Syksy Rasanen wrote:Talking about Einstein-de Sitter... This paper claims that observations of cluster counts prefer a high \Omega_m as opposed to \Lambda plus low \Omega_m (no statistical significance is given), and that this conclusion is consistent with all previous analyses of X-ray selected samples performed with the same methodology.

Can someone who knows about clusters enlighten me as to what is the general opinion in the field about this? Is there something clearly wrong with these results, do other analyses lead to different conclusions, is there controversy?

Well, on this particular paper, i'll quote my own response in our internal Toruń cosmology group:

http://cosmo.torun.pl/pipermail/cosmo-t ... 00180.html

"Omega_matter = 1 (was Re: gas question)"

http://www.astro.uni.torun.pl/sympa/sha ... 00005.html

The article by Blanchard et al:

http://arxiv.org/abs/astro-ph/0311381
Look carefully at the figures. The triple-dotted-dashed line is the

model for an (Om_m=0.3, Om_Lambda=0.7). It's hard to find, but once you

find it, you'll see it matches the observations very, very nicely.

According to the authors, this triple-dotted-dashed line is for eq.~(6),

which replaces eq.~(2). The difference between the dtwo eqns is a factor

of (1+z).

The authors' derivation of eq.~(2), in other papers, obtains this (1+z)

factor from the density of the Universe at the time that the cluster

virialises. So unless you throw out rho = rho_0 (1+z)^3 , it's hard

to change the derivation.

However, if you just look at another derivation of the equivalent of

eq.~(2), i.e. a T-M relation, and look at the z evolution of,

e.g. equation (73) of

Niayesh Afshordi, Renyue Cen

http://arxiv.org/abs/astro-ph/0105020
then the (1+z) factor becomes essentially (1+ not much) over the

relevant redshift range.

In other words, Blanchard et al's figures, with Afshordi & Cen's

version of the T-M relation, give the concordance model.

Again in simpler words, Blanchard et al think that clusters form

in one way, Afshordi & Cen think they form in a somewhat different way,

leading to moderately different T-M relations and hence totally different

local curvature parameter inferences.

It's good to have dissidents around :). Sometimes they're right, sometimes they're wrong.

In answer to your question: do Afshordi & Cen better understand the T-M relation than Blanchard et al? This I don't know, my physical intuition here is lacking.

BTW: Alain Blanchard, at least, is continuing to defend this claim:

astro-ph/0502220
astro-ph/0503426