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.

Related https://physics.stackexchange.com/questions/2361/how-long-will-the-universes-hydrogen-reserves-last-for — ProfRob, May 12 '22 at 07:49
https://en.wikipedia.org/wiki/Timeline_of_the_far_future is a fun read. — chepner, May 13 '22 at 14:21
Another fun one is "Manifold Time." I like the sense of scale they manage in that one — Cort Ammon, May 13 '22 at 23:38

score 12 · Accepted Answer · answered May 12 '22 at 06:11

12

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.

answered May 12 '22 at 06:11

Allure

20,501

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 – sku May 12 '22 at 06:34
11

Cyclic universes are possible in theory but it's very much outside the regime of well-understood physics. See https://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. – Allure May 12 '22 at 06:52
8

@sku Yup, everything you ever do will become irrelevant, completely forgotten, in some trillions of years or more. Try not to think about it. – user253751 May 12 '22 at 09:48
5

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. – jawheele May 12 '22 at 14:49
@jawheele you sure? I am under the impression we don't know why the Big Bang produced lots of hydrogen (= protons). BBN explains why we observe helium and lithium in the abundances we do, but it doesn't explain why the universe started out with all hydrogen. – Allure May 12 '22 at 15:57
1

@Allure I guess it depends on what one means, but that's a fair point to make. BBN tells us why there's so much more hydrogen relative to other elements, but you're definitely correct that it's unknown what baryogenesis physics set the overall scale through the baryon-to-photon ratio. – jawheele May 12 '22 at 16:11
So the last civilization would start directing relativistic hydrogen jets towards their last bastion, not least because their source will go out of scope in a few billion years in an expanding universe... – Peter - Reinstate Monica May 12 '22 at 20:17
1

@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? – MSalters May 13 '22 at 13:16

score 8 · Answer 2 · answered May 12 '22 at 07:55

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.

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? — peterh, May 12 '22 at 20:45

score 5 · Answer 3 · edited May 13 '22 at 08:43

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.

What happens when the universe runs out of fuel?

3 Answers3