Why didn't the big bang just turn into a big black hole?

Factual Questions
1

Okay, clearly I am missing something somewhere so let me start by saying what I think I know.

  1. The big bang was the start of time and space. (I am also aware of the expanding balloon analogy.)
  2. Nothing can travel faster than the speed of light.
  3. Black holes are black because their gravity is so strong that nothing, not even light can escape, their gravitational well. Also, they are thought to be infinitely dense.
  4. If you squeeze stuff tight enough, it will turn into a black hole. If it is not a lot of stuff this black hole will not last long due to hawking radiation but if it is as much stuff as in a large star, the black hole will last a long time.

So apparently the universe started out as a singularity. Is this not enough stuff squeezed tightly enough to form a black hole? I realsie there was a lot of energy involved but given that nothing, no matter how much energy you give it, can move faster than light and not even light can escape the gravitational field of so much stuff all in one place, how did the universe overcome it’s own gravity?:dubious:

btw, I do not doubt that the big bang happened.

2

My guess is that the universe expanded such that there was more “place” for all the “stuff”.

3

The space in the universe was expanding, and still is in fact.

There is a section of the universe called the “observable universe” which means exactly that, it’s the part of the universe we can see, because it’s close enough that light can reach us. The further away things are, the faster they are moving away from us, until the expansion of space is actually faster than the speed of light. The “stuff” doesn’t actually accelerate faster than the speed of light, and no information is traveling faster than the speed of light, so there is no violation of the laws of physics.

At the time of the big bang, the universe was expanding extremely fast. Faster than gravity could crunch it all back together. If you were at one edge of the expanding universe, you would not be able to see the other edge, because it would have been expanding faster than the light from the other edge could reach you.

4

This is a very good explanation, thank you.

5

There is one school of thought that the universe IS a gigantic black hole.

6

Do you have a reference that describes this school of thought?

7

Formula for the Schwarzschild radius

If you chuck the average density of the observable universe into the equations, get the Schwarzschild volume, and convert that to a radius, you get a number that’s somewhat bigger than the radius of the observable universe.*

*IIRC, It’s been 5 or 6 years since I tried this, so I could be mis-remembering. It sure can’t be smaller or we’d all be in trouble.

8

it was explained to me that, yes, nothing can move faster then the speed of light inside the universe. right after the big bang, the universe expanded faster then the speed of light. it could do this because it wasn’t inside itself and wasn’t restricted to it’s own laws.

9

Some say that the big bang did turn into a black hole and we’re inside it.

ETA: Damn you, BrotherCadfael. The usual threats. :smiley:

10

Except that space can expand faster than that. If any kilometer of space expands a cm a second (numbers made up, but space does expand even now, and the expansion is accellerating), then any 2 “stationary” particles more than X kilometers apart will move away from each other faster than the speed of light.

As far as I know it still is.

11

A closed universe with no cosmological constant is exactly equivalent to a Schwartzschild black hole: The Big Crunch singularity at the end of time is the singularity in the center of the hole.

Now, our actual Universe currently appears to be flat (or very close to it), not closed, and the cosmological constant (or whatever the heck else the Dark Energy is) also complicates things. But the Universe being a black hole isn’t a crazy or crackpot idea by a long shot; it’s just a plausible idea that appears to turn out to be wrong.

12

two points to consider.
during the earliest moments (fractions of the first nanosecond) gravity was repulsive. In fact this repulsion puts the bang in the big bang. So far from overcoming gravity, gravity is the force that created the universe.
During this period the universe expanded faster than the speed of light.

13

Not really. You’re probably referring to cosmological inflation, here, but so far as we can figure out, that started some time into the history of the Universe. Admittedly it was a very short time, but the Universe was already expanding before that. Even at that, it’s not so much that gravity itself was different, but that the stuff that gravity was acting on was different.

Only in the same sense that it’s currently expanding faster than the speed of light. The larger a scale you look at, the greater the speed of the expansion, and there’s always some scale beyond which the speed of expansion is greater than c. That required scale was much shorter during inflation, but qualitatively, it’s the same phenomenon.

14

thanks!
I have Brian Greene’s “The Fabric of the Cosmos” on my nightstand. But it is late when I get a chance to read it. :slight_smile:
I see that the universe was expanding before the inflaton field gave up it’s energy. I assume that is what you mean.

I am always amazed that the weight of the entire universe-before inflation was only about 20 lbs. From that everything appeared. amazing.