Conceptual Problems IV

On Dec 6, 9:00 am, (Phillip Helbig---
remove CLOTHES to reply) wrote:
In article , Knecht

I would welcome a clear and succinct synopsis of your reasoning for
the contention that these two phenomena are unrelated. I am wondering
if there might be some disparity in our starting assumptions.

Special relativity says that the laws of physics have the same form in
all frames of reference, whatever their relative state of motion. This
holds whether or not there is some frame---such as that of the
CMB---which is special in some sense. The laws of physics look the same
in it and in frames moving relative to it.

General relativity and cosmology are of course related. Relativistic
cosmology usually assumes that the universe is homogeneous on a large
enough scale. Perhaps there is now some evidence that we have
underestimated this scale, but that is a quantitative problem, not a
qualitative one.

Let me put it this way: what do you see as the influence of one on the
other?-

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.

To me the drumbeat of deviations from homogeneity (in distributions,
flows and anisotropies) which have appeared at 20 Mpc, then 50 Mpc,
then 100 Mpc, and now = or 1,000 Mpc over the last 3 decades is very
much related to which relativistic models we chose (there are
inhomogeneous models) and whether or not there is a universal frame of
some sort that violates the foundations of general relativity (which I
strongly doubt, but only on grounds of natural philosophy).

Maybe we would do better to take our assumptions with a larger grain
of salt and focus more on what can be said with confidence on the
basis of empirical evidence?

Yours in science,
Knecht
www.amherst.edu/~rloldershaw

In article , Knecht
writes:

Let me put it this way: what do you see as the influence of one on the
other?-

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.) There is the CMB, there is the distribution of
extragalactic radio sources (visible at very large distances), there is
the distribution of normal galaxies. This is not to say that there is
NO structure; indeed, large-scale structure is a very active field. But,
on a scale less than that of the observable universe, there is
homogeneity. These observations remain even if there are others which
indicate inhomogeneity. Thus, any explanation has to account for both
(assuming all observations are correct).

To me the drumbeat of deviations from homogeneity (in distributions,
flows and anisotropies) which have appeared at 20 Mpc, then 50 Mpc,
then 100 Mpc, and now = or 1,000 Mpc over the last 3 decades is very
much related to which relativistic models we chose (there are
inhomogeneous models) and whether or not there is a universal frame of
some sort that violates the foundations of general relativity (which I
strongly doubt, but only on grounds of natural philosophy).

Can you elabourate on how a universal frame (assuming it exists) causes
deviations from homogeneity?

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