Vacuum genesis (zero-energy universe) is a scientific hypothesis about the Big Bang that questions whether 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 had not 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 expanding at an accelerated rate, but was originally thought to be slowing down. Furthermore, the rate at which this is changing gives 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 will 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 halt. Tryon's theory requires that omega be equal to or less than one. Through calculations it has been found that omega is 1 as far as instruments are able to determine. However, the universe could be so vast that its curvature is undetectable. The universe's rate of expansion is accelerating and is projected to continue accelerating due to dark energy, thus omega must be lower than 1.
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