Vacuum genesis (Zero-energy universe) is a hypothesis about the Big Bang that states that the universe began as a single particle arising from an absolute vacuum, similar to how virtual particles come into existence and then fall back into non-existence.
The concept of vacuum genesis was first proposed in 1969 during a seminar being conducted by cosmologist Dennis Sciama. Edward Tryon, in the audience, was seized by an idea and blurted "Maybe the universe is a vacuum fluctuation." This was treated as a joke at the time, but Tryon hadn't been joking. In a 1984 interview, Tryon recalled that three years later, sitting at home, he had a further revelation; "I visualized the universe erupting out of nothing as a quantum fluctuation and I realized that it was possible that it explained the critical density of the universe."
The critical density of the universe is dependent upon the rate at which the universe is still expanding. The universe is still expanding, but thought to be slowing down, and the rate at which it is slowing may give the overall mass density of the universe which is denoted by the greek letter omega. If omega is less than one, the mass density would be insufficient to stop the universe's expansion and it would go on expanding forever. If omega is more than one, the universe will eventually stop expanding and thus collapse in on itself to again form another fireball not unlike the one from which it came. If omega is exactly 1, the universe's expansion will continuously slow, but never quite halting. Tyron's speculation requires that omega be equal to or less than one. Thorough calculations it has been found that omega is, in fact, 1, or almost exactly 1.
This omega value gives an almost precise value of unity. This means that cosmic space is neither dramatically open or dramatically closed, but is almost perfectly flat.
1 ^ Tryon, Edward P. (December 14, 1973). "Is the Universe a Vacuum Fluctuation?". Nature 246 (5433): 396–397. Bibcode 1973Natur.246..396T. doi:10.1038/246396a0.