[walru]

It does seem like natural thought - that at some point even a black hole reaches it's maximum density.

However, I do seem to recall (some) black holes shoot a stream of particles from either pole (eg. an Astrophysical Jet (http://en.wikipedia.org/wiki/Astrophysical_jet). In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode.

Of course there's another theory that they're dumping that matter elsewhere.. but who knows. I imagine much of what we currently believe will be disproved in the decades and centuries to come.

[Udo]

> It does seem like natural thought - that at some point even a black hole reaches it's maximum density.

There is no compelling reason, neither theoretical nor observational, that black holes should have a maximum mass. In fact, we've observed some extraordinarily heavy black holes and so far no upper bound has become visible other than factors stemming from the amount of time active and the environment in their vicinity that allowed those holes to grow.

> However, I do seem to recall (some) black holes shoot a stream of particles from either pole

That happens whenever a lot of matter accretes around a source of gravity - a hot disk is formed and matter is accelerated out of the poles. It happens with heavy objects other than black holes as well. The ejected matter comes from the accreted stuff, not from the black hole itself.

> In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode

No, only a small part of the accreted matter is ejected.

> Of course there's another theory that they're dumping that matter elsewhere..

There is no plausible theory at this point that suggests anything of that nature. Matter as we know it ceases to exist in a black hole, it becomes compressed in a way that its usual properties are lost - so much so that in fact, how and if information loss happens in a black hole is still a matter of debate and has led to some credible theories on how information and entropy might be preserved at the border to ordinary space.

There is also nothing to suggest that black holes are losing mass at any appreciable rate, other than possibly through Hawking radiation. Combined with the fact that the known cosmos does not exhibit any regions where measurable amounts of matter simply come into existence, the idea that opposite of every black hole is a white hole spewing out the stuff both ignores the fact that black holes get bigger by ingestion and the lack of any evidence for matter spewers.

Of course that does not prove it's not happening, but there is zero reason at the moment to believe this is going on. And that pertains to this article as well, the content being presented here is not supported by any evidence or current theoretical modeling.

[Tyrant505]

So will the universe eventually just have black holes?

[Udo]

Not strictly speaking. Black holes are just ordinary gravity wells. They're strong, and stuff falls into them, but they don't actively suck stuff in anymore than planets or stars do. However, over very long time spans, orbiting systems should lose energy by radiating gravitational waves. That means after an inordinately long period of time, and after ejecting a lot of its mass, each galaxy will likely end up just one big black hole. But there is always stuff traveling through the universe that is not gravitationally bound to anything, and since space itself is expanding, a lot of the matter in the universe will just keep on drifting without ever colliding with anything ever again.

Not that it matters from a practical perspective. Once the last stars have gone cold, that's pretty much as good a definition of "the end" as any other, and that will happen long before maximum entropy is reached.

There might not be a definite end state to the universe itself, but there are certainly some thresholds past which everything will become so boring that we may just as well consider them to be final.

[lloeki]

This wikipedia article[0] is a nice entry point about the possible futures at geological and astronomical scales.

[0]: http://en.wikipedia.org/wiki/Timeline_of_the_far_future

[serf]

Is it no longer a valid concept that 'cold' universes are just another part of a cyclical 'era' in our universe, awaiting the quantum interactions to start the 'Big Bang' anew?

With expansion, and the fact that there are objects in space without a practical gravitational bounds, does that imply a form of finality which may inhibit the QM interactions that took place during the initial 'Big Bang'? I've been of the understanding for some time now that the 'Big Bang' could not have happened without the vacuous void which was to exist preceding it, is my understanding incorrect?

[swombat]

Your understanding is indeed incorrect. According to the big bang theory, there is no vacuous void preceding the big bang. I know that it's hard to wrap your head around that, but there is nothing before the big bang. Not even time. Time itself begins with the big bang. The idea of "before the big bang" has no meaning in the big bang theory as we understand it.

Yes, cosmology is hard.

[Udo]

I've been thinking about why this idea of cyclicality is so powerful and persistent in the absence of any evidence, and it's probably not only inspired by notions of "spirituality" but also because everything on Earth seems to be cyclical during the normal course of human experience. Maybe we're tuned to expect things to always work that way, even though not even the conditions on our home planet are really cyclical when you look at longer time spans.

Still, I find it interesting. Recently a hypothesis was floating around that after the universe reaches maximum entropy this would somehow reproduce the conditions where a quantum fluctuation could produce another universe. It's irrational on several levels to just postulate that without any concrete reason, and all the more astonishing since even if it were plausible, no living thing would be in existence at that point, and not even the building blocks of matter would "survive" such an event.

Even if the universe worked that way, there would be absolutely no reason to feel comforted by it.

I get that people are looking for cosmic harmony or maybe a sense of meaning when they postulate these cyclicalities, but in fact over superhuman time spans almost nothing in the universe is actually recurring. We're all just ephemeral patterns helplessly sliding down the big hill of entropy. Our universe is incompatible with the notion of permanence, even if it's introduced through the back door in the form of eternal cycles.

[chunky1994]

Well that's not necessarily true in the sense that we have no idea what was before the big bang. Time as we know it did not exist before the big bang, however there could've been almost anything before and it doesn't affect our universe. (Except in certain theories where brane collisions could possibly have driven the big bang, in which case the specific type of collision does matter etc.)

[privong]

> In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode.

In the current understanding of jets from black holes, the jets originate from outside the black hole. So the jets are not reducing the mass of the black hole. The amount of material launched in a jet is smaller than the amount of material which falls into the black hole. So black holes with jets would gain mass, rather than losing mass or staying the same mass.

[pingou]

Is it a natural thought? I'm not an expert but I thought it would just bigger and bigger.

For the dumping matter part, they talk in the article of the hawking radiation, I think the astrophysical jet wouldn't reduce the mass of a black hole but only slow its rotation, as the matter haven't reached the event horizon it would not be considered part of it. Again, I'm no expert, so you shouldn't trust me.