On 3/8/16 2:17 AM, Steve Willner wrote:
Nature doi:10.1038/nature17140
"The dispersion measure and redshift, in combination, provide a
direct measurement of the cosmic density of ionized baryons in the
intergalactic medium of Omega_IGM = 4.9 +/- 1.3 percent"

which happens to be exactly the Concordance value for the total
baryon density. The uncertainty is fairly large, though.

In article ,
"Richard D. Saam" writes:
It is important to note 4.9 percent refers to ionized baryons and
is not an indicator of non ionized baryons, if they exist,

Neutral gas would show up in the 21 cm hydrogen line. I don't know
whether this particular line of sight has been observed, but all the
H I masses I know of are far smaller than 4.9%, i.e, neutral gas
seems to be a trivial fraction of the "baryon budget."
This assumes neutral hydrogen in gas form.
What if the neutral hydrogen were in aggregate or clumped form?
For example:
let neutral hydrogen gas with density 1x10^-24 g/cm^3
(on the order of galactic density)
be in clumps with 1 g/cm^3 density and 3,000 cm diameter,
then the distance between clumps would be 1x10^6 cm
defining a mean free path of 1x10^25 cm
(much greater than the galactic diameter typically of 1x10^22 cm
and rendering the neutral hydrogen clumps optically undetectable
by Beer's law)
The 21 cm line would not detect these 3,000 cm clumps here on earth.
I do not know of any current analytical procedure
to detect neutral hydrogen in such aggregate or clumped form.
One can argue that such neutral hydrogen aggregates
would have to be extremely cold (2.7K) to form
and the present BBN models "baryon budget" do not allow them
beyond the 4.9% ionized form
but the fact is that current analytical methods
(other than gravitational lensing methods)
cannot detect them if they exist.

Richard D Saam