I don't think the other answers here are quite right. It started very hot and also expanding. Space itself was expanding. It's sometimes compared to a time reversed black hole (a "white hole").
If it was stationary it would have collapsed. When Einstein developed his theory of general relativity - he found it very hard to get a stationary solution - in all his solutions, either the universe was expanding or collapsing. He thought that meant he had got it wrong, because the established view of astronomers at the time was that the universe was in steady state - and so to correct his model, he added a "cosmological term" to keep it stationary.
EINSTEIN'S "GREATEST BLUNDER" OR DID HE SAY THIS?
As the story goes, he later called this his greatest blunder. Gamow recalled: "Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder of his life"
(Potentially he could have predicted the later expanding universe theory just from the equations of relativity - but he wasn't bold enough to do that :) ).
But - turns out he may not have said that after all. We only have the word of Gamow. One of Einstein's biographers, and himself an astrophysicist, Mario Livio cast doubt on whether he actually said this, asking the question why he would say this to Gamow who he was not particularly close to, only in speech, not in letters, and not to any of his other colleagues and friends.
Einstein Likely Never Said One of His Most Oft-Quoted Phrases
The nearest he could find to this in all his published works and his letters is a sentence:
"Historically the term containing the 'cosmological constant' ƛ was introduced into the field equations in order to enable us to account theoretically for the existence of a finite mean density in a static universe. It now appears that in the dynamical case this end can be reached without the introduction of ƛ."
He did say he had made one great mistake, but not in his mathematics and physics. This is in a conversation reported by Linus Pauling in his diary soon after it happened, so that seems to be on good evidence. "He said that he had made one great mistake -- when he signed the letter to Pres. Roosevelt recommending that atom bombs be made; but that there was some justification -- the danger that the Germans would make them."
ANYWAY ONE SOLUTION IS AN EXPANDING UNIVERSE
So one solution to the equations of general relativity is an expanding universe that starts off immensely dense.
The thing is that there is a time component rather than just a purely space description. At the beginning in the Big Bang the universe is expanding rapidly so each moment it is different from the previous moment. The Schwartzchild radius for a black hole only applies in a static universe.
At which point pass you over to a physicist to explain it properly: Is the Big Bang a black hole?
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Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
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