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I was wondering, if every single hydrogen in the universe originate from the time about ~3 min after the Big Bang.

I know there are nuclear fusion processes going on in stars like the pp-chain reaction which 'emit' hydrogen, too, but those reactions needed several hydrogen atoms to begin with.

So are there really processes which create hydrogen 'from scratch'?

Now, assuming that the human body consists of ~60% water and a taking a body weight of ~75kg => 45kg water => 5kg hydrogen (taking m(H) = 1u & m(O) = 16u => m(H2) / m(H2O) = 1/9) [I'm no biochemist so I don't really know if there are significant amounts of hydrogen in other compounds within the human body], does this mean that roughly 95% (70kg/75kg) of me is - like Carl Sagan said - made of stardust, but even more mesmerizing, ~5% of the stuff that makes up my body made shortly after the Big Bang, nearly 14 billion years ago?

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    $\begingroup$ Today's APOD addresses this question. $\endgroup$
    – rob ♦
    Jan 26, 2016 at 3:02
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    $\begingroup$ I've deleted a number of obsolete comments and/or responses to them. $\endgroup$
    – David Z
    Apr 19, 2020 at 6:56

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Free neutrons can be generated by spallation reactions between cosmic rays and heavy metal targets. Such processes do occur in the cosmos.

The free neutrons will then beta decay to yield protons (hydrogen nuclei) and electrons. This is a weak reaction, so the protons produced here were not present shortly after the big bang.

So not all hydrogen atoms originate from protons made in the big bang, though other sources are negligible, so your calculation looks OK to me.

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    $\begingroup$ And there are any number of (n,p) reactions (like 14N (n,p) 14C) that occur in neutron environments. But, they aren't even a rounding error in the universe's inventory of hydrogen. And, here on Earth they are mainly driven by those cosmic-ray induced neutrons, so perhaps its even a rounding error on a rounding error... $\endgroup$
    – Jon Custer
    Jan 18, 2016 at 23:42

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