Brian Cox and the end of the Universe?

[KDP]

TL;DR Summary

Brian Cox stated the Universe will end with the remaining black holes evaporating to nothing leaving eternal darkness. Is that the consensus opinion? What exactly is emitted by Hawking radiation? Is it real physical particles?

Professor Brian Cox was on the TV last night. He stated that eventually everything will end up in black holes. When there is nothing left to absorb they will start evaporating via Hawking radiation until eventually they all disappear in a small flash of light and then there will be eternal darkness. He then digresses to how there wont be exactly nothing left but that all the information that went into the black holes will be recovered in some form.

What I disagree with is the statement "eternal darkness". I have always assumed when the black holes evaporate they emit real physical particles along with real photons. I presume the universe doesn't simply lose all its mass and energy. Those physical particles that have been emitted might eventually clump together again under the influence of gravity forming gas clouds and eventually new stars giving off light, not to mention all the photons that were emitted by the evaporating black holes in the first place that will be a new cosmic background radiation. That does not sound like eternal darkness to me.

Or is it assumed that that the emitted particles will be so dispersed by the expansion of the universe that there is no possibility of clumping together again and the new background radiation will be red shifted away to insignificance by that same expansion? Or is it assumed that the Hawking radiation is not real physical particles that can gravitate together again and form new stars? Just wondering

 

[PeroK]

You could always do some research, starting here perhaps:

https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe

https://en.wikipedia.org/wiki/Hawking_radiation

 

[Halc]

'Eternal darkness' is relative. The radiation produced today will mostly still exist, but will (relative to the cosmic frame) fade to ever longer wavelengths which is the same as the universe 'going dark'. Sure, the light is still there, but it ain't visible to a nonexistent human. If anything evolves late in the game like that, it will perhaps be sensitive to those super-low frequencies, but it's hard to imaging what sort of thing might be able to intercept such light.

As for Hawking radiation, it emits mostly stuff without proper mass like photons and gravitons, and it does it now, not just when the black hole ceases to absorb new energy.

 

[KDP]

As for Hawking radiation, it emits mostly stuff without proper mass like photons and gravitons, and it does it now, not just when the black hole ceases to absorb new energy.

I get that black holes have Hawking radiation even right now, but they usually accumulate mass and energy faster than they lose it until there is no mass left in the universe to absorb and then the equilibrium changes and they start losing mass.

So its mass in and energy out. The entire mass of the universe converted to energy at an exchange rate of E=Mc^2 seems like one hell of a big atom bomb.

 

[PeroK]

So its mass in and energy out. The entire mass of the universe converted to energy at an exchange rate of E=Mc^2 seems like one hell of a big atom bomb.

Isn't that what are the stars are currently doing? The Sun is a giant thermonuclear reaction.

 

[Ibix]

one hell of a big atom bomb.

But very low density. I think the point about black hole evaporation is that the end point is incredibly far in the future - thousands of times the current age of the universe. So I think the point is that when they do finally evaporate to nothing (if that's actually what they do) everything is so spread out that new star formation events are rare.

 

[PeterDonis]

thousands of times the current age of the universe

More like $10^{60}$ or more times the age of the universe. Way, way, way, way beyond any epoch where star formation happens.

 

[Ibix]

More like $10^{60}$ or more times the age of the universe. Way, way, way, way beyond any epoch where star formation happens.

Ok. So the answer to "what happens to the stuff that comes out of the black holes when they explode" is just that the universe is so low density that it can just keep spreading out and never clumps together to form new stars (not in significant numbers, anyway)?

 

[PeterDonis]

So the answer to "what happens to the stuff that comes out of the black holes when they explode" is just that the universe is so low density that it can just keep spreading out and never clumps together to form new stars (not in significant numbers, anyway)?

In our best current model, yes, that's what would happen.

Note that radiation, which is what is going to come out of evaporating black holes, doesn't clump anyway. Clumping requires emission of radiation in order for a system of matter to become gravitationally bound. Radiation can't emit radiation. So even the radiation that is in the universe now, such as the CMBR, will never clump. So it actually doesn't matter when black holes evaporate; what the evaporate into won't form new stars.

 

[KobiashiBooBoo]

Brian Cox is pop-sci - elsewhere in this forum we are instructed to be wary. Just a thought.

 

[phinds]

everything is so spread out that new star formation events are rare.

I think that's overly optimistic. In the long run it is "never" rather than "rare".

By the time any of the last remaining black holes evaporate, they will each be the only object in its observable universe.

EDIT: and, I should add, even if all of the mass of the evaporating BH were to be emitted in the form of massive particles (which of course will not be the case) the speed at which they are ejected will exceed the escape velocity of the relatively small mass that is there at the final evaporation, so there is no possibility that they would fall back together.

 

[Ken G]

I would also add, extrapolations of our current models of universal dynamics and black hole physics to futures like that must be taken with a giant grain of salt. There are so many things we could have wrong that seem pretty insignificant now but could become completely dominant in a future like that. I wouldn't even spend a moment imagining such speculation is the actual future, the chances of being correct are infinitesmal in my mind. Instead, one could view Cox's remarks as a kind of lens through which to look at our current theories, rather than at our universe.

 

[ShadowKraz]

Brian Cox is pop-sci - elsewhere in this forum we are instructed to be wary. Just a thought.

That's because he is trying to explain physics to non-physicists who for the most part don't want to know the underlying physics, just the 'cool stuff'. I run up against that in History; people think it's funny that Czarina Catherine the Great died on a German throne ala Elvis Presley, but don't care about why she had a German throne with a hole cut in the seat.

 

[Grinkle]

This pop-sci video is very engaging, well worth the time to watch it. It brings home what @PeterDonis points out in post 7 in a very visceral way, at least it does for me.

There is a lot more narrative than physics in it, but I'd be very interested in hearing if what physics is presented is consistent with mainstream consensus regarding the future evolution of the universe.

I'm not saying this is the right way to learn cosmology, just making a weaker claim that if you watch this expecting to be entertained in way that makes you want to learn more about what you just saw, you won't end up feeling like you wasted your time.