On 25 Jun, 14:05, Chalky wrote:
On Jun 23, 9:17 am, George Dishman wrote:

"jacob navia" wrote in message

...
What physical evidence we have that the CMB is not just
radiation coming from a diluted hydrogen cloud whose mean
temperature is 2.7K

I assume here the implied qualification: "post redshift"

I think Jacob meant the actual temperature of the gas.

and whose radius is (say) 20 billion
years?

The evidence is the observed Hubble red shift. Take
the cloud and divide it into thin spherical shells.
We see some light from each shell but more distant
shells are red shifted to a greater degree. If the
cloud is at a uniform temperature, we should see
the integral of the redshifted curves. Ned Wright
produced a similar curve based on the CMBR being
red shifted starlight to illustrate the problem:

http://www.astro.ucla.edu/~wright/Stolmar_Errors.html

And if we were in the middle of such a HUGE cloud?

Yes, we would have to be at the centre, but the
model only works if the temperature is proportional
to the radius from us so that the redshifted
temperature is 2.7K for all shells.

Absolutely. This is the crux of the matter.

The cloud has
to have a spherically symmetric temperature profile

And what is wrong with that? The same is true, in essence, within
classical BB theory, for increasing z, no matter what cosmological
parameters you choose.

The difference is that Jacob was proposing a
gas cloud as an alternative to expansion,
essentially a steady-state cosmology.

with the lowest temperature of 0K at the centre,

Why?

The Hubble Law, redshift is proportional to
distance. If properly thought through it would
be more difficult because there are extra
factors of (1+z) in the standard model compared
to a steady state model but the point is that
a very specific temperature profile is needed
if we are to measure a Planck curve in all
directions. The dipole moment might be
explainable as uot position being offset from
the centre but even that might not work out.

exactly where our galaxy is situated, but the
temperature 13.7 billion light years away is 3000K.

And that would be just *one* of the explanations for the
CMB that doesn't require a big bang or a "beginning".

Why is this so hard for some people to accept?

xanthian.

Yes, why would that be so difficult to accept?

It is difficult to accept because there is no good
explanation for how a cloud could maintain such a
strange temperature profile without the gas at the
centre being warmed to equilibrium,

Expansion and accelerating expansion potentially account for that.

It is a given of Jacob's suggestion that there is no
expansion since he is offering it as an alternative.

Otherwise you are essentially re-introducing Olber's paradox at a
lower EM frequency range.

Yes, the cloud should reach equilibrium given
sufficient time, or alternatively the cloud
should cool from the outside and we should see
a high temperature here in the middle further
exagerrated by the redshifting of the radiation
from the distant edges of the cloud if they are
observable.

Incidentally, what, precisely, is the z/T relationship predicted by
EFE? (I honestly don't remember)

The intensity is proportional to (1+z)^4 which
maintains the black body curve. Of those, some
of the factors come from relativistic effects
but offhand I can't remember either. Look up
the Tolman Test for more on that, I think Ned
Wright had a page mentioning it.

and personally
I find the idea that the universe is centred on the
Earth even more distasteful than a beginning with a
homogenous and isotropic universe.

AHA! Back to the foundations of natural philosophy, I am pleased to
see!

Just pointing out that argument from philosophy
wasn't in favour of Jacob's proposal any more
than the Big Bang.

It has been pointed out (by prior authors) that Einstein's
general principle (also attributed {by some} to Galileo), puts the cat
amongst the pigeons in this respect. Stated pregeometrically, all
bodies of ref. are equiv. for formulating general laws of nature.
Hence coordinates centred around the observer are as good as any
other.

Or to state that the other way round, all other
coordinates are as good as the observer's.

Thus, the pre-Copernican and Copernican approaches are
equivalently valid in this relativistic respect. This is the "sting in
the tail" of GR theory, and greater authors than me have made this
point before.

What was that point again? ;-)

George