Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   IIRC the Universe only expands (ie. new space is formed) at intergalactic
> > space, not inside galaxies. The gravity of galaxies is high enough to stop
> > (or at least greatly diminish) expansion from happening inside them.

> FWIW, that's not what I've heard. However, I'm not sure anyone actually 
> knows the answer there. :-)

  Thinking about it, I can think of one argument of why that would be so:
If space was expanding evenly everywhere, it would mean that there should
be an extra "force" pulling planets away from the Sun, and an even "stronger
force" pulling stars away from thir galaxy's center, and this extra "pulling"
factor would have to be taken into account in orbit equations. But is it?

  (OTOH maybe the expansion inside our solar system is so minuscule that
it basically unnoticeable and doesn't affect orbits in practice...)

  Wikipedia (the always trustworthy source) seems to confirm this idea.
For example at http://en.wikipedia.org/wiki/Metric_expansion_of_space it
says:

"Metric expansion is a key feature of Big Bang cosmology and is
modeled mathematically with the FLRW metric. This model is valid in
the present era only at relatively large scales (roughly the scale of
galactic superclusters and above). At smaller scales matter has
clumped together under the influence of gravitational attraction and
these clumps do not individually expand, though they continue to
recede from one another."

"In addition to slowing the overall expansion, gravity causes local
clumping of matter into stars and galaxies. These stars and galaxies
do not subsequently expand, there being no force compelling them to do
so."

"However the only locally visible effect of the accelerating expansion
is the disappearance (by runaway redshift) of distant galaxies;
gravitationally bound objects like the Milky Way do not expand."

  The page http://en.wikipedia.org/wiki/Hubble%27s_law could perhaps have
more on this, but I can't easily find anything to corroborate either claim,
except perhaps for:

"In systems that are gravitationally bound, such as galaxies or our
planetary system, the expansion of space is a much weaker effect than
the attractive force of gravity."

-- 
                                                          - Warp

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