# Why isn't the universe a big black hole?

1. Jun 1, 2012

### ncarron

In the early days after the Big Bang, the universe was very dense and of relatively small radius. Those are the conditions for a mass to be a black hole.
So the early universe should have been a black hole.
When did it stop being a black hole, or is it still one?

2. Jun 1, 2012

### Mark M

Well, there are few things to clear up. Firstly, keep in mind that the universe has no edge, boundary, or center. To understand what this means, imagine that the two dimensional surface of the earth was an entire two dimensional universe. There would be no center, and traversing the entire diameter will return you to your position. Generalizing this to three dimensions isn't visualizable, but it just means that, if you froze the expansion of the universe, traversing enough distance in any direction will just bring you back to your starting position.

Now, remember what exactly a black hole is. It's just a mass with a gravitational force strong enough to curve a ray of light back onto itself, creating a black horizon that surrounds it. In the context of relativity, since light defines that quickest route between two points, the light-cone of an object is curved back onto itself. This translates into this dark horizon being an event horizon, a point at which nothing on one side can directly affect something on the other side.

With these facts in mind, it doesn't really make sense to ask if the universe was a black hole. However, because of the rapid rate of expansion in the very early universe (particularly inflation) everything around an observer would be shrouded in an event horizon, as even relatively nearby objects would recede with superluminal speed.

3. Jun 1, 2012

### Imax

Space time is intimately related to the Big Bang. Donâ€™t think of the BB as an explosion into infinite space. The BB caused inflation in space and time.

4. Jun 2, 2012

### phinds

There was no "radius". As Mark has already said, the BB has no edge or center. It might have started off infinite in size, but smaller than it is now, or it might have started off finite but unbounded, but smaller than it is now. In neither case does your statement and subsequent question apply.

5. Jun 2, 2012

### dimension10

There was NOTHING before the big bang, that is why. Not even a singularity. Nothing. 0 energy. Even now, the total energy of the universe is 0, because gravitational energy is negative.

0=-1+1+i-i.

Black holes classically have 0 radius but with quantum mechanical corrections, not anymore.

6. Jun 2, 2012

### phinds

I don't understand the relevance of the lack of anything BEFORE the singularity to what happened just AFTER the singularity. That is, I don't understand how your statement is relevant to the OP's question. I'm not saying you're wrong, I'm just saying I don't see how you're right.

7. Jun 2, 2012

### dimension10

I'm just citing the difference between a big bang "singularity" and a black hole singularity.

8. Jun 4, 2012

9. Jun 4, 2012

### Staff: Mentor

I'd say that just because the math says it is possible doesn't make it so and to take it with a grain of salt.

10. Jun 4, 2012

### Whovian

While I highly doubt it, it would make for a really interesting concept for a multiverse.

11. Jun 4, 2012

### dimension10

What? That was my imagination when I was like 5 or 6 years old, and I doubt that statement.

12. Jun 4, 2012

### phinds

Sounds ridiculous to me. In fact, I'm pretty sure that that topic has been here before and was roundly criticized by most as ridiculous. None of that MAKES it wrong of course, but I STILL say it's ridiculous.

13. Jun 4, 2012

### Darken-Sol

how do we know this?

14. Jun 4, 2012

### phinds

We absolutely do NOT know this. It is one model ("finite but unbounded") and COULD be correct but there is zero observational evidence for it.

15. Jun 4, 2012

### Mark M

We know that it must be true *if* the universe is finite. I think that phinds is saying that we do not know if the universe is finite or infinite. That is an open question. What I was saying is that if the universe is finite in size, it must wrap back around on itself, not have an imaginary edge or boundary.

16. Jun 4, 2012

### phinds

Yes, I agree w/ that

17. Jun 6, 2012

### pervect

Staff Emeritus
try the sci.physics.faq http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

I'll quote a relevant part of the original source under "fair use" provisions here - it's better if you read the original source in its entirety, but hoepfully reading just the selected quote will convince you as to its relevancy.

To summarize the above argument, it's the stress energy tensor that curves space-time in general relativity, it is NOT"mass" or "energy". (The idea that it's mass that curves space-time is an incorrect conflation of Newtonian physics with GR - it's unfortunately a common misunderstanding that since "mass' caused gravity in Newtonian physics, it's also responsible for gravity in General Relativity).

Because the stress-energy tensor for expanding matter is different from the stress-energy tensor of non-expanding matter, you can have compact, dense, expanding objects that are not black holes.

18. Jun 6, 2012

### ncarron

19. Jun 7, 2012

### GeorgeDishman

Possibly, but you will find it commonly discussed regarding Penrose Diagrams, for example:

Andrew Hamilton is well known in the field:

20. Jun 9, 2012

### Alex1998

as pointed out by mark M, the universe has no boundary edge, or centre. the centre of the universe is analagous to the centre of the surface of a sphere. also, it doesn't have an edge, because if you walk around a sphere (say, the earth), you could do so infinitely.

I have made a model which is called "The Pebble and the Pond". this model explains that the big bang is a pebble being thrown in a pond, this creates a circular shockwave, this shockwave is 3 dimensional space, and the movement over the water is the passing of time. this means that in the beginning of the universe, the radius over the time dimension was very small, so in 4D it has an edge, but in 3D, it doesn't.

I also talk about quantum dimensions in my youtube videos, there I say that all those rolled up quantum dimensions, are the same as the 3D space, but 3D space is just an inflated roll, or a very big roll, this is showed in the model of the pebble and the pond. This is the same with when the universe started, it was very small, but it is actually the same as when it is very big, so if the universe was a black hole when it started, it will still be one.

the only question is, where is the black hole, because there is no centre of the universe. maybe the big bang is the opposite concept of a black hole, because a black hole sucks up matter, and the big bang created matter. then it would sit in the centre of the 4D universe, and the black hole is the big bang.

if you are interested in my YouTube videos, visit my channel "Fathom the Universe".

21. Jun 9, 2012

### Staff: Mentor

Alex, I was impressed by your video on dimensions, especially given the fact that you are 13!

Just so you are aware, your description of the "Pebble and the Pond" makes much more sense in your video than it does in your post, and saying that you have created a "Model" could mean to many that you have developed your own personal theory, which would be against PF posting rules. I would suggest labeling it as an analogy instead of a model.

22. Jun 9, 2012

### Alex1998

ok, thanks for pointing that out, you are right..it is indeed an analogy I used to explain the current big bang theory.. I find it useful to scale down hyper-dimensional space to two or three dimensional models for clarification.