Relative Amount of Hydrogen in the Universe

[Jim Lundquist]

Did hydrogen reach some maximum level in the early universe before fusion began creation of helium and thus decreasing the amount of hydrogen, or do fission reactions maintain some kind of balance? If the universe is constantly expanding, the density of remaining hydrogen would seem to decrease and no longer fuel fusion into helium. If the amount of hydrogen does decrease, what of all other elements?

 

[mathman]

Outside of stars hydrogen to helium stopped very soon after the big bang. H1 made up about 75% of baryonic matter and He4 about 25%.The big bang also produced trace amounts of H2, He3, and Li isotopes. Everything else comes from stars, including supernovae. I don't know the proportions these days, but I believe H1 and He4 still dominate.

 

[Buzz Bloom]

Hi mathman and Jim:

I don't know the proportions these days, but I believe H1 and He4 still dominate.

The following references have some information about the current relative abundance of elements with respect three contexts: (1) the Earth, (2) the Solar System, (3) the Milky Way. I could not find any reference about the universe as a whole.

1, 2, and 3. https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
2. https://en.wikipedia.org/wiki/Nucleosynthesis
​

If the universe is constantly expanding, the density of remaining hydrogen would seem to decrease and no longer fuel fusion into helium. If the amount of hydrogen does decrease, what of all other elements?

Eventually, some trillions of years, there will be no more free H₁ in stars ,so no more fusion to He.. There will also be no stars that can make elements from H₁ but they may for a while be able to make elements from other sources, e.g. H₂. There may be clouds of free H₁ gas which will be unable to form stars, so they will not be involved in any fusion .Many isotopes of some elements are unstable, so for a while they will decay to lower mass elements. Eventually, this process will also stop. There are more theoretical details that are likely to be correct about the state of the very far future, but I think they are not in the context of the OP.

Regards,
Buzz

 

[mfb]

There is no process that would produce ¹H in relevant amounts. Its total amount goes down over time.
The amount of ⁴He should go up, and the amount of all stable (or long-living) heavier elements goes up as well.

 

[Jim Lundquist]

There is no process that would produce ¹H in relevant amounts. Its total amount goes down over time.
The amount of ⁴He should go up, and the amount of all stable (or long-living) heavier elements goes up as well.

Thank you, so please let me follow-up. Theoretically, if an atom of ⁵⁶Fe were to fall into a black hole, would the atom be stripped of its fundamental particles, become super compressed, or something totally different as it approaches the singularity? If the mass is considered to be infinite in the singularity, is this mass undifferentiated in that the "identities" of the components of this mass is lost forever?

 

[mfb]

The atom gets destroyed. Shortly before its the singularity (or whatever might be there) it is ripped apart by tidal gravity.

If the mass is considered to be infinite in the singularity

It isn't. A black hole has a finite mass.

in that the "identities" of the components of this mass is lost forever?

Yes.

 

[Jim Lundquist]

The atom gets destroyed. Shortly before its the singularity (or whatever might be there) it is ripped apart by tidal gravity.It isn't. A black hole has a finite mass.Yes.

Thank you again. I really meant the singularity of the black hole having infinite density, not mass. So, in the distant future of the universe, is it theoretically possible for the entire universe to be reduced to many black holes and then those black holes combining into fewer and fewer black holes, so that given "infinite" time a state replicating the original "Big Bang" exists that would conserve all existing mass while becoming infinitely dense?

 

[PeterDonis]

in the distant future of the universe, is it theoretically possible for the entire universe to be reduced to many black holes and then those black holes combining into fewer and fewer black holes, so that given "infinite" time a state replicating the original "Big Bang" exists that would conserve all existing mass while becoming infinitely dense?

Not if our current best fit model of the universe is correct. What you are describing is basically a "Big Crunch" scenario, and that would require the universe to recollapse; but according to our current best fit model of the universe, it won't.

 

[kimbyd]

Did hydrogen reach some maximum level in the early universe before fusion began creation of helium and thus decreasing the amount of hydrogen, or do fission reactions maintain some kind of balance? If the universe is constantly expanding, the density of remaining hydrogen would seem to decrease and no longer fuel fusion into helium. If the amount of hydrogen does decrease, what of all other elements?

I don't know how useful this question is. What is your purpose in asking it?

Super short version: early universe had a quark-gluon plasma. This condensed into a fluid of protons and neutrons which were rapidly decreasing in temperature. There was enough time for just a teeny tiny amount of nuclear fusion, where some of the protons combined with neutrons and other protons. But most of the protons stuck around (any neutrons that didn't bind with protons decayed into protons). The remaining lone protons became hydrogen much later when they combined with electrons.

Over time, that fixed population of protons and neutrons per comoving volume has dropped as it has formed heavier and heavier elements. Eventually the quantities of those elements will drop as they fall into black holes or the protons within them evaporate.

The details of the first part are described above in the links Buzz Bloom posted. The details of the first bit (and many, many more) are described here:
https://en.wikipedia.org/wiki/Future_of_an_expanding_universe