Phillip Helbig---remove CLOTHES to reply wrote:
In article , Knecht
writes:

[...]
Currently some cosmologists approve of the assumption that the
Universe is homogeneous, if not within the observable universe, then
on larger scales, or somewhere over the next horizon.

Another group of cosmologists have begun to have very serious doubts
about the usual assumptions of homogeneity and isotropy, which were
instituted more for mathematical simplicity than on the basis of well-
grounded physical argument.

Originally, yes. However, we now have many observations indicating
homogeneity. (Actually, we observed from only one place, so what we
observe is isotropy, but that implies homogeneity unless we are in a
special position.)

We can actually do a bit better. We can observe the CMBR temperature at
distant locations, both by the SZ effect and by looking at CMBR excitation
of low-energy atomic transitions. The agreement with standard predictions
-- in particular, the red shift dependence -- puts some useful limits on CMBR
anisotropy far from the Earth: if the CMBR were very anisotropic, it would
not behave as a black body spectrum with a constant temperature, and this
could be observed. The limits are not yet very strong, but they exist.

There's a very nice discussion of this in a paper by Jeremy Goodman,
astro-ph/9506068. A more recent proposal for future observations that can
test homogeneity is by Clarkson et al., arXiv:0712.3457.

Steve Carlip