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After some X billion years, one would think the stars in the entire universe will run out of hydrogen. What would happen next? Is there any way to get hydrogen out of heavy metals (extreme fission)? Just curious.

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Then star formation ceases and the universe goes dark. At this stage of the universe's evolution, there'll still be plenty of hydrogen, they just don't form stars.

In theory you can create hydrogen out of heavy metals, but it's a process that requires energy. If you have the energy banked somewhere (and you'll need a LOT of energy to make enough hydrogen for a new star) then it's possible.

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  • $\begingroup$ Would you say then that the notion of a cycle cannot exist? If that is the case, what was the state at big bang? Was there one single large mass with lots of hydrogen? and what was the state before big bang ( if you will allow me). It appears the star formation business more or less can happen only once and it stays dark for ever. This is crazy! Thanks $\endgroup$
    – sku
    May 12 at 6:34
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    $\begingroup$ Cyclic universes are possible in theory but it's very much outside the regime of well-understood physics. See en.wikipedia.org/wiki/Cyclic_model. We don't know why the Big Bang produced lots of hydrogen because we don't have laws of physics that hold at the Big Bang. Star formation happens much more than once, every star you see went through star formation. But chronologically there is a time when it can't happen anymore, yes. It's not that uncommon, e.g. if you buy a new car today you'll be able to drive it, but 100 years in the future it'll cease to run. $\endgroup$
    – Allure
    May 12 at 6:52
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    $\begingroup$ @sku Yup, everything you ever do will become irrelevant, completely forgotten, in some trillions of years or more. Try not to think about it. $\endgroup$
    – user253751
    May 12 at 9:48
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    $\begingroup$ I don't think it's fair to say we don't know why the Big Bang produced lots of hydrogen. Hydrogen production (Big Bang Nucleosynthesis) happened late enough that we have a fairly good idea of what the universe and its physics were doing at that time, and we're able to predict light element abundances based on that idea impressively well. A bit before nucleosynthesis, though, we start to lose confidence. $\endgroup$
    – jawheele
    May 12 at 14:49
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    $\begingroup$ @Allure: Hydrogen was created by protons and electrons recombining. That happened after about 370.000 years; free neutrons have a half-life of 15 minutes before they decay into protons and electrons. What else did you expect to happen with all the hadrons that were created earlier? $\endgroup$
    – MSalters
    May 13 at 13:16
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Star formation will die out long before all the hydrogen runs out. Much of it will be trapped in very low mass stars and lots more will be in the very sparse intergalactic medium (where $\sim 50$% of it is now).

In terms of possible production mechanisms, almost all the hydrogen that exists now comes from the big bang. However, neutrons can be generated by spallation reactions between cosmic rays and heavy metal targets. The free neutrons then beta decay to yield hydrogen nuclei and electrons.

This isn't a very efficient production mechanism and is also likely to cease if as thought, supernova remnants are the sites of cosmic ray acceleration.

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  • $\begingroup$ If some cosmic rays are coming from a not yet discovered source, whose intensity does not decrease with time, then we might have some chance to avoid the bitter end? $\endgroup$
    – peterh
    May 12 at 20:45
  • $\begingroup$ @peterh as I said, there will be no lack of hydrogen. $\endgroup$
    – ProfRob
    May 12 at 21:19
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Well, fate of the universe has a lot of possible scenarios, from which periodical expansion/contraction is very unlikely, because universe expands in an accelerated fashion, and it doesn't looks that it will change, unless dark energy will run-out too, which is highly unlikely due to quantum mechanical laws. So most probable scenarios of fate would be Big Rip and Big Freeze. As for Big Freeze,- it's about thermodynamic equilibrium, when all supermassive black holes will finally evaporate due to Hawking radiation in about $10^{100}\text{years}$ and universe will become a cold soup of sparse gas of photons and leptons. The good thing is that there's a probability that some place in this "particle fog" can quantum tunnel into a new inflating universe, after about $\Large {10^{10^{10^{56}}}}$$\text{years}$. Btw, keep in mind that this is not the same as repeating inflation model, it's more like a multiverse theory,- spawning child branches of new universes in some "root tree" of process.

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