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)
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- For crying out loud, this article doesn't deal with the flatness problem almost at all! There's a lot of Big Bang theory and inflationary universe, but what does it mean "flatness"? How are these theories related to it? How can we tell or observe if the universe is flat or not, from our POV? This should be merged somewhere else and the article rewritten from scratch or so.
- The article's actually okay, if a little jargon-y and not very in-depth. I'll try and tackle it a bit later. WilyD 23:15, 21 December 2006 (UTC)
[edit] Evolution of density
I'm glad to see some work being done on this article. I'd still like to better understand the evolution of Ω. In universe without dark energy / cosmological constant, is Ω constant in time? It would seem as though the answer must be no, and the article seems to suggest that. But what is the mechanism for this evolution? -- Rsholmes 23:46, 22 December 2006 (UTC)
- Okay, I can try to make that clearer - the article is only talking about Ω today. If Ω is less than one, then in the past it was larger (but still less than one), similarly if it's greater than one at the big bang, it gets larger as time goes on. But I'll have to do some math or research to address the issue for generic universes.
- Generally, the density of radiation decreases as the fourth power of the redshift, or the fourth power of the scale factor of the universe. For matter, it's the third power (and this is the easiest to understand, as matter density is just mass/volume, volume is the third power of distance, and distance is just a constant times the scale factor. Curvature, if thought of as having a Ω component, goes as the second power of Ω, and Dark Energy doesn't depend on Ω (assuming w = -1 for Dark Energy, which isn't shown, but is probably true. Nonetheless, everyday astro-ph has a paper or two on w maybe isn't -1, so ... WilyD 16:08, 23 December 2006 (UTC)
- Actually, if you can give me a lot of feedback on what's confusing, I can try to make the article clearer. I understand the flatness problem okay, but I'm not a great communicator without jestures, and I'm not really aware of the sticking points of this, as I've never taught or even TA'd a cosmology course. WilyD 16:12, 23 December 2006 (UTC)
