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Re: 3 K Background Radiation



Practically, yes. In principle, not necessarily. You must make the
assumption that the expansion of the universe is isotropic, which it
seems to be. However, that is an assumption built in to the
Friedmann-Lemaitre-Robertson-Walker model, not a physical
requirement. You can make anisotropic cosmological models perfectly
consistent with general relativity but they behave in a very bizarre
fashion.

Also, defining a center of mass for the universe is somewhat
problematical since isotropy implies the nonexistence of boundaries
-- all points are physically equivalent. Think of trying to find the
center of mass of a two dimensional surface. For a finite plane, it
is easy but for an infinite plane (a 2 dimensional analogy to an
asymptotically open universe), where is the c.o.m.? If you distort it
into a spherical topology (imagine cutting a very thin shell from the
surface of the Earth), you have the same problem. You might argue
that it is the center of the sphere still but suppose your coordinate
system is restricted to exist only on the surface (a 2 dimensional
analogy to a closed universe). Where on the surface is the center of
mass? You can see that this presents problems and they all flow from
the requirement that all points be equivalent.

The original argument, supposing a gas of particles flying into a
preexisting geometry for which their gravitational effects are
negligible is not a very good model for thinking about the Hubble
expansion (though it was fine as an analogy for thinking about a
description from different reference frames). Galaxies don't fly
out into space. Rather, the geometry changes and the galaxies
just surf.


Would a reference frame in which the center of mass of the Universe was
at rest also be a reference frame at rest with respect to the background
radiation?

Joe D. Darling

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
(803)349-2227 The Name of the Rose
fax: (803)349-2926