🚀 go-pugleaf

metzler@pablo.physics.lsa.umich.edu (Chris Metzler) writes:
>It is true estimates of Omega_o from cluster galaxy dynamics or from
>cluster X-ray emission modelling generally result in values smaller than one.
>However, it has been shown numerous times, through N-body and hydrodynamical
>simulations, that both methods of estimating Omega_o are based on serious
>assumptions that affect the analysis.  

Yes, but most people acknowledge these estimates as lower bounds.
I guess I'd phrase it differently: I figure the assumptions are
probably true, but couldn't set upper bounds on Omega_0, regardless.
They're consistent with Omega_0 = 1 if you assume some kind of
biasing, which you have to assume anyway.  The arguments for Omega_0 = 1
are of a more global variety (eg horizon problem; structure formation,
which you outlined nicely, thanks!)

> [ dynamics <-> substructure problem ]
> [ x-rays <-> thermalization assumed ]

>Both of these are very questionable.  In particular, the so-called "beta
>discrepancy" suggests strongly that cluster gas has not thermalized since
>infall.

The substructure problem is well recognized.  Not that that stops
anybody.  For X-rays, the situation is more complex, because the beta
problem (brief summary: extended distribution of gas implies that
the gas should have a higher specific energy than the galactic matter,
but this is not observed, in eg Perseus cluster) isn't always a problem.  
In my hot little hands I have a paper (Watt et al MNRAS 258, 738) which gives 
references saying that there is no beta problem for the Coma cluster.

>N-body, and N-body + hydrodynamic simulations have shown several times that
>such models underestimate cluster masses.  For example, Evrard (1990) showed
>results for clusters grown in an Omega_o = 1 cosmology which, when virial
>and binding mass estimates were applied to the final cluster, showed an 
>apparent Omega_o = 0.2.

This is the key.  I just heard a talk by David Weinberg in which he said the
same thing (paper in newest Ap J Lett).  Basically it comes down to
this: when you talk about dark matter, you can never rule out that
there's more of it on the larger length scales.