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Re: Cosmology

Boy, I'm confused, as you'll no doubt determine from the following
questions and statements.

Bob Z. said gravitationally bound systems do not undergo the
expansion. My question: The expansion is of space-time, is it not? I
know you'll correct me if I'm wrong, but is not the expansion of space
time a non-Newtonian concept, and isn't gravity a Newtonian concept,
so is it proper to be invoking Newton to explain the non Newtonian?

I liked Bob's analogy of the tiles flying off the roof while the tiles
themselves remain intact. In our galaxy, I translate this to mean the
effects of the cosmological expansion are present but are overwhelmed
by local effects. Would it be correct to say that within a galaxy
Newtonian gravity applies because general relativity yields Newtonian
gravity in the limit of a relatively small amount of mass?

In GR, do you not explain the orbits of planets in terms of geodesics?
Are these geodesics not curved as a result of the mass distribution in
our solar system? Yet, is not the space time in our solar system
flat, because there isn't that much mass contained within it? Is
flatness defined strictly in terms of the geodesic followed by light?

Philip Zell

______________________________ Reply Separator _________________________________
Subject: Re: Cosmology
Author: " Forum for Physics Educators"
<> at internet
Date: 2/11/01 6:16 PM

My small mind is having trouble thinking about what it means to have a
that is stretching the fabric of space-time as we speak but somehow the patch
of fabric that we occupy remains un changed. Any help??

Cliff Parker

Cliff I am reposting what I said about this below. I hope this helps.
When considering the expansion of the Universe you must make a distinction
between gravitationally bound structure and unbound structure. The Structure
of our
Universe appears as follows. There are seven levels of cosmic structure
(Varies depending on how you classify the levels)
1) Planetary systems like our own solar system. ( It appears indisputable
now that there are other planetary systems.)
2) Galaxies such as our Milky way.
3) Groups of Galaxies such as our local group.
4) Clusters measuring 10 to 20 million light years across which consist of
hundreds to thousands of galaxies.
5) Structures called clouds (Not accepted as an actual level of structure
by some) which measure 30 million light years across often linked together in
strings called filaments and spurs.
6) Super Clusters measuring 100 million light years across and contain
upwards of ten thousand Galaxies.
7) Super Cluster complexes which measure a billion light years in length.
These complexes form walls of huge voids measuring about 300 million light
years across.
All structures smaller than a cloud are gravitational bound and are not
expanding. Structures larger than clusters are undergoing expansion.

To add to this while it is true that the Universe scale factor gets larger as
a function of time this process does effect cosmological structures which
have a sufficient gravitational bounding. They remain at the same scale. When
the wind blows shingles off your house they rush away from each other but
remain intact themselves. This isn't a wonderful analogy but maybe it will

Bob Zannelli