Am I mixing up some concepts or what?

Question

My latest attempt to understand the big bang theory: so if all the matter was in this tiny space...wouldn't it all condense to form a black hole...you know the process of gravitational collapse.

*sigh* if only I took physics as a major in college......

Answer 1

No, you are right.
In fact at one point it was believe that the entire universe would finally collapse into a black hole forming a new cosmic egg which would explode into a new universe.

We don't know WHY the big bang happened or WHAT was going on before it. We don't even know the original size of the big bang. We can project it to an atom size point 13.7 billion years ago, but what if it was larger, and what existed before. Those questions are unanswerable since we don't know anything about the prior universe and we can't since nothing of it exists in our universe.

According to the theory the big bang started with the entire contents of the current universe located at one point and then it exploded; forming the current expanding universe. The explosive front is the edge of the universe and it is receding from us quite quickly (currently it is 13.7 billion light years away). If we could venture into that area then we might be able to learn about the prior universe, but we can't do that, and we can't see light from that far; it would be drowned out by the background radiation of the universe.

Answer 2

No you're absolutely right.

The thing is, and nobody else seems to have said it yet... There's a good chance black holes don't exist.

Sure you get gravitationally collapsed stars, they have event horizons and they suck matter and radiation in, but personally I've never found the concept of a singularity as physically tenable. There are alternative ideas such as black branes, gravastars and dark energy stars for idea of what a black hole might really be but they're very tentative for now.

Similarly the 'ekpyrotic model' of the big bang is a cyclic model of universes colliding - and although the phenomenon has huge energies and densities there is no singularity.

If it makes you feel any better, I don't think any of this would be covered in a physics major. This is the cutting edge of theoretical physics.

kind regards,

Answer 3

It is somewhat hard to understand, but a black hole and the big bang cosmology do not share the same geometry. They do not even share all the same equations... just some of them.

A black hole is a highly curved region of space on a mostly flat background. The big bang is a mostly flat space with only tiny variations of the curvature on any but the quantum level.

A local observer during the big bang would experience enormous energy densities but only slight gravitational discomfort.

A local observer around a black hole sees zero energy density but enormous gravitation... so its just the opposite.
There is no interaction mechanism in empty space to stop matter from falling closer to the bottom in a black hole while the local observer in the early big bang will be pushed around by inhomogeneities in the energy density and the resulting radiation pressure.

When a gravitational wave goes through a universe of high energy density, it will locally superheat the radiation field and the radiation pressure might just dissipate the energy of the wave away rather than to let it amplify itself into a black hole solution. It all depends on the details... so one has to do the detailed calculations.

In comparison, during a black hole collapse there is no mass outside of the critical radius itself and the mass on the inside can not push back with enough force, no matter what happens.

Both cases are, of course, solutions of the same shared equations for the gravity part, but one has to throw more variables into the game to describe matter and radiation in case of cosmology while one can abstract from that for the outside of a black hole.

I wish I could explain it better... but I can't without having to go through the whole lecture cycle on general relativity.

If you feel a real need to understand these things in detail, you will probably have to learn some of the mathematical apparatus of the theory, at least enough to understand how to solve for a Schwarzschild and the Friedmann-Lemaître-Robertson-Walker metric. Moreover, you would probably have to go through the calculations of the stability criteria for black hole collapse and cosmologial expansion.

I have done the calculations of the black hole and cosmological solutions myself as student exercises, but I never bothered to read the papers with the stability analysis for these solutions for myself. Maybe one day when I have more time... and more brains to work through the math.

I would think that the naive physical explanation is that in the cosmological solution the expansion always happens much faster than the time scale of the potential local gravitational instabilities which can create black holes and that radiation pressure and radiative transport for most part can smooth out local instabilities.

People did speculate about the possibility that tiny black holes get created early which are just starved of growing because the energy density falls faster than the black holes can grow themselves and by the time its all over, there is nothing left for them to "suck in"... I am sure Hawking must have thought and written tons of material about this scenario.

Hope this helps to point out a few differences between the scenarios and why the cosmological models depend on the total energy of the universe as another parameter.

Answer 4

Very good question! I did major in physics, but I still can't explain why the big bang took place.

You are right. At the instant of the big bang, the whole universe was in a black hole singularity. Gravity should have kept it confined to a point. But somehow it expanded. Inflation theory describes it from about 10^-43 seconds onward. But to understand why it expanded from a point in the first place, you would need to use quantum gravity theory. And no one has yet discovered a correct quantum gravity theory.

Answer 5

there was no matter in the beginning, it was all energy. that energy later cooled and formed matter. and yes it would create massive gravity if there had been space. but the theory is that space was created when it exploded. so if there is no space there is no gravity.

Answer 6

I believe that the big bang was the explosion of a black hole.