Talk:Friedmann–Lemaître–Robertson–Walker metric

message: There's been a merge tag on Einstein's radius of the universe to merge it here. They don't seem related but I'm not a physicist. Does anyone know if they should be merged, or should the tag be removed? Tocharianne 16:23, 14 July 2007 (UTC)
thread_title:  Merge with [[Einstein's radius of the universe]]? 

message: It looks to me as though Lemaitre metric and Lemaitre-Tolman metric ought to be merged here.  Or if it's not appropriate to merge them, then at least those two articles should discuss the relationship between the various models, along the lines of the discussion in section 4 of this article. Gdr 14:39, 28 November 2007 (UTC)
thread_title:  Some possible merges 

message: Aren't two Naviagational templates too much in an article ? I propose to remove the GR template, as I did already one month ago !(Sheliak (talk) 15:35, 7 February 2008 (UTC)
thread_title:  Two Naviagtion templates 

message: Why is there no criticisms section in this article? That seems horribly peculiar. 66.69.194.16 (talk) 15:18, 3 June 2008 (UTC)
replies: text: The article is entirely about the mathematical nature of the metric itself and not about its physical interpretations. It mentions that it is the basis for the big bang cosmology only in passing; were this in fact an article about the FLRW cosmologies with physical interpretation, then one would expect opinions about interpretation, while this page gives none. When you assume homogeneity and isotropicity, you always get the FLRW metric - it also occurs as the solution to a collapsing sphere of relativistic dust; I am not sure if that is mentioned on the page. Atonita (talk) 17:04, 1 December 2008 (UTC)
replies: text: In response to this concern raised by 3 June 2008, I have written an abstract referring to a contribution under construction that is meant to be a new free standing article or else a segment to be appended to the FLRW Metric article or maybe to the Friedman Eqautions or perhaps to both. This proposed contribution has the working title “Critique of the Universe”.
In the full text contribution, also available on my Talk Page, the references are numbered in “belt and suspenders” duplicate style because, at present, some of them contain a comment as well as a reference or else the reference is incomplete – being only a reference to Wikipedia itself – so the citation would need to be edited and upgraded. I hope I can attract some editors who might like to do this. 
As 3 June 2008 said of the FLRW Metric, both of the articles are defective in that they contain no criticisms, no caveats, no cautionary dicta. They lack a critique. This violates Wikipedia’s own NPOV policy because, to the reader, it appears as if Wikipedia sanctions or tacitly accepts the statements and conclusions given in these articles as scientific fact, when they are both just hypotheses. 
As an editor pointed out, the FLRW metric is indeed just a mathematical object but, there are alternate mathematical expressions that do not require the same assumptions and do not lead to the same derived set of equations as a model. So, a caveat or two would be in order. Just because it is itself mathematically internally logical, does not mean that the math must actually make sense, particularly with regard to the required assumptions.  
See my full text proposed contribution and help edit it at:
Kentgen1 (talk) 22:16, 29 October 2010 (UTC)
thread_title:  Criticisms 

message: To : You revised several parts of the article and in doing so put in a reference to the Sinc function with which I have only a slight familiarity. Could you please explain how you get that
:r \; \mathrm{sinc} \, (r \sqrt{k}) = \sqrt{|k|}^{-1} \sinh (r \sqrt{|k|}) \,
when k < 0. Thank you. JRSpriggs (talk) 05:48, 29 November 2008 (UTC)
replies: text: FWIW this seems to me to be a non-standard extension of the meaning of "sinc". I like the rest of the changes made by BenRG but I think we should remove reference to sinc. PaddyLeahy (talk) 16:46, 1 December 2008 (UTC)
replies: text: It's completely standard mathematically (sinc is defined over the whole complex plane), but I'll admit I've never seen anyone write the FLRW metric using sinc. Is the rewrite okay? -- BenRG (talk) 20:14, 1 December 2008 (UTC)

text: Here's one way to derive it: \frac{\sinh (r \sqrt{|k|})}{\sqrt{|k|}} = \frac{e^{r \sqrt{|k|}} - e^{-r \sqrt{|k|}}}{2 \sqrt{|k|}} = r \frac{e^{i(ir \sqrt{|k|})} - e^{-i(ir \sqrt{|k|})}}{2i(ir \sqrt{|k|})} = r \frac{\sin (ir \sqrt{|k|})}{ir \sqrt{|k|}} = r \; \mathrm{sinc} \, (ir \sqrt{|k|}) = r \; \mathrm{sinc} \, (r \sqrt{-|k|}). -- BenRG (talk) 20:14, 1 December 2008 (UTC)
replies: text: To BenRG: Thank you for your explanation and for improving the article. JRSpriggs (talk) 07:53, 2 December 2008 (UTC)

text: Ditto. I do prefer it this way even though you are right about sinc...writing r sinc(r) for sin(r) is not very transparent for those of us of little mathematical brain and the extension to implicitly imaginary arguments was just plain confusing, as my comment above showed! PaddyLeahy (talk) 20:22, 2 December 2008 (UTC)
thread_title:  The sinc function 

message: I have just replaced the false statement "the FLRW metric is the only one on a Lorentzian manifold that is both homogeneous and isotropic" with "the FLRW metric is the only one on a spacetime that is spatially homogeneous and isotropic."  Metrics on homogenous/isotropic (maximally symmetric) 4D Lorentzian manifolds are the metrics of de Sitter space, anti-de Sitter space, and Minkowski space.  In the FLRW model, only space is maximally symmetric, not spacetime. —Preceding unsigned comment added by 128.36.149.21 (talk • contribs)  19:57, 1 December 2008 (UTC)
thread_title:  Note: homogeneity/isotropy applies to space, not spacetime 

message: The equations use undefined terms: sigma, omega, etc.
These require explanation. If we're going to assume that all of the readers of the article are already familiar with the topic then there's no point in having the article in the first place. It also needs work from someone who can provide a physical description. I don't know the topic well enough to fix this page but I know enough to know that this page is seriously incomplete.  Michael McGinnis (talk) 17:58, 9 September 2010 (UTC)
replies: text: They are defined. Just read the article straight through. JRSpriggs (talk) 09:15, 10 September 2010 (UTC)
thread_title:  INCOMPLETE EXPLANATIONS: The equations need work 

message: I think there's an error in the second equation in the "Solutions" section of the article. I was working this metric (k, lambda=0) with Mathematica and it tells me the second equation should be
$$-2a \ddot a -\dot a^2=8\pi G_N p$$
which differs by an overall factor of $a^2$ on the left-hand side.  — Preceding unsigned comment added by 76.20.12.36 (talk) 08:43, 9 June 2011 (UTC)
replies: text: Could you explain that in plain mathematics and English without using the crutch of Mathematica? JRSpriggs (talk) 10:04, 9 June 2011 (UTC)
replies: text: Apparently not!  67.162.165.126 (talk) 05:10, 20 May 2013 (UTC)
thread_title:  Error in Field Eqns? 

message: In the section entitled "Solutions", p is not defined. 
  — Preceding unsigned comment added by 190.46.106.200 (talk) 20:10, 26 May 2014 (UTC)
replies: text: p is the pressure, a part of the Stress–energy tensor#Stress–energy of a fluid in equilibrium, as one could discover by following any of several links in the article. JRSpriggs (talk) 02:12, 27 May 2014 (UTC)
thread_title:  Equations of the solution 

message: In the intro it says: "This model is sometimes called the Standard Model of modern cosmology", yet the Lambda-CDM model article say it is "...frequently referred to as the standard model of Big Bang cosmology". I imagine that this is just a matter of some small clarification required.Snori (talk)
:This is not a contradiction, because the Lambda-CDM model is a special case of the Friedmann–Lemaître–Robertson–Walker metric. Everything said here about FLRW also applies to Λ-CDM. The difference is that Λ-CDM gives a more specific relationship between the pressure and the density. So there are versions of FLRW which are not versions of Λ-CDM, but they are non-standard. JRSpriggs (talk) 17:31, 10 July 2015 (UTC)
thread_title: Standard model

message: :\mathrm{d}\mathbf{\Sigma}^2 = \frac{\mathrm{d}r^2}{1-k r^2} + r^2 \mathrm{d}\mathbf{\Omega}^2
it is not possible to add or subtract 1 and kr²[m²] Ra-raisch (talk) 00:17, 1 January 2016 (UTC)
replies: text: Read the paragraph below the equation. It explains what the units of the various constants and variables are. You will see that they are consistent. JRSpriggs (talk) 14:23, 1 January 2016 (UTC)
thread_title:  wrong units 

message: made a correction to Friedmann–Lemaître–Robertson–Walker metric#Spherical coordinates by changing the sign from negative to positive. He was right, but the reason he gave was wrong. The negative sign was not due to an erroneous calculation in the source; rather it is due to the fact that we have chosen the (-+++) signature for the metric while the source was using the (+---) signature. Specifically, the change of the sign of the metric does not affect the Christoffel symbols nor the Ricci tensor, but it does change the sign of the Ricci scalar. JRSpriggs (talk) 02:32, 24 December 2016 (UTC)
thread_title:  Sign of Ricci scalar 

message: I'm pretty sure there's a factor of 1/a missing in the spatial components of the Ricci tensor. What currently reads
: R_{xx}= R_{yy} = R_{zz} = c^{-2} (a \ddot{a} + 2 \dot{a}^2) 
should probably be
: R_{xx}= R_{yy} = R_{zz} = c^{-2} \frac{a \ddot{a} + 2 \dot{a}^2}{a} \ . 
Otherwise, the Ricci scalar does not come out correctly.
Julian.eicher (talk) 21:05, 29 December 2021 (UTC)
thread_title:  Possible mistake in section "Curvature" under "Cartesian coordinates"