Talk:Flatness problem
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Okay, here's something I ought to understand but don't, and would like some clarification of.
- The critical density at which the expansion rate of the universe will tend asymptotically towards zero is about 1×10^-29 grammes per cubic centimeter, and the ratio of the actual density of the universe to this value is known as Ω.
This value of 1x10^-29 g/cm presumably must either be a function of time, or rather of the size of the universe, or be measured in some sort of co-moving coordinate system in which a cm today is larger than a cm yesterday, right? Otherwise, if the density were slightly above 1x10^-29 g/cm early in the history of the universe, it would fall below that value as the universe expands, and you'd go from a universe that's supposed to fall back into a big crunch to a universe that is supposed to expand forever, and that makes no sense.
- Currently, observations indicate that Ω is between 0.98 and 1.06 - in other words, that the universe's density is very close to or exactly the critical value. ... The fact that approximately 14 billion years after its formation, the universe still has an Ω so close to unity indicates that Ω must have been within one part in 10^15 of unity when the universe formed.
Is it the case that in a matter dominated universe, Ω is constant? If it isn't, how can it vary? And in a universe with dark matter, Ω does evolve... right? -- Rsholmes 18:40, 20 February 2006 (UTC)
