternatively, can we measure/ estimate a drop in the hydrogen? I presume that little hydrogen is being created. I am unaware of any common nuclear process which creates lone protons. (Please correct me if appropriate.)

Clarification: The Wikipedia Big Bang article mentions helium being created after a few minutes in the Big Bang nucleosynthesis. So, it is helium created after this point that I am asking about. I am presuming that negligible helium was created between this point and the first stars much later. (Again, please correct me if I am wrong.)

  • $\begingroup$ "Just after" is a bit tricky, since 0 to 1 nanosecond covers a lot of things happening! $\endgroup$ – Carl Witthoft Mar 25 '19 at 17:29
  • $\begingroup$ Yes, I was wondering how to phrase that. Was there a point at which helium production dropped to zero or close to (prior to the stars forming and creating more)? $\endgroup$ – badjohn Mar 25 '19 at 17:32
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    $\begingroup$ @CarlWitthoft Thanks for spotting and correcting my embarrassing loan / lone mistake. $\endgroup$ – badjohn Mar 25 '19 at 17:34
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    $\begingroup$ well, it was funny the other way. $\endgroup$ – Carl Witthoft Mar 26 '19 at 11:42

We can estimate the answer by considering the "mass to light ratio" for baryons, and assuming that ratio hasn't changed too much over the time that there has been galaxies, say for ten billion years. The Wiki (https://en.wikipedia.org/wiki/Mass-to-light_ratio) on the mass to light ratio says this ratio for the universe as a whole is about 100 times what it is for the Sun right now, but that includes dark matter. If we restrict to baryon mass-to-light, that would be more like 16 times the Sun right now. The Sun's main-sequence lifetime is about ten billion years, conveniently, and in that time it will convert perhaps 20% of its hydrogen, so a mass-to-light ratio 16 times larger would convert only about 1% of its mass from hydrogen to helium. So that says roughly 1% of all the hydrogen in the universe has already been converted to helium.

Since it started out 75% hydrogen and 25% helium by mass, now it should be more like 74% hydrogen and 26% helium by mass. So although the estimate is crude, I think it's fair to say we are just now beginning to see a trace of an effect, which would be hard to see after only 1 billion years, and will be quite obvious after 100 billion years.

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