What happens when the universe runs out of fuel?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.