wrote in
...


: Does your model predict anything about the past CMB
: temperature? This can be measured -- for a very recent
: and apparently quite accurate result, see the preprint
: http://arxiv.org/abs/0804.0116, or for a different method,
: http://arxiv.org/abs/astro-ph/0208027.
:
: Steve Carlip

I would consider the results presented in these papers as anything but
accurate. Most of the measurements presented in the first paper
(Srianand et al.) represent anyway only upper limits, and in addition
they use the COBE measurement of the present temperature to constrain
the data. Without the latter, one could fit the data virtually by any
z-dependence, e.g. with a constant temperature of about 8 K. See for
instance my adaption of the corresponding result from an earlier paper
by Srianand et al. at http://www.plasmaphysics.org.uk/imgs/srianand.gif
(where I have also added the actual error bars to the upper limits). The
new publication merely adds two more data points which hardly manage to
constrain the data any further (as they would both be consistent with a
constant temperature at 8K as well (as are the results of the other
reference)).
So although the data don't rule out an increase of the excitation
temperature with z, they can't confirm it either. This means the
observed excitation might probably not be due to the CMB radiation field
at all but due to other processes (e.g. collisional excitation by
electrons) which simply may have been mis-modelled here.

Thomas