If space is expanding, is the space within a galaxy expanding as well?

On Sat, 8 Apr 2000, Neil wrote:

> Please could you tell me if the space
> within galaxies is also expanding as is the space between galaxies? If
> this is so, then is it at the same rate per unit volume and, if not,
> then why not? If it is not then could it be that expansion within the
> galaxy is constrained by the mass of the galaxy? But if this is so,
> then would this not be reflected in the values of the red shifts of
> galactic objects which we observe?

Hi,

The expansion of the universe is a tough concept to get. Yes, all space
everywhere is expanding. But the rate at which it is expanding is very
slowly - around 70 kilometers per second per million parsecs (one parsec
is a little over three light years). So, on small scales, such as our
solar system, our galaxy, and even our local group of galaxies, gravity is
strong enough to counteract the expansion, and so things don't fly apart.
It's a little like pulling on taffy and then pulling on a piece of wood.
Taffy (empty space) pulls apart, but the wood holds itself together, so
the force of pulling doesn't visibly do anything.

Most of the universe is empty space, and so it expands. If the universe
had enough matter in it, then the expansion would slow down and eventually
the universe would recollapse. However, there doesn't appear to be enough
matter in the universe to stop the expansion, and so the universe will
probably fly apart forever.

If we look far enough backwards in time, we can tell that the rate of
expansion was different in the past than it is today, but the effect is
hard to notice.

I hope this helps explain things a little. Thanks for writing!

sincerely,
Kurtis Williams

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