How Could the Big Bang Avoid Being a Black Hole?

[darkfall13]

So if a black hole in summary is anybody massive enough to succumb to it's own gravity. And before the Big Bang all the Universe's matter was in one location. How was this not a black hole? Clearly if you have all the mass of the Universe you kind of pass that threshold of "just enough matter."

I talked to Dr. Andrea Ghez here at UCLA about this and she said that without getting too involved in it that 1) space is warped by gravity/mass so in the beginnings it was not as we know it now 2) our normal laws of physics don't really apply anymore for various reasons.

I was just wanting to hear some more opinions on this as this has always been a question on my mind.

 

[Wallace]

Have a read of http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html" faq.

That is a well written and comprehensive answer, here is my brief attempt.

Massive objects attract other objects not only because they have a lot of mass but more importantly, they have more mass then their surroundings. Think about this, the Earth is attracted to the Sun because over 99% of the mass in the solar system is in the Sun, so almost all of the local pull of gravity is in that direction. If the entire solar system was full of material the same density as the Sun we would not longer be pulled towards it. Its mass hasn't changed, but the contrast between it and its surroundings has.

In technical terms, gravitational acceleration is produced by there being a gradient in the gravitational potential. In turn such a gradient is caused by a non-uniform mass distribution. A black hole surrounded by empty space is clearly a very non-uniform mass distribution and hence has a large gravitational potential and hence acceleration.

Now onto the Big Bang. The first thing to realize is that 'the Big Bang' is a terrible name for the theory and gives the impression that the Universe started with an explosion from a single point. This is not what the scientific theory says. In fact the Universe can in principle be infinite in size. The Big Bang didn't happen at a point, rather it happened everywhere. Don't think about an expanding Universe as material rushing out from a central point, this will lead you to many misconceptions.

Importantly, the early Universe was extremely uniform. This is the key. If there is the same amount of stuff everywhere, then even though there is a higher density of gravitating material, the gravitational potential is flat, there is no gradient and hence no acceleration towards any particular point. The entire universe does expand, and the amount of material in the Universe dictates that expansion rate, but to form a black hole you need to have much more material in a small spot than in the space around it, not just have a lot of material.

I hope that helps, feel free to ask more questions if something isn't clear.

 

[darkfall13]

ah, surprisingly short for all the information included in that but it answered many questions thank you!

 

[Matthieu]

If the big bang had the physical structure of a Black Hole I don't see how it would be expanding today. Exepted if the time we know goes actually backward (our future trully being our past) but the causality law forbid this.

 

[Chronos]

It is useless to attempt to define causality before time arose.

 

[darkfall13]

If the big bang had the physical structure of a Black Hole I don't see how it would be expanding today.

Yeah that's exactly what I had in mind, but that's from just looking at a large mass small volume system.

 

[marcus]

If the big bang had the physical structure of a Black Hole I don't see how it would be expanding today. Exepted if the time we know goes actually backward (our future trully being our past) but the causality law forbid this.

Yeah that's exactly what I had in mind, but that's from just looking at a large mass small volume system.

Without some unusual extra assumptions, classic General Relativity wouldn't allow a gravitational collapse to bounce, creating a new region of spacetime. As you both indicate, classic GR would normally run into a singularity and break down. But there are various ways to quantize GR to get a quantum cosmology (QC) model of a collapse. Quantizing GR can cause the singularity to be avoided.
Researchers now run QC computer models, exploring various cases.
Fairly often what happens is quantum effects make gravity repulsive at near-planck density and the collapse turns around and re-expands into the future (so there is something like a new bigbang or new expanding region.

Most of the QC modeling has been in the past 2 or 3 years, much of it by Ashtekar's group at Penn State. Let me know if you want links to some of the papers, or just
use arxiv search engine

Here's the search engine
http://arxiv.org/multi?group=grp_physics&/find=Search
Here's what happens when you put Ashtekar in for the author
http://arxiv.org/find/grp_physics/1/au:+Ashtekar/0/1/0/all/0/1

Many of these paper, since 2005 or 2006, report QC modeling where a bounce was observed
I guess because gravity becomes non-classical at very high density.
So the classical singularity or breakdown which you would expect with classical GR does not happen

Another name to try in the arxiv search engine is Bojowald. he is at Penn State too.

gravity at very high densities is different, due to quantum corrections
It is very much work in progress, and not as yet conclusive. But apparently you can't just naively use classic formulas like the Schwarzschild radius etc, because at some point they stop applying realistically.

Other names are Gambini, Pullin, Vandersloot, Param Singh-----but if you try Ashtekar's papers you will get the flavor of current research along these lines.