Addressing Impossibilities in the Standard Cosmological Model

[Nereid]

Perhaps this thread seems disjointed because there are posts in the GD thread from which it was split that were not moved over?

In any case, what are we discussing now - the 'impossibility' of concordance cosmology wrt (non-baryonic) DM? If so, in what way is a discussion of DM in spiral galaxies relevant? After all, there's so little of the universe's (apparent) mass in spirals that however 'spiral galaxy rotation curves' gets explained, it will have but a trivial impact on cosmology, surely?

I mean, aren't the key (cosmological) observations those (SZE, lensing, X-ray, Zwicky+Virial Theorem, ...) of rich clusters?

 

[Garth]

I think that second abstract answers your question.
The number you quote is actually \Omega_{b,g}, the total baryonic content of galaxies. The reason I say there isn't a concordance value for \Omega_{M,observed}, or equivalently, \Omega_{b, observed}, is that a large fraction of the baryonic content of the universe is thought to lie in gas in between galaxies. There have been some crude measurements of this that indicate that most of the remaining baryonic mass lies in the WHIM (Warm-Hot Intergalactic Medium). Here's one such reference:

http://lanl.arxiv.org/abs/astro-ph/0501126"

Thank you ST, that was what I wanted. My confusion has been exactly how much WHIM has been detected. It was a little difficult to determine exactly what limits on the WHIM density your last link paper was stating:

This gives a combined \Omega_b estimate
\Omega_b^WHIM (≥ 7 × 10¹⁴) = (2.4^+1.9_−1.1) × 10^−[O/H]−1 %, consistent with both model predictions and the actual number of missing baryons.

So we need [O/H]; now earlier in Table 1 they state at:
z = 0.011 [O/H] > -1.47 and
z = 0.027 [O/H] > -1.32,
so am I correct in reading the upper limit:
\Omega_b^WHIM > 4.3 × 10^0.47 % = 12.6%
and the lower limit:
\Omega_b^WHIM > 1.3 × 10^0.32 % = 2.7%?
Which indeed is consistent with about 4% cosmic baryon density, but also with a much higher \Omega_b if BBN allows.

So at least, 7.5% of the baryon content is in the form we are familiar with, galaxies etc., and 92.5% in the inter-galactic medium, there is a lot of 'stuff' out there!

But if we now combine this with Chronos' link in an earlier thread: You Can't Get There From Here: Hubble Relaxation in the Local Volume

Imposing an end condition on the visible matter in the Local Volume has revealed a problem with the currently-visible structures: they are moving too slowly and often in the wrong direction for the observed peculiar velocities to have produced them from a smooth distribution at the time of recombination. Setting limits (rather than performing detailed calculations), it has been found that no known dynamical process explains the deficit.
The most straightforward explanation is that much or most of the baryonic matter in the Volume is in the form of ionized gas, or for some other reason not now presently detectable; and that it is distributed largely in the places where galaxies are not. In that case it might be observed soon, as X-ray technology improves.
This is not the only possible explanation, however. Some previously unsuspected way of modifying peculiar velocities might be at work, slowing everything down greatly. This solution, though, is only speculative.

So perhaps the WHIM is not too far away after all? Although do we not now have to explain why it is not observed?

(Note: I have continued a discussion on DM in SCC in the Self Creation Cosmology thread.)

Garth

 

[SpaceTiger]

So perhaps the WHIM is not too far away after all?

Sure, I would certainly expect some WHIM within 10 Mpc of the sun. We're not in a dense cluster, so any local hot gas might have a low enough emissivity that it wouldn't be observed.

Although do we not now have to explain why it is not observed?

Is this not explained in the paper you linked?

In principle the gas might be detectable in X-rays, in emission or absorption. Some galaxy groups of quite modest mass (less than 100 km s−1 dispersion) have an observed X-ray flux (Helsdon, Ponman & Mulchaey 2005), though none are in the Local Volume, and
for those the inferred gas mass fraction is on the order of 10%. It is unclear how much gas would be necessary to explain the peculiar velocity deficit (there is no obvious way to run the “treacle” procedure with an unknown additional field of matter), but certainly it would be several times that of the (visible-light) luminous matter. It might soon be possible to detect much more tenuous material (Yoshikawa et al. 2003).

 

[Garth]

Although do we not now have to explain why it is not observed?

Is this not explained in the paper you linked?

Agreed - so it is just a matter of waiting for better X-ray detectors.

Garth