Talk:Flatness problem

Comments
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)


 * 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)


 * 2nd the motion -the diagram with 3 images do nothing to illustrate flatness! Juror1 (talk) 07:54, 29 February 2016 (UTC)


 * The article assumes that one knows what "critical density" is. The link, which points to Friedmann equations is not very helpful. Maybe someone can explain "critical density" in simple terms. —Preceding unsigned comment added by 193.78.112.2 (talk) 15:37, 14 May 2008 (UTC)
 * I've just begun working on a new version of this article. I'll try and explain that (among other things) more clearly. Olaf Davis | Talk 16:31, 14 May 2008 (UTC)

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)

Rewrite
I've just done a complete rewrite of the article I'd been working on in user space for a while. There are still some things left on my to-do list, but it seemed ready for mainspace. Here are some things I'll try and work on - feel free to give a hand!


 * Why physicists dislike the AP - Anthropic Explanations has some on this.
 * Discuss dark energy? Not much to say but it might be nice
 * Include some stuff on recent developments - some of these look promising
 * Add a second diagram including the inflationary era
 * claims it wasn't discovered until 1979 - some further sources to confirm one of the dates would be nice
 * Non-breaking spaces are needed in a few places for equations ✅

Olaf Davis | Talk 11:21, 7 September 2008 (UTC)

My edit to Current value of Ω
Originally, the article stated that: Data from the Wilkinson Microwave Anisotropy Probe (measuring CMB anisotropies) combined with that from the Sloan Digital Sky Survey (observing Ia supernovae) constrain Ω0 to be 1 within 1%. This is misleading because the main contribution of SDSS to measuring the spatial flatness is through the galaxy power spectrum and it is this data that was used in the study cited. Although, as Olaf Davis pointed out in his edit summary, the SDSS telescope has been used more recently to find type-1a supernovae, this data was not used in the study cited. Even if it had been, it would be misleading to single it out for mention, since the Sloan Supernova Survey data form only a part of the overall sample of supernovae with measured redshifts.

I changed the above sentence to read: Data from the Wilkinson Microwave Anisotropy Probe (measuring CMB anisotropies) combined with that from the Sloan Digital Sky Survey and observations of type-Ia supernovae constrain Ω0 to be 1 within 1%. I think this is a more accurate description - at least of the study cited. I'll admit to being out of the loop for the past year though, so if anyone knows of a more recent study in which spatial flatness has been measured to a higher degree of accuracy using only WMAP data and type-1a supernovae, then please update the citation.

Cosmo0 (talk) 22:52, 15 September 2009 (UTC)


 * Good point. I reverted your edit when I saw the summary said the SDSS didn't observe SNae, without thinking to check what the particular citation was about. Sorry! Olaf Davis (talk) 19:09, 16 September 2009 (UTC)

Geometric problem with the diagram
The first diagram that shows three different geometries based on curvature: positive, negative and flat. For each of these geometries, a triangle is shown. The triangle for the negative curvature is wrong. The sum of the degrees for the three angles in a triangle on these three geometric surfaces should be respectively: >180, <180, =180. The triangle shown for the negative curvature erroneously has a angles adding to >180, like the positive curvature triangle. BuzzBloom (talk) 13:34, 17 March 2015 (UTC) BuzzBloom

Mistake and correction
Universal flatness is the law. Other artefacts are secondary. All the data suggest that the universal flatness is fundamental.Dark Matter, Dark Energy and Big Bangs are secondary effects that urge to maintain that permanent flatness. Some people like mistakes because are weird. A mistake is always stupid and against the data though. — Preceding unsigned comment added by 2A02:587:4103:AE00:D47:B687:8172:7D1E (talk) 23:13, 1 July 2016 (UTC)

Copernican Principle
How about a section exploring the possible solutions if we relax the Copernican Principle? — Preceding unsigned comment added by 157.14.234.194 (talk) 11:11, 6 April 2019 (UTC)

Status of article
Just read some old comments about the status of the article and so on. No offense, but if one isn't sure whether Omega is constant in time (in general, it is not), should one even attempt writing such an article?

This is an important problem (or lack of a problem, since current research suggests that it is based on a misunderstanding), but even most people who work broadly within the field of cosmology don't understand it, much less astrophysicists in general, much less scientists in general, much less the general public. — Preceding unsigned comment added by 193.29.81.233 (talk) 11:40, 17 March 2020 (UTC)

Where is the problem?
What I don't understand at this flatness problem is that on one hand the universe seems to be flat today. Most cosmologist would indeed prefer this case. But if it is really flat it must have been so since the big bang because in this case omega needs to be the exact integer 1 and thereby would never deviate from that number (zero deviation from 1 cannot be inflated). If, for instance, the universe was spherically closed since the beginning, the observable universe also tends to increasingly getting flat like the limited surface on an inflating balloon. In that case omega only gets close to 1 but always deviates from 1 by some error. So what's the problem? Is it considered that it is highly improbable that nature started with an exact integer for omega? Is that the problem? Please explain! EternalAsker (talk) 18:31, 20 July 2023 (UTC)