## Is the entire universe a black hole?

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Derek
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### Is the entire universe a black hole?

So something I've known for awhile is that the density of blackholes decreases with their mass. Recently I got to thinking what this means on the scale of the universe. So I checked some numbers with Wolfram Alpha:

The Schwarzchild radius of the mass of the observable universe = 4.975 * 1027m

Radius of the observable universe = 4.4 * 1026m

On a naive level, this would imply that the entire universe is, in fact, a black hole. I also find it interesting, though it's probably just a coincidence, that these values are almost within an order of magnitude. But I know that there are a lot of potential problems with this analysis on the scale of the universe, which I don't have nearly the knowledge in physics to answer. So I ask those of you who have a better physics background than me: Is the entire universe a black hole? If not, where does this analysis breakdown? If so, are there any interesting implications? Or finally, is this one of those questions that physics is not yet prepared to answer?

Neil_Boekend
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### Re: Is the entire universe a black hole?

No. The universe is not a black hole. You can be certain of it because it is expanding. Matter is leaving our observable universe (the speed (with earth as a reference point)of far away galaxies exceeds the speed of light).
If anything of the sort, the universe is a white hole.

AFAIK the reason is that the space inside is expanding faster than the speed of light (on the scale of the observable universe). This expansion makes it impossible to collapse. You could say that a black hole messes with the fabric of space-time but the universe messes with it's own space-time even more.
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Frenetic Pony
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### Re: Is the entire universe a black hole?

Neil_Boekend wrote:No. The universe is not a black hole. You can be certain of it because it is expanding. Matter is leaving our observable universe (the speed (with earth as a reference point)of far away galaxies exceeds the speed of light).
If anything of the sort, the universe is a white hole.

AFAIK the reason is that the space inside is expanding faster than the speed of light (on the scale of the observable universe). This expansion makes it impossible to collapse. You could say that a black hole messes with the fabric of space-time but the universe messes with it's own space-time even more.

Technically, neither a black hole nor a classical white hole start out with all the matter in it at once then slowly entropying. They would add mass as the black/connected white hole feeds, then lose mass as hawking radiation bleeds out. Neither of which is an effect we see. (How does a white classical white hole even account for hawking radiation? Does energy just vanish from the universe for no reason?)

Though a non traditional white hole, one spawning from a critical mass quantum black hole, could work. The idea goes something like: quantum effects exist and somehow work with relativity in some sort of ultra grand unified theory. We know this to be true and we don't know how so handwaviness on the specifics is considered a sin worth overlooking for the sake of the idea. Anyway inside a black hole is not something going towards infinity forever and losing all information, but something we don't quite understand that uses quantum mechanics. And when this unknown things reaches some critical point of non equilibrium the gravity of the black hole is overcome, blasting out minimal entropy energy from what appears to be a nigh singularity, AKA something that looks remarkably like the big bang (or is the big bang, if this hypothesis turns out to have merit).

jewish_scientist
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### Re: Is the entire universe a black hole?

What if the observable universe were inside a gigantic black hole? I mean HUGE; like a google^gram's number times bigger than the observable universe. The formula for a mass's Schwarzschild radius is r=2GM/c^2. That means that the radius of a black hole is proportional to its mass. However, the volume of a sphere is proportional to its radius^3. Density is mass/volume; all together this means that the density of a black hole is proportional to its mass^2 (citation). In other words, the more massive a black hole is, the less dense it is. If we were in a huge black hole, the density would be very little, which is why there is so much 'empty' space in outer space.
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sevenperforce
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### Re: Is the entire universe a black hole?

No.

Actually, wait.

If you have fallen into a black hole and have already crossed the event horizon, you will fall faster than everything "above" you, and slower than everything "below" you. So it will seem to you...as if everything is moving away from you.

Huh. That actually makes a little sense.

Wait, no, that's only if there's an actual singularity at the center that you're being pulled toward.

Tub
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### Re: Is the entire universe a black hole?

Derek wrote:On a naive level, this would imply that the entire universe is, in fact, a black hole.

This depends on your definition of a black hole. It does not mean that there has to be a singularity or something in here.

Being part of a collection of mass that's inside its own Schwarzschild radius only means one thing: somewhere out there is an event horizon which we cannot pass through from this side. In other words, we cannot leave the observable universe to take a peek at the unobservable parts. That's it. There's no other consequence.

jewish_scientist wrote:What if the observable universe were inside a gigantic black hole?

Same as above. If that the universe is big enough and roughly uniform even beyond the observable part, we probably are. But that doesn't affect us at all. If we cannot pass the event horizon that Derek calculated, we certainly don't need to worry about your event horizon even further away.

sevenperforce wrote:Wait, no, that's only if there's an actual singularity at the center that you're being pulled toward.

Bingo. Since there isn't, nothing special happens to us. The observable universe's mass isn't concentrated in a singularity, but spread evenly across the observable universe. None of the scary things you read about the inside of a black hole apply to us, except for the existence of an event horizon.

Derek wrote:I also find it interesting, though it's probably just a coincidence, that these values are almost within an order of magnitude.

No coincidence. Those values match because our universe is flat.

p1t1o
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### Re: Is the entire universe a black hole?

sevenperforce wrote:If you have fallen into a black hole and have already crossed the event horizon, you will fall faster than everything "above" you, and slower than everything "below" you. So it will seem to you...as if everything is moving away from you.

Wouldn't this mean though, that objects at the same "height" as you will move towards you?

jewish_scientist wrote:google^gram's googol^Graham's number times bigger than the observable universe.

Holymoly that WOULD be big.

KarenRei
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### Re: Is the entire universe a black hole?

Some interesting reading on the subject:

http://cds.cern.ch/record/348993/files/9803052.pdf

They posit an inflation-driven effect at high densities (a dilation gravity) that would mean that there's no such thing as a singularity, and that both black holes and our universe would be versions of the same phenomenon. In their model, dilation/inflation causes infalling particles onto a tangential path with the event horizon without the formation of a singularity, converging to a topologically flat space. The evolution of such a state begins superinflationary (shifting Hubble parameter) before settling down as the expansion proceeds. This could be seen as corresponding to both the evaporation of a black hole and the Big Bang.

If my reading of this is correct.

It's worth mentioning, as Tub wrote:

No coincidence. Those values match because our universe is flat.

Indeed, our universe is flat. But there's certainly reason to ask ourselves why.

Neil_Boekend
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### Re: Is the entire universe a black hole?

Tub wrote:Being part of a collection of mass that's inside its own Schwarzschild radius only means one thing: somewhere out there is an event horizon which we cannot pass through from this side. In other words, we cannot leave the observable universe to take a peek at the unobservable parts. That's it. There's no other consequence.

There is no reason an object on the edge of our observable universe can't leave our observable universe. This even happens all the time, since the space inside our observable universe expands faster than the observable universe itself
Space expands at 74.3 km/megaparsec*second.
The diameter of the observable universe is 29000 megaparsec.
This means the space in the observable universe is expanding at 2,154,700 km/sec.
Lightspeed is 300,000 km/sec. The diameter of the observable universe expands at twice that speed (logically the radius expands at light speed) => 600,000 km/sec.
Ergo, the space in the observable universe expands at just over seven times the rate of the observable universe itself. => Mass is leaving our observable universe constantly. And a lot of it.

However, if we leave now at light speed we would never leave our current observable universe because our observable universe also expands at the speed of light. We'd be 14500 megaparsecs behind and never catch up.

A black hole is slightly different. There is a Schwarzschild radius that nobody can leave once they are in it. It doesn't change where you are within that radius, you are not going to leave (well, until the black hole evaporates during heat death of the universe. If you still live by then).

Scratch that. The observable universe expands with the space in it. The observable universe is 93 billion lightyears in diameter but not 46.5 billion years old.
Mikeski wrote:A "What If" update is never late. Nor is it early. It is posted precisely when it should be.

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Derek
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### Re: Is the entire universe a black hole?

Tub wrote:Being part of a collection of mass that's inside its own Schwarzschild radius only means one thing: somewhere out there is an event horizon which we cannot pass through from this side. In other words, we cannot leave the observable universe to take a peek at the unobservable parts. That's it. There's no other consequence.

So based on the mass and radius of the universe, this would be true even if the universe were not expanding, correct?

If we assumed a static universe of infinite (or sufficiently large) size and even density, there would still be some radius beyond which we could not reach or send signals due to the Schwarzchild radius of our region of the universe. Is that correct? Actually, that doesn't quite seem right as I think about it more. I keep coming up with contradictions, and it seems like the problem probably lies somewhere in the assumption of an infinite universe (infinities are troublesome) and also probably that Newtonian gravity probably doesn't work on this scale.

Bingo. Since there isn't, nothing special happens to us. The observable universe's mass isn't concentrated in a singularity, but spread evenly across the observable universe. None of the scary things you read about the inside of a black hole apply to us, except for the existence of an event horizon.

Scary popular science aside, modern physics doesn't say that the mass of a black hole must be concentrated at a singularity, right? Just that, to the outside observer, it is as if the mass is concentrated, but that is also equivalent to mass that is evenly distributed in a sphere. Additionally, within the black hole all matter (and massless particles for that matter) must move towards the center, but if the black hole is billions of light years in radius, that process would take billions of years.

No coincidence. Those values match because our universe is flat.

But as the universe expands, won't this ratio change? In particular, the radius of the observable universe increases while the mass remains constant, right?

KarenRei wrote:Some interesting reading on the subject:

http://cds.cern.ch/record/348993/files/9803052.pdf

They posit an inflation-driven effect at high densities (a dilation gravity) that would mean that there's no such thing as a singularity, and that both black holes and our universe would be versions of the same phenomenon. In their model, dilation/inflation causes infalling particles onto a tangential path with the event horizon without the formation of a singularity, converging to a topologically flat space. The evolution of such a state begins superinflationary (shifting Hubble parameter) before settling down as the expansion proceeds. This could be seen as corresponding to both the evaporation of a black hole and the Big Bang.

I skimmed that as best as I could, being way above my level and all. So are they basically proposing that there is no distinction between our universe and the interior of a black hole, because the interior of a black hole can experience spatial expansion the same as the universe?

Tub
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### Re: Is the entire universe a black hole?

Derek wrote:So based on the mass and radius of the universe, this would be true even if the universe were not expanding, correct?

The thing with expansion is that the schwarzschild radius can increase. Technically, nothing passed through the event horizon in the wrong direction, but when the event horizon keeps moving further and further away from you, it doesn't really restrict anyone's movement at all.

I don't know if an expansion based on a metric expansion of space (rather than outward momentum) makes a difference here. Schwarzschild assumed a cosmological constant of 0, so I don't know how many of his formulas remain useful in our universe at the largest scales.

Derek wrote:If we assumed a static universe of infinite (or sufficiently large) size and even density, there would still be some radius beyond which we could not reach or send signals due to the Schwarzchild radius of our region of the universe. Is that correct?

Yes and no. If the universe keeps expanding, then there are parts of the universe we can never communicate with. If the universe is contracting, we can eventually communicate with everything. Of course our signals won't reach anything outside of the event horizon, but everything outside of the event horizon will eventually fall in, joining us for a very communicative group hug in a central singularity. There's no such thing as a static universe, especially not when you want to talk about long-term evolution.

Scary popular science aside, modern physics doesn't say that the mass of a black hole must be concentrated at a singularity, right? Just that, to the outside observer, it is as if the mass is concentrated, but that is also equivalent to mass that is evenly distributed in a sphere. Additionally, within the black hole all matter (and massless particles for that matter) must move towards the center, but if the black hole is billions of light years in radius, that process would take billions of years.

Don't confuse "mass inside its own schwarzschild radius" with "the result of a heavy collapsing star". The latter will lead to spacetime geometries like the Schwarzschild metric or Kerr metric which have singularities. I don't think they apply when the mass is evenly distributed inside the event horizon with an outward momentum. They also don't consider a metric expansion of space, so you shouldn't try to apply them to our observable universe.

In particular, the radius of the observable universe increases while the mass remains constant, right?

To my understanding, those two values must match as long as the universe remains flat. In our universe, objects keep moving out of the observable part due to the metric expansion of space, so the mass decreases.

Derek
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### Re: Is the entire universe a black hole?

Tub wrote:To my understanding, those two values must match as long as the universe remains flat. In our universe, objects keep moving out of the observable part due to the metric expansion of space, so the mass decreases.

Well all the more then. The volume of the observable universe can only increase, right? So if the mass remains constant or decreases, the density must decrease. I don't see how the values could remain close to each other.

jewish_scientist
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### Re: Is the entire universe a black hole?

Tub wrote:
sevenperforce wrote:Wait, no, that's only if there's an actual singularity at the center that you're being pulled toward.

Bingo. Since there isn't, nothing special happens to us. The observable universe's mass isn't concentrated in a singularity, but spread evenly across the observable universe. None of the scary things you read about the inside of a black hole apply to us, except for the existence of an event horizon.

In my gigantic black hole, there is a singularity; we just have not reached it yet.

Tub wrote:
Derek wrote:I also find it interesting, though it's probably just a coincidence, that these values are almost within an order of magnitude.

No coincidence. Those values match because our universe is flat.

When we say that the universe universe is flat, we mean that it follows Euclidean geometry at a large scale. Why does that imply that the mass of the observable universe be equal to its Schwarzschild radius?

p1t1o wrote:
sevenperforce wrote:If you have fallen into a black hole and have already crossed the event horizon, you will fall faster than everything "above" you, and slower than everything "below" you. So it will seem to you...as if everything is moving away from you.

Wouldn't this mean though, that objects at the same "height" as you will move towards you?

Maybe that is what the Axis of Evil is.

jewish_scientist wrote:google^gram's googol^Graham's number times bigger than the observable universe.

Sorry, I turned the language port of my brain of because the physics and cosmology is so far beyond me.
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Rococo
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### Re: Is the entire universe a black hole?

All hail Sean Carroll