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### [astro-ph/0603647] The Hubble Constant: A Summary of the HST

Posted: February 27 2007
This HST Hubble parameter summary paper gives a lower value than the widely cited Freedman et al paper from 2001, astro-ph/0012376, which found $$h=0.72\pm0.08$$, consistent with the CMB parameter fits of the $$\Lambda$$CDM model.

These authors find $$h=0.62\pm0.05$$. They claim that the difference with Freedman et al is due to the "untenable" treatment of Cepheid distances by the latter. (The authors note that with the improved treatment of Cepheids, Freedman et al would get $$h=0.60$$.)

Sandage and Tammann also have a more recent paper supporting the low value of $$h$$, astro-ph/0608677.

It seems that people have not paid much attention to these results. (For example, the Freedman et al value is still commonly applied as an external prior in CMB analysis.) is there some physics reason for this? The paper appears authoritative and careful. (This of course does not guarantee correctness - and I know nothing about Cepheids.) Is there something wrong with the analysis?

### [astro-ph/0603647] The Hubble Constant: A Summary of the HST

Posted: February 28 2007
h = 0.72 ?
h = 0.62 ?

Why don't we just split the difference and call h = 0.67, which is within the error bars of both values.

Wait a minute, haven't we seen that value before??

Garth

### [astro-ph/0603647] The Hubble Constant: A Summary of the HST

Posted: February 28 2007
I found this strange (from the conclusions):
The constancy of H0 from global cosmic scales down to 4.4Mpc or even 2.5Mpc (see also Ekholm et al. 2001) in spite of the inhomogeneous mass distribution requires a special agent. Vacuum energy as the solution has been proposed by several authors (e.g. Baryshev et al. 2001; Chernin 2001; Chernin et al. 2003a,b; Thim et al. 2003; Teerikorpi et al. 2005). No viable alternative to vacuum energy is known at present. The quietness of the Hubble flow lends support for the existence of vacuum energy.
What is going on here? I was left unsure if the authors were claiming that, the predictions of linear theory for the velocity field on largeish (e.g., 30 Mpc) scales were being called into question.

I took a quick look at Baryshev's paper from 2001, and do not yet understand the claims. It seems actually to be a rather subtle thing (as far as I can tell) that is being claimed: that velocity dispersions in the "translinear" regime (< 10 Mpc) are lower than "expected" (relative to what, I am not yet sure.)

But there are two questions here:

1. is the dispersion of the measured Hubble constant at a particular distance lower than expected?

2. are there systematic deviations due to coherent velocity flows that show up as a distance-dependent H_0? ("Hubble bubble", etc.)

Baryshev's paper seems to address #1 (which talks about velocity dispersions on the outskirts of the local group -- do we really understand the velocity profile of clusters well enough to say this is a problem yet?), whereas the conclusion of the HST paper seems to address #2 -- and #2 should really be something that we can address with linear theory. Is the apparent constancy of H_0 given the HST error bars consistent with what you would expect from the standard LCDM?

Anyway, sorry if this is a somewhat confused comment. Just poking around in the paper and this caught my eye.

### [astro-ph/0603647] The Hubble Constant: A Summary of the HST

Posted: March 01 2007
One can separate two things: the coincidence of the local mean and global Hubble parameters, and the small dispersion around the local mean. The latter issue is known as the "coldness" of the local Hubble flow.

How much of a problem the dispersion is, I don't know (see astro-ph/0107104, astro-ph/0201524, astro-ph/0212538, astro-ph/0308415, astro-ph/0412583, astro-ph/0603226 for analyses). However, the agreement between the global and local values is certainly surprising. Given that the scale of homogeneity is at least 70-100 $$h^{-1}$$ Mpc, having the global Hubble parameter at already 4 Mpc seems strange.

Sandage et al (and Baryshev, Chernin and Teerikorpi, astro-ph/0011528, astro-ph/0603226) advocate that the coldness is local evidence for vacuum energy. However, it's not clear to me whether the coldness could not be due to some peculiarity of the local region. (See e.g. the conference proceedings by van de Weygaert and Hoffman, http://adsabs.harvard.edu/cgi-bin/nph-b ... c12bf05039 .)

At any rate, the theoretical explanation of this problem is separate from the issue of the observational global value of the Hubble parameter.

### [astro-ph/0603647] The Hubble Constant: A Summary of the HST

Posted: March 01 2007
I guess what I don't understand about the problem here is "cold relative to what" -- what about our current understanding about how velocities work on 4-30 Mpc scales fails? Yes, the distribution of matter is getting very inhomogenous on these scales, but is it sufficiently inhomogenous to require some new homogenizing mechanism for velocities?

Just browsing the papers on this subject -- including Sandage's remarks -- I haven't seen where someone goes through the "standard" picture to show that the constancy of the Hubble parameter given the error bars is unusual.

For example, we've been doing velocity-mass relationship studies for many years (POTENT) -- did those studies discover anything unusual about the mass-velocity relationship?

I totally get that this is separate from the determination of the global H_0, though -- just taking the discussion in a different direction here.

### Re: [astro-ph/0603647] The Hubble Constant: A Summary of the

Posted: March 01 2007
Simon DeDeo wrote:Just browsing the papers on this subject -- including Sandage's remarks -- I haven't seen where someone goes through the "standard" picture to show that the constancy of the Hubble parameter given the error bars is unusual.
You're right - people have looked at the velocity dispersion for different models (both for the background cosmology and the local environments), but I haven't seen similar work on the Hubble parameter.

Naively the agreement of the Hubble rates seems surprising, because there are regions of a few Mpc in size which are collapsing, and 20 Mpc Mpc underdense regions which expand faster than the average (and occupy a large fraction of the universe), so one would expect the variance on Mpc scales to be large. (Also, the galaxy distribution is very inhomogeneous on scales below 100 Mpc or so, astro-ph/0411197, astro-ph/0406202, astro-ph/0501583, so one would expect the same from the expansion rate.)

So happening to be located in a region around which things are very smooth seems surprising. But I don't know whether anyone has actually calculated the odds.