Talk:Lambda-CDM model

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Contents 1 Older comments 2 finiteness 3 100,000 4 Parameters section values 5 Tensor-to-scalar ratio 6 Page name

[edit] Older comments

"This component makes up 26% of the energy density of the present universe. The remaining 4%"

I'm not qualified to fix it, but that sure looks wrong to me...

-Robin

70% are alrady accounted for (Lambda) in the preceding paragraph. --Pjacobi 17:48, 2005 May 4 (UTC)

The latest results from the WMAP probe have estimated these numbers to be, indeed, 74% Dark Energy, 22% Dark Matter, and (still) 4% normal matter. --130.253.135.140 04:57, 5 June 2006 (UTC)

[edit] finiteness

I didn't understand your edits to ΛCDM. It really says nothing about the finiteness of the universe, other than that it is substantially larger than the observable horizon. Can you please discuss on the article's talk page? Thanks –Joke137 19:59, 16 Jun 2005 (UTC)

It is the simplest model that is in agreement with all the observations, apart from the subjective debate concerning whether or not an infinite Universe is simpler than a finite Universe. The model is not a complete model, because it is only local and, if interpreted literally, represents the Universe to be infinite in spatial volume.

The ΛCDM is a Friedmann-Lemaître-Robertson-Walker model. It is a scientific model which assumes that general relativity is correct. If we ignore topology (hmmm we should probably add this as an implicit assumption to the article content), then a flat or hyperbolic model is infinite, and a spherical model is finite, in spatial volume. See shape of the universe for more about possible shapes of a spatial section at constant time in this model.

The ΛCDM model, if interpreted naively (literally) very definitely represents the Universe to be infinite. No serious scientist really tries to overtly claim that the Universe is necessarily infinite, but we agree that it's a property of the model. Please read wikipedia pages on cosmology or any introductory cosmology text on the web or in print.

These are the simplest assumptions for a consistent, physical, local model of cosmology. However, ΛCDM is a model, and it is only a local model.

general relativity is a local theory. It is about the limit (mathematics) towards a point. ΛCDM is fundamentally local, especially in the way stated in which it says nothing about global properties. It makes no claim at all to be a global model. The inflationary extrapolation says that the ΛCDM model concerns just one extremely tiny sphere of space-time, whose radius is the particle horizon, outside of which we have (virtually) no information.

i can explain more if this is unclear. To me this is a very sensitive point, because IMHO it's extremely unfair of scientists to give the false impression that we have a scientific model of everything when really we only have a scientific model out as far as we can go in any objective sense.

BTW, it still remains possible that the ΛCDM model + topology just possibly might be the correct model of the whole Universe and there a bunch of us working on this - see http://adjani.astro.uni.torun.pl/cgi-bin/twiki/view/Cosmo/TopologyMeudon2005March for (limited info on) our most recent conference, but that's independent of the ΛCDM article.

Please put back my corrections, or else ask for more explanations. Certainly i could be wrong (my ability to make errors has been proven beyond the faintest doubt ;), but i would be surprised to be wrong on this particular issue. Boud 11:38, 17 Jun 2005 (UTC)

The size of the universe is a metaphysical problem at best. OTOH the size of our local patch is finite for an universe which is asymptotically de Sitter. No signal from beyond will reach us nor can we signal to any point beyond our patch, provided Einstein locality holds. --Pjacobi 11:52, 2005 Jun 17 (UTC)

Wrong about the first sentence. If the Universe is much larger than the horizon, then the size of the Universe is a metaphysical problem. Certainly, this is a reasonable possibility given present observations, but it is wrong to state at best. However, even if the size of the Universe is a metaphysical problem, it is part of the model. The model is a methematically described model. It is called the Friedmann-Lemaître-Robertson-Walker model and naively or literally interpreted, it is a model for the entire Universe. In this sense, using your terminology, the FLRW class of models are in many cases very metaphysical. We cannot have this both ways. Boud 13:41, 17 Jun 2005 (UTC)

OK, finding an observable "edge of the universe" would make the size question a valid question of physics. And tests, whether there is a wraparound in observable distances have been done.

But whether we should give any signifance to the fact, that the model describes something outside the local patch, isn't that clear to me.

Pjacobi 14:10, 2005 Jun 17 (UTC)

Our obligation to the reader of an encyclopaedia page is to make it clear, correct and avoid misleading the reader. If we fail to mention the fact that the model (interpreted literally - and after all, it is a 'mathematical model, it's not just poetry) describes something outside of the local patch, then we are misleading the reader. If we fail to say that the model is incomplete, we are also misleading the reader. If we suggest that an infinite model is simple, we also mislead the reader (e.g. in a truly infinite universe, the probability of another wikipedia being written somewhere else in the Universe, at some time, with exactly the same sequence of editing steps as here on Earth, except that every occurrence of the word Universe is replaced by pink elephants, is unity: infinity is not simple). Boud 14:55, 17 Jun 2005 (UTC)

You have stated a common misunderstanding about the nature of infinity. Infinite space does not guarantee that all concievable events will occur. Only events which have non-zero probability per unit space are guaranteed to occur. By analogy, consider the real number interval [0,1]. Now pick a number on that interval. Since there are an infinite number of choices, the probability of you picking any particular number is exactly 0, and yet, I am sure you managed to pick one. (Was it 0.2351532?) Furthermore, since the real numbers are uncountable, even if you continued to pick numbers at random for an infinitely long time, there would still be numbers that you would never manage to pick. It is the same thing with the universe. Even if it is infinite in extent, there can still be possible sequences of events which will never happen to occur, and just because something has happened once is no guarantee that it will ever happen again anywhere else in the universe. In other words, just because you picked the number 0.7345312 today does not mean you would ever pick that number again even if you went on picking at random forever. So, in short, you are right, infinity is most definitely not simple, but I wouldn't hold out for the pink elephants. Dragons flight 00:16, Jun 18, 2005 (UTC)

---

I really don't know what to make of this. ΛCDM is a model used to compute, among other things, big bang nucleosynthesis, the cosmic microwave background radiation and the spectrum of the large-scale structure of the cosmos. Any model, so far as I know, that is in clear quantitative agreement with these data is based on general relativity (or very simular theories) and assume a perturbed Friedmann-Lemaître-Robertson-Walker background. ΛCDM happens to be the simplest, because, in addition to fundamental physics parameters, it adds only six parameters to the model. It does this by choosing special values for the others, where it is possible (i.e. a cosmological constant, dark matter has vanishing thermal pressure, etc...). One such choice is that the size of the observable universe is much larger than the particle horizon.

I really don't understand this point about a local model. The only people I know of advocating for a non-local model of physics are the holographic principle people. –Joke137 15:03, 17 Jun 2005 (UTC)

I made another edit. Is this satisfactory? The main point I want to make is that the ΛCDM model is more a physics-inspired mathematical model than a complete physical theory. –Joke137 15:23, 17 Jun 2005 (UTC)

[edit] 100,000

Not to say I understand the rest of it, but the most mysterious phrase in the article is this: "spatial curvature at the level of one part in ten or one hundred thousand". One hundred thousand what? I can kind of picture spatial curvature of 1/10 as the shape of the top of an umbrella in n dimensions, and it probably corresponds to spatial curvature Ωk = 0.086 at the end of the article. But I can't imagine why that picture would be described with the number 100,000, or any other number unrelated to 1/10. Art LaPella 20:45, September 2, 2005 (UTC)

I may have written that part. Ten or one hundred thousand (Ω_k = 10⁻⁴ or 10⁻⁵) corresponds to the level of spatial curvature predicted by cosmic inflation. It comes from the fact that inflation will have generated fluctuations that are much larger than the particle horizon and these show up as perturbations in the spatial curvature for the universe. The numerical value comes from the 10⁻⁵ amplitude of primordial perturbations produced in inflationary models. –Joke137 23:34, 4 September 2005 (UTC)

I think I understand the part I need to. Does that mean you would support this change: "...one part in ten thousand to one hundred thousand"? Or better, "...at the level of 10⁻⁴ to 10⁻⁵" ? Art LaPella 00:04, September 5, 2005 (UTC)

Absolutely. –Joke137 00:11, 5 September 2005 (UTC)

[edit] Parameters section values

Values given in the article do not correspond to values given in WMAP. For example, value for Hubble constant says 69.5 ± 4 here, while in the WMAP it's 71 ± 4. Values for the age of Universe are 13.55 and 13.7 Gyr. Values for Ωb, Ωm, ΩΛ, are also different. Where are they correct? 217.114.151.228 18:48, 13 September 2005 (UTC)

I strongly assume at both places. In the WMAP article there should be the estimates based on WMAP data alone and here the best concordance estimates (taling into account all data). There are quite lot choices to mix the together (and also to choose which parameters should be estimated an which are fixed), so that there are always some disagreements. --Pjacobi 18:57, 13 September 2005 (UTC)

When I added these values I used the values in the Tegmark et al. 2003 paper, which includes both WMAP and the Sloan Digital Sky Survey. The original WMAP paper included WMAP and 2dF GRS, and got slightly different values. The Tegmark values are better for this page, but hopefully we'll be able to update to WMAP 3yr + SDSS in a few months. –Joke137 03:31, 14 September 2005 (UTC)

I suggest that the parameter values used in the article should be those given here http://lambda.gsfc.nasa.gov/product/map/current/params/lcdm_all.cfm as that is the most complete data set (WMAP 3-year, 2dFGRS, BOOMERanG, ACBAR, CBI, VSA, SDSS, SNLS and SN Gold) at this time and has the smallest errors. 128.214.205.44 08:40, 20 March 2006 (UTC)

[edit] Tensor-to-scalar ratio

This term is not explained adequately, I feel, yet a value is given. Does it relate to the text mentioning gravitational waves?

The abbreviation CMB is used without explanation. I assume it is the initials of Cosmic Microwave Background?

203.12.158.34 15:19, 27 September 2005 (UTC)

Where to you see "CMB"? Are you reading an old version on a mirror?

Yes, tensor-to-scalar ration should get some explanation, especially as it is not explained in gravitational wave.

Pjacobi 16:24, 27 September 2005 (UTC)

Hmmm, my bad! I can only find "CMB" on the copy of the Non-Standard Cosmology page I saved last night, when Wikipedia was frequently failing to return anything (server problems I guess). And even there I find that it IS properly explained several paragraphs prior, just not linked back.

Yes, please DO explain "Tensor-to-scalar ratio"! Thanks.

203.12.158.44 07:46, 28 September 2005 (UTC)

[edit] Page name

Should it be at ΛCDM model? Rich Farmbrough 23:30, 11 December 2005 (UTC)

Not without first considering Wikipedia:Naming conventions (use English). Art LaPella 02:48, 12 December 2005 (UTC)

ΛCDM is already a REDIRECT. Should be good enough. --Pjacobi 07:43, 12 December 2005 (UTC)

Should it be "Flat Lambda-CDM model"? If not, then the model should have seven base paramters rather than six, IMO. Spebudmak 06:19, 24 September 2006 (UTC)

No. The ΛCDM model is a minimal model, and has six parameters. –Joke 15:39, 29 September 2006 (UTC)

Well the WMAP papers call it the "Power Law \Lambda CDM" model. I have also seen it called the "Flat Power Law \Lambda CDM" model. I think whether it is "the" minimal model depends on how you define the penalty for adding additional parameters. Take a look at e.g. Table 3 in Spergel et.al. 2006. Spebudmak 18:44, 16 November 2006 (UTC)