h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If the Universe and everything in it began as a small, highly condensed object. How is it that gravity didn't prevent its expansion? When you consider a black hole, which is less than the original mass of the Universe, prevents anything from escaping, shouldn't the mass of the Universe, when all four forces were effectively one, have prevented this expansion.
Please note I do not believe in gods etc, so therefore there must be a logical explanation.

Good question, we might have somewhat of an answer once they get the LHC at CERN up and running.

Well, p'raps it was (is) a black hole and we're the other side of it?

P'raps it's now about 90% of the way along its course & is now busy filling us up with dark matter?

When we're about half full...

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Oh, ignore him, he's got nothing better to do for an hour.

Well, one theory states that the Big Bang wasn't *the* Big Bang, merely *a* big bang... which just so happens to have exploded with the right amount of force and had the right amount of gravity so that the infant universe wasn't either dispersed into the void or damn near immediately sucked back in on itself.

As for "black hole... prevents anything from escaping"; according to Hawking's radiation theory if a particle and anti-particle were created just outside a black hole one would fall in and the other would 'escape'. So it's not necessarily true that nothing can ever escape from that high a level of gravity, and as the universe would have been nothing but particles at the very beginning it's not at all unreasonable to accept that it started with a singularity.

Please note that I am quite willing to accept the possibility of a higher power and think that the existance of gods would not negate a logical explanation for things. We rationalise the supposed actions of gods in a way that we can understand, and it is not at all unreasonable to think that a being that powerful would've been messing about with elementary particles rather than building stuff out of clay and ribs.

I’m not sure I can accept (but then on the other hand who am I to argue with the great and the good) Hawking radiation. It seems to me that if two particles come into existence just outside the event horizon (which isn’t a problem as energy and matter are two sides of the same coin) and one falls in then the black hole is getting bigger. Just because the other particle doesn’t fall in cannot mean that the black hole is leaking.

The thing is that the particle/anti-particle pair was created from something. The Universe doesn't allow for a free lunch. The energy that created that pair has to come from somewhere; and that somewhere is the black hole itself. Which is why black holes eventually "evaporate".

I was under the impression that the energy for these particles came from the residual background radiation from the Big Bang, which I think is about 1.9 Kelvin, and not from the Black Hole. This question seems to raise more questions than answers! .

I was under the impression that it was a zero-point energy thing, so the particles are supposed to annihilate to conserve the universe's energy?

Well beyond my mandate here and into the realms of things I have absolutely no clue about.

I think the difference between the Big Bang and a Black Hole is that the Object at the start of the universe was High Density but low entropy where as a Black Hole has High Density and High Entropy.

Hawking radiation is weird but if energy is leaving the Black Hole the Black Hole must be shrinking. I think it might be that when the one of the pair falls into the Black Hole it will be able to annilate with something.

The book The Feathered Onion, Clive Trotman quotes (someone) on the laws of thermodynamics:

1 You can't win
2 You can't break even
3 You can't stop playing

If we're on the *inside* of the black hole then we don't have to worry about how it formed. Apparently we are if the universe is closed, but not if it's open. Easy.

The problem you appear to have is that you are assuming that the big bang was an explosion of matter in space, like a normal explosion would be. However, this is not true. The big bang occurred at all points in space and since then it has not been matter that has been expanding into space, but space(time) itself that has been expanding.
And, the early universe is homogeneous (the same everywhere) so even though the density is high it is the same everywhere so there is no preferred point for everything to collapse to.

Now, the deal with hawking radiation is that a particle - antiparticle pair, like it has been said are "created" outside of a black hole (according to the uncertainty principle, you cannot have zero energy, so particles popping out of itty-bitty packets of energy is just fine, quantum mechanically) and instead of immediately inalating, one falls into the black hole and the other one wanders off into space (and to follow the law of conservation of energy the particle that fell in must be negative and therefore deplete a bit of the black hole's mass). So, this "radiation" that Hawking predicts does not actually come from the black hole, it is merely a by-product of virtual particles being popped into existence near a black hole.

I hope my explanation sort of puts together what everyone else is trying to say coherently.

Yes I obviously had the wrong end of the stick here.I think I get what you're saying and it is logical. Thus the early universe couldn't collapse on itself, nowhere to collapse to, hense it goes on expanding. Once it had expanded enough to develop matter from the plasma, pockets of it could collapse to form black holes.

Yup, thanks, it does help to make this just a tiny bit clearer in the thick fog of quantum machanics

"The problem you appear to have is that you are assuming that the big bang was an explosion of matter in space, like a normal explosion would be. However, this is not true. The big bang occurred at all points in space and since then it has not been matter that has been expanding into space, but space(time) itself that has been expanding.
And, the early universe is homogeneous (the same everywhere) so even though the density is high it is the same everywhere so there is no preferred point for everything to collapse to."


Reviving this thread...

One other thing is that heavy black holes have a very large radius. From http://en.wikipedia.org/wiki/Supermassive_black_hole :-

"The average density of a supermassive black hole (measured as the mass of the black hole divided by its Schwarzschild volume) can be very low, and may actually be lower than the density of air"


The statement above that we may be inside the black hole is another way of stating that a hole the weight of our universe should be approximately the size of our universe (with a density massively lower than air).

We will be in such a "closed" universe if the average density of matter across the whole universe is above a critical threshold. We know it's pretty close, but this is currently an open question.

I guess when the universe was tiny, it could expand hugely before the event horizon/Schwarzschild radius became a limiting factor. There is some weird accountancy in General Relativity by which a closed universe (ie the event horizon) can continue to expand, slowing all the while, until it reaches a limit and starts to collapse towards a big crunch.

This is all hugely complicated by the question of whether "dark energy" is actually accelerating the expansion of the universe, overriding the above effect.

cheers, Martin

Hi Martin, not sure whether to thank you or thump you!
I appreciate that I had the wrong end of the stick when I was assuming (and what is it they say about assumtion and and mothers!) that the Big Bang was an explosion in the normal sense of the word.

I, however, do now realise that I now know a lot less than when I started this.
If, as you say, it is space time that is expanding, which I must accept, for who am I to argue with the good and the great (this is a neat bit of groveling eh!). Where did the matter/energy originate? or am I asking the wrong question?
regards
Davers6

That is the bit that the LHC is trying to discover (along with many other things). Physics as we understand it breaks at the very earliest moments of the Universe - the moment at which the Big Bang 'began' (timeline from W*k*pedia - http://en.wikipedia.org/wiki/Graphical_timeline_of_the_Big_Bang ).

t.

*BUMP!*
"Where did the matter/energy originate? or am I asking the wrong question?"
Matter, space, time, and energy were all wrapped up in this singularity "in the beginning". The issue is, because space and time were indistinguishable, it is hard to ask "where" or "when" the matter/energy came to be as one cannot talk about a "where" and a "when" at the point of singularity.