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Comment by trhway | Hacker News Reader

>Things become black holes by being dense, not by having large mass.

not really. The larger the mass the less dense it has to be when it fits into its Schwarzschild radius. For example, our visible Universe - 46B ly radius - has Schwarzschild radius of 13.7B ly (that number sounds strangely familiar, isn't it? and may give rise to various speculations :), ie. it would have to be just 64 times denser that it is now, ie. like 64 atoms/m3 instead of the current 1 atom/m3.

According to https://en.wikipedia.org/wiki/Schwarzschild_radius :

"The Schwarzschild radius of an object is proportional to the mass. "

Thus we can see that given the same density mass grows as a cube of the radius of containing sphere, and thus Schwarzschild radius of such a mass would grow as a cube of that sphere radius too. So if a mass grows it is possible for it to lose density pretty fast, like the sphere's radius square fast, and still fit into its Schwarzschild radius.

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Whoa so there's a possible that the visible universe is a black hole?

Yes, absolutely - black holes don’t need singularities inside them in order to be black holes. Just enough mass in a small enough volume.

(Of course, we don’t actually know whether stellar or galactic centre black holes have singularities inside them - we just don’t know of any process that will prevent one from forming in current physics.)

If our universe is a very big black hole, that means light cannot leave it.

So where does it go?

My intuition - which is almost certainly wrong - says it would turn back around, and be visible as a faint background radiation.

> black holes don’t need singularities inside them in order to be black holes.

They do according to the standard GR model. Speculations about holes not having singularities inside them are quantum gravity speculations and we have no way of testing them experimentally any time soon.

True! Hmm. I retract my statement: Penrose/Hawking predict that all GR black holes contain singularities.

In our universe, it looks like inflation is the fly in that particular prediction: The universe is going to tear itself apart instead.

> In our universe, it looks like inflation is the fly in that particular prediction: The universe is going to tear itself apart instead. reply

I assume you're referring to the "Big Rip" scenario? That's not considered very likely, at least according to our best current data.

No, just inflation taking the rest of the universe over the horizon - I believe it’s the dynamic effects of being in an inflationary universe that mean that the standard GR results for black holes don’t apply, at least according to the references I read. (I can’t pretend to be able to derive this stuff personally.)

*possibility (dang typos)

IANAP, but see: http://www.preposterousuniverse.com/blog/2010/04/28/the-univ...

> so there's a possible that the visible universe is a black hole?

No. The universe is expanding. If it were a black hole, it would not be expanding. The calculation of how much mass has to be inside what radius in order to form a black hole assumes that the matter is static (or collapsing); if it's expanding, the calculation is meaningless.

That's actually a super-interesting result.

It would be totally plausible to have supermassive black holes inside of universe-sized black holes. Neat.

I'm not sure because the above result doesn't say anything about the distribution of such mass, and intuitively since the light cone tips over once you're past the event horizon, everything inside is converging towards the singularity following a law that may prevent any black hole inside black hole from existing (but that's way over my head now).

>everything inside is converging towards the singularity

singularities are result of the pure abstract model of [mathematically] continuous spacetime. In real live at small scales the continuity gets broken and as one of the results of it we have QM - if you think about a typical large, star sized, black hole than you can see how degenerate matter in the center would provide the opposite pressure against collapse into singularity similarly like it happens in neutron star. The difference here is that in neutron star the limited existing mass of the star limits the resulting neutron degeneracy level achieved. In case of a black hole the process doesn't stop and as more and more mass comes in the degeneracy goes further/higher with the neutrons being pushed into even higher energy levels and starting to be teared apart into separate quarks and gluons, and as even more mass/energy comes in the quarks get teared apart even further one from another with the new pairs of quarks appearing as a result... That would provide the pressure opposing collapse into singularity. The more incoming gravitational mass/energy pressure the stronger the response. Btw, that boiling quark-gluon soup looks suspiciously like something that is hypothesized as what was in the beginning of our Universe ( what could have triggered the Big Bang explosion/expansion? - may be merge with another super-hyper-gigantic black hole).

> In real live at small scales the continuity gets broken

This is a common speculation in quantum gravity, but at this point that's all it is: a speculation. We have no evidence that spacetime is not continuous. And given the predicted scale at which continuity would break down if the quantum gravity speculations are correct (about twenty orders of magnitude smaller than the smallest scale we can currently probe), we aren't likely to be able to test those speculations any time soon.

> if you think about a typical large, star sized, black hole than you can see how degenerate matter in the center

There is no degenerate matter in the center of a black hole; it's vacuum inside. A black hole is not an ordinary static object with "stuff" inside, that happens to have a radius smaller than the Schwarzschild radius for its mass. It's a fundamentally different kind of thing: it's made of spacetime curvature, and that's all.

> In case of a black hole the process doesn't stop and as more and more mass comes in the degeneracy goes further/higher with the neutrons being pushed into even higher energy levels and starting to be teared apart into separate quarks and gluons, and as even more mass/energy comes in the quarks get teared apart even further one from another with the new pairs of quarks appearing as a result... That would provide the pressure opposing collapse into singularity.

There have been speculative models along these lines, but none of them have any experimental support.

> that boiling quark-gluon soup looks suspiciously like something that is hypothesized as what was in the beginning of our Universe

Same comment--there are speculative models along these lines, but none of them have any experimental support. Figuring out a way to test these models, as well as many other different speculative models of the very early universe, is an open area of research in cosmology.

> It would be totally plausible to have supermassive black holes inside of universe-sized black holes

No, it wouldn't. You can't have one black hole inside another one. A black hole is a region from which light can't escape to infinity. There can't be such a region inside another one; that makes no sense.

You're playing semantics here.

If you define a black hole as a clump of matter entirely contained within its own Schwarzschild radius, then it seems that yes, you can have one inside of another.

Even if you define a black hole as "a region from which light can't escape to infinity", a theoretical black hole inside of another black hole doesn't violate this definition.

> You're playing semantics here.

No, I'm using the term "black hole" as it is actually defined in physics.

> If you define a black hole as a clump of matter entirely contained within its own Schwarzschild radius

Then you are not defining the term "black hole" as it is actually defined in physics.

> Even if you define a black hole as "a region from which light can't escape to infinity", a theoretical black hole inside of another black hole doesn't violate this definition.

Yes, it does, because once you are inside the boundary of one such region, that's it. There can't be a second boundary further inside; if there were, there would have to be a region from which light can escape to infinity (outside the second boundary), inside a region from which light can't escape to infinity (inside the first boundary). That's not possible.

In other words, if one black hole falls into another, what you get is not two black holes, one inside the other. What you get is one black hole. The two holes merge into a single hole.

> > If you define a black hole as a clump of matter entirely contained within its own Schwarzschild radius

> Then you are not defining the term "black hole" as it is actually defined in physics.

Again, you're playing semantics here for literally no point other than to be a pedant on the Internet. JUST AS COOL AND INTERESTING is a clump of matter that fits inside its own Schwarzschild radius, that's inside a universe-sized black hole. In other words, the exact kind of structure I was talking about before you decided to derail the conversation.

What the hell happened to you that you feel the need to remove people's wonder and enjoyment of science by pedantically correcting non-precise word choice?

> Yes, it does, because once you are inside the boundary of one such region, that's it.

Being inside the boundary of such a region does not preclude a subregion from itself having an escape velocity greater than the speed of light. Such an object seems to me that it still has all of the interesting properties of a black hole, while existing inside of a black hole.

Again, do you feel like being a pedant contributed positively to this conversation in any way?

> you're playing semantics here for literally no point other than to be a pedant on the Internet

Physics insists on precise terminology for a reason. It's not just pedantry.

> JUST AS COOL AND INTERESTING is a clump of matter that fits inside its own Schwarzschild radius, that's inside a universe-sized black hole

The only difference is that the clump of matter contains matter, while the universe-sized black hole can be idealized as vacuum (or at least as being so much lower in density than the clump of matter that this density can be ignored when you're analyzing the clump of matter). But that has nothing to do with any properties related to black holes. See below.

> Such an object seems to me that it still has all of the interesting properties of a black hole, while existing inside of a black hole.

And my point is that this is not correct, as a matter of physics, not words. There are no "interesting properties of a black hole" which change in any way when you cross the boundary from the universe-sized vacuum region to the clump of matter region. The only property that changes is the density of stress-energy. As far as any "interesting properties of a black hole" are concerned, there is just one region.

Here's another way to put it: when you think of the clump of matter inside the universe-sized black hole as "fitting inside its own Schwarzschild radius", you are calculating the Schwarzschild radius based on the mass of the clump of matter. But that calculation only has physical meaning if the clump of matter is isolated--i.e., if it is not inside the universe-sized black hole. If the clump of matter is inside the universe-sized black hole, then the only Schwarzschild radius that has physical meaning is the one you get by plugging in the total mass of the universe plus the clump of matter. (And even that is only meaningful if the "universe" in question is not expanding, as I've posted elsewhere in this thread.) So if you are thinking of the clump of matter as being a black hole because it fits inside its own Schwarzschild radius, you are thinking of it wrong, as a matter of physics. That's why the term "black hole" is not appropriate to describe it. It's not just a matter of words.

So if it turns out that the whole universe is at least 64 times larger than the visible portion then we are in fact already in a black hole!

It would give an "intuitive" explanation to why we can only see the inside of the universe.

No explanation needed if the age of the universe is finite.

The universe is expanding in volume, but not mass. Thus some years ago the current visible portion did fit inside a 14Bly radius.

> if it turns out that the whole universe is at least 64 times larger than the visible portion then we are in fact already in a black hole!

No. The universe is expanding. If it were a black hole, it would not be expanding.

Actually I think I just gave the Big Crunch hypothesis. The universe can be expanding so long as it eventually collapses. Even in a normal black hole forming out of a star, some particles will happen to be heading out as the star collapses and then be dragged back in after the black hole forms.

> Actually I think I just gave the Big Crunch hypothesis. The universe can be expanding so long as it eventually collapses.

Even in this case it's not correct to say that the universe is a black hole. The spacetime geometry is very different. A black hole, as I've posted elsewhere in this thread, is a region from which light can't escape to infinity. But in a universe that will end up in a Big Crunch, there is no infinity: space has a finite volume (which increases up to maximum expansion and then decreases back to zero).

Then can any black hole exist in such a universe?

In a closed universe that collapses to a Big Crunch, no, no black holes can exist. Everything will eventually come back together at the crunch.

Wait, what?

So admitedly, a few (bilion) years back, when the universe had a radius smaller than ~14B ly, it was sufficiently dense to be a black hole?

I mean, what ? Our universe did not lose mass in between (if we admit that it is a "closed" system), so at some point in the past, it grew out of being a black hole when it expanded above its Schwartzschild radius?

As long as you have ~uniform density out to the edge of the observable universe you get zero net gravity. Move over 1 light year and that particle also sees ~uniform density out to the edge of it's observable universe and you don't get a vast black hole.

> a few (bilion) years back, when the universe had a radius smaller than ~14B ly, it was sufficiently dense to be a black hole?

> at some point in the past, it grew out of being a black hole when it expanded above its Schwartzschild radius?

No and no. The calculations of the "Schwarzschild radius" corresponding to a given mass assume that the mass is static. They do not apply to an expanding universe.

Yes, the "density" of a typical supermassive black hole at the center of a galaxy is roughly the density of water.

I'm not following. Why is that number familiar? Can you explain this in layman's terms?

It's the current age of the universe.

> that number sounds strangely familiar, isn't it?

I don't know much about universes.

Why should this number sound familiar?

The universe is roughly 13.8 billion years old.

Where do you get the 46B ly radius from?

Never mind, I figured it out. ("Size of the universe" on Wikipedia, if you're curious.)

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