Expert: James Gort - 9/30/2006

Question
If I understand correctly, a black hole results when a star more that a few times the mass of our sun ehausts its supply of hydrogen fuel and can no longer maintain internal pressure to counteract the effect of gravity, causing it to undergo a complete gravitational collapse.  The result is an 'object' occupying a spacial singularity, a point in space with no size whatever (or a point whose size is 'infinitely small', as I've seen it described) from wich nothing, not even light, can ever escape.

What I'd like to know is, if an amount of matter a few times greater than the sun is all that is needed to generate a black hole, how could the mass of all the matter/energy in the universe have exploded outward from a concentration smaller than a hydrogen atom the way I hear the Big Bang described?  Wouldn't such a mass be subject to the same gravitational collapse?  How could it ever achieve a "Bang?"

Answer
Hi Ben,

Your idea of black hole formation is essentially correct, except the star starts with a mass over about 25 times the mass of the sun.  After it collapses, if the degenerate core is a few times greater than our sun's mass, gravity is greater than its internal pressure and it continues to collapse.  The reason people think the mass gets concentrated into a singularity (it may not be, but we need a quantum theory of gravity, which doesn't yet exist!) is because space-time is so bent that light can't escape. That means that all matter within a certain radius must travel AT THE SPEED OF LIGHT (relative to a distant observer) just to remain stationary (relative to the star).  Einstein said that would require an infinite amount of energy.  So that means no matter can remain stationary within the star - it must continue to collapse.  That's a direct consequence of relativity.

The "Big Bang" is another story.  In fact, the name is pretty bad.  It wasn't a "bang" at all.  It was the beginning of space-time.  Outside the Big Bang, there was no space-time - the "bang" was the expansion of space-time into what was previously "nothing".  So space-time couldn't be bent, if it didn't yet exist.  When it came into existence, there was no matter - only some form of energy.  And it isn't clear if there was gravity yet - the force is actually carried by particles called Gravitons.  So you can't relate today's physics to the time shortly after the "Big Bang".  Most of the physical laws that we know today would not have come into being yet.  

I hope I've answered your question, and not confised things too much.  It's a very fundamental question, and one which no one has all the answers to.  But I've tried to summarize our current state of understanding of the problem.  Continue to ask those interesting questions!

Prof. James Gort