Dear moderators, Thanks for moderating this newsgroup!
Sorry to bug you, but here is a version of a post I just submitted,
with two spelling mistakes corrected.

Thomas, I have linked to your page
http://www.plasmaphysics.org.uk/research/redshift.htm as a
plasma redshift theory along with Ari Brynjolfsson's and mine.

The basis of your theory, as I understand it, is that a
wavefront of light (or microwaves etc.) which has a coherence length
shorter than the average inter-particle spacing will be stretched by
the electric field between those particles. I can't see exactly how
that would occur, but I think it warrants consideration.

Lets think of a left-to-right travelling wavefront as having energy
and momentum - and therefore mass - distributed along it. (With a
short enough wavelength the wavefront can congeal into an electron-
positron pair. Likewise, a flashlight in a spaceship loses mass to
the light beam it creates and the light beam deposits energy and
therefore mass on whatever part of the spaceship absorbs it, but the
total mass of the spaceship remains the same.)

The question is whether the electric field between a particle A, to
the left, which the wavefront is receding from and particle B, which
is is moving towards, can stretch the wavefront. Particle A has more
of an effect on the trailing end of the wavefront and B has more of
an effect on the leading edge. All we need is a mechanism by which
individual parts of the wavefront are attracted to a charged particle
of either polarity. Not much of an effect is required to explain the
cosmological redshift - one part in 13 billion or so per year the
light travels in the inter-cluster medium.

Maybe it is not an electrical attraction, but a gravitational one.

Whatever the nature of the stretching process, it would need to be
shown that the wavefront wasn't similarly compressed to the same
degree when it has one or more particles in its middle.

Such a stretching, redshifting, process would be subject to various
challenges, such as whether it would predict sideways scattering of
the light to a degree greater than that which is observed.

I disagree with your statement that the coherence length of the light
from stars is 100 microns. I estimate that an impulse which has the
spectral characteristics of the Sun's black body light would have
most (say 90%) of its energy in about 2 to 4 microns. The peak
energy is at about 0.5 microns. The coherence length of the
emission and absorption lines would be much longer than this.

I agree in broad principle with the notion of emr being redshifted
pervasively in the inter-galactic or inter-cluster medium by some
kind of plasma redshift process until it attains a wavelength or
coherence length which prevents further redshifting. How well that
would explain the CMB, I am not sure. When thinking about the CMB
and plasma redshift, the the Sunyaev-Zeldovich Effect may be worth
bearing in mind. This involves the CMB seeming to be slightly
shorter wavelength when looking towards galaxy clusters, supposedly
due to CMB being altered by the inter-cluster medium.
(http://www.astro.ucla.edu/~wright/distance.htm#SZ). I tentatively
suggest that this could be explained by the CMB emanating from the
cluster (by whatever mechanism) being not plasma redshifted at that
point compared to the CMB from more distant galaxies having been
plasma redshifted by its passage through a greater distance of
inter-cluster medium.

- Robin http://astroneu.com