Thomas Smid wrote:
In order to
have this coherent scattering one needs at least a density of about
10^18 m^-3, which can with certainty be ruled out for the intergalactic
medium

Just a clarification on this:

With 'coherent' I meant 'phase-coherent' here, not 'frequency-coherent'
as in my later statement regarding the coherency of spectral lines.
'Phase-coherent' has nothing to do with the spectral coherency as it
merely affects the spatial redistribution of the radiation but not the
frequency spectrum. Even an infinitely sharp (i.e. frequency-coherent)
line will be scattered phase-incoherently by randomly distributed
individual scatterers (but still maintains its spectral coherency).
Radiation will only be scattered phase-coherently by a random medium if
the scattering is not due to individual particles but a collective
process. The latter is the case if the average distance between two
scattering particles is less than the wavelength (in this case one has
a 'continuous' medium and the random nature disappears in this
respect). As mentioned above, this happens only at particle densities
higher than about 10^18 m^-3 for the visible region of the spectrum.
For shorter wavelengths the required density would even be higher
(~wavelength^-3).

Thomas