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What should be the expansion speed of the universe for it to overcome its own inward gravitational pull?
Continuing on from Paul Davies' The Last Three Minutes (1994):
"... the escape velocity will be about the velocity of light. This is a very significant result, because the most distant galaxies appear to be receding from us at close to the speed of light. Taken at face value, the galaxies seem to be flying apart so fast that they may indeed just 'escape' from the universe, or at least from one another, and 'never to come down.'
"In fact, it turns out that the expanding universe behaves in a manner closely analogous to a body projected from Earth, even if there is no well-defined edge. If the rate of expansion is fast enough, the retreating galaxies will escape from the cumulative gravity of all the other material in the universe, and the expansion will continue forever. On the other hand, if the rate is too slow, the expansion will eventually be brought to a halt and the universe will start to contract. The galaxies will then 'come down' again, and the ultimate cosmic catastrophe will ensue, as the universe collapses."
The Big Rip or the Big Crunch?
Well for now, it looks like it's the former; observations point to an ever accelerating expansion of the cosmos.
[Image credit: NASA]
Related post: Escape velocities
Technorati Tags: Big Chill, Big Crunch, Big Rip, cosmology, Hubble constant, universe expansion
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