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Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.

If the matter contained within our galaxy were concentrated within a small radius wouldn't that lead to the entire universe being a black hole? If so how is it that all the matter of our observable universe could have originated from a region of infinite density?

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4 Answers 4

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This is actually a common question. Many websites have been setup to try to explain this. I like this one for instance. I shall attempt to do my own layman explanation.

First of all, in order to have a black hole, you need to have a place for it to be in. Since there was no such thing as a universe, there isn't a place for the black hole to actually exist in. It's like asking, what is north of the north pole. There is no reference for an answer.

Secondly, gravity and other fundamental forces didn't act the way we are used to them acting. All four fundamental forces were combined in one basic force. Therefore there was no such thing as gravity to actually act on the mass as it existed. And then there is the problem that there was no actual mass. It was energy, which was creating the density of the universe. I know it's kind of counter-intuitive to how we are used to understanding these terms. Actually, Brian Greene has a good explanation of all this in his book Elegant Universe. Suffice it to say, at the start of the universe, the fundamental forces really acted very differently from how we see them now. As gravity was separating itself from the other forces, it actually had a repulsive effect as opposed to attraction.

Finally, the big bang wasn't really an explosion or a bang as one would think of it. It's actually the rapid expansion of space itself. Instead of things themselves speeding away from each other in a fixed space, the motion is actually caused by the space between the objects themselves getting bigger. Visualizing this is usually presented as a balloon expanding, and seeing how two dots on the surface of the balloon get further away from each other without actually moving on the surface. Of course, it's harder to visualize this in three dimensions (or actually four). While we are limited to the speed of light for any objects, the expansion of space itself is not limited by this (Brian Greene's book also has info on this).

The bottom line is that anything that you think you know should probably be discarded when thinking about the big bang. The reason that there is "string theory" and many other things is that classical quantum physics and relativity physics break down at the Plank Epoch which is at the heart of the big bang.

I hope that helps.

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wouldn't energy also attract itself due to gravity even if there was no matter in the beginning? –  Jus12 Mar 19 '12 at 22:48
No, not exactly. Energy is not exactly a "thing". One way to describe it is a field (sometimes transported by a particle), however that still doesn't quite explain it in the context of this answer. –  Larian LeQuella Mar 19 '12 at 23:32
No one has any evidence that there was "no universe" before the big bang. This is simply conjecture, or misinterpretation from popular explanations of the big bang theory. –  B T Aug 8 '14 at 21:01
You don't need to invoke the breakdown of general relativity to show that the big bang wasn't a black hole. General relativity, alone, already predicts that the big bang singularity is distinct from black hole singularities. –  Jerry Schirmer Feb 11 at 15:31

Good question! I often pondered that myself. As this website explains, the Big Bang wasn't a black hole basically because it couldn't be! A black hole is the mathematical solution to Einstein's equations of General Relativity that describes a pre-existing region of spacetime that has gravitationally collapsed and formed a singularity. Since there was no spacetime before the Big Bang, it couldn't possibly be a black hole.

But what was the Big Bang? It turns out, if you flip the sign of t (time) in the black hole solution to Einstein's equations, what results is also a perfectly valid mathematical solution. It describes a black hole in reversed time, or in other words, well, the Big Bang! A singularity explodes outwards into a nice, flat(tish!) region of spacetime. This solution is also known as the white hole solution.

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yields an infinite density and temperature at a finite time in the past.

Not quite, the extrapolation stops sometime around a Planck length, nobody is saying what happened before then

wouldn't that lead to the entire universe being a black hole?

No, even if the universe was originally a singularity after inflation it wasn't and isn't

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During the initial stage of the Big Bang known as the inflation stage (hence the name "Inflation Theory") the laws of physics were different than they are today. The foces of electromagnetism, the strong and weak nuclear forces and gravitation were unified in a single force.

In the first 10-35 seconds the forces separated and the universe as small as it was at this stage, filled with what's known as the vacuum energy. As a consequence of this vacuum energy density (which plays the role of an effective cosmological constant), gravitation became repulsive for a period of about 10-32 seconds. This caused the universe to expand an an astonishing rate.

By the time gravitation phased towards becoming an attractive force, the universe has expanded beyond the critical density required for it to collapse back in on itself to form a singularity.

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This is all true, but doesn't answer the question of Why didn't all that super-dense stuff collapse into a black hole? –  Andrew Jun 25 '11 at 10:46
The vacuum energy didn't collapse because gravitation was a repulsive force during the phase transition period causing a push outwards rather than attract energy inwards. –  Fergal Jun 25 '11 at 11:19

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