🚀 go-pugleaf

In article <C15vrI.6yp@well.sf.ca.us> metares@well.sf.ca.us (Tom Van Flandern) writes:
>
>     The hypothesis that the initial explosion was an explosion of
>spacetime and not of matter is a big bang hypothesis, not part of GR.

This is the basic problem with your argument, Tom --- this statement
is not really right.  (At least, my interpretation of your interpre-
tation of this statement is that you're wrong.)  Your criticism of
ad hoc assumptions seems to me to come from a confusion between
what is assumed and what is derived.

General relativity gives a set of field equations that determine the
geometry of spacetime (curvature *and* expansion or contraction) from
the matter distribution.  If you assume a homogeneous, isotropic
distribution of matter with standard relationships between density
and pressure, you *derive* Friedman-Robertson-Walker cosmology,
with a *predicted* relationship between the expansion rate of the
universe and the density of matter and radiation.

If, on the other hand, you assume an *approximately* homogeneous and
isotropic distribution, with local areas of higher or lower density,
you can again solve the field equations (at least in a systematic
approximation) and look at the predictions.  In this case, you find
an average expansion rate, but you can also look at the detailed
behavior of regions with higher or lower densities.  I have not
done these calculations myself, but I have no reason to doubt Ethan
Vishniac's statement that when this is done, you find that dense
enough subsystems do not participate in the expansion.  In any case,
this is something that can be checked by looking at the predictions
of general relativity with the appropriate matter distributions.  
It is *not* anything new that needs to be brought in from the outside.

I gues I don't really understand why this is confusing.  In general
relativity, space doesn't just expand because it wants to; the
expansion is the (predicted) response of spacetime geometry to the
presence of matter.  If you change the matter distribution, why
shouldn't you expect the expansion to change?  

Steve Carlip
carlip@dirac.ucdavis.edu