Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
3 minutes, I was thinking 380,000 years old at Recombination period (Cosmic Microwave Background period). – Ed Yablecki Jan 19 at 0:04
@EdYablecki Except the hydrogen in your body hardly remained neutral (or monatomic) between the early universe and today, so if you think post-recombination hydrogen is different from that of pre-recombination, you must also think the hydrogen in you has nothing to do with that initial batch of neutral atoms. – Chris White Jan 19 at 0:27
@ChrisWhite, my assumption its not a Hydrogen until the state when an energy of an electron was low enough to bind to a proton to make Hydrogen. – Ed Yablecki Jan 19 at 0:33
Wikipedia has charts of the atomic composition of the human body, both by weight and by number. By number, hydrogen looks is around $62\%$. By weight it is about $10\%$. – Ross Millikan Jan 19 at 3:53
@EdYablecki: But if the questioner means atomic hydrogen, then any material that has previously been part of a star isn't "the same hydrogen" as before the star formed, since it has been a plasma in between (as it was a plasma before recombination). If by "hydrogen" the questioner in effect means, "protons", then we don't have to wait for recombination :-) Unless there's a kind of hysteresis, where primordial plasma "isn't hydrogen", then recombination "turns it into hydrogen", and then it remains hydrogen once re-plasma-ed in a star. Which is a bit odd, but OK... – Steve Jessop Jan 19 at 9:57
up vote 14 down vote accepted

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.

share|cite|improve this answer
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... – Jon Custer Jan 18 at 23:42

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.