An Expanding Universe

[Bazile]

If space is expanding ( as we are told ) shouldn't the distance between the planets in our solar system , and the distances between the sun and the planets ; be getting increasilgly larger from each other ?

[ToraToraTora]

no, essentially anything smaller than a galaxy is considered insigfnificant size wise, at the tiny scale within galaxies there is no effect, they are held together by dark matter. The ballon analogy is often used to explain expansion because it is easier to grasp but it doesn't really explain the expansion as it is. The space between the galaxies is generally increasing due to the repelling of dark energy. Though of course I will point out before some pedant does that our closest galactic neighbour is actually blue shifted.

[Bazile]

What , so the space between stars in a galaxy is insignificant ; however the space between galaxies is what's really expanding to any appreaciable degree ?

[ToraToraTora]

correct

[jim360]

If you take the balloon analogy a bit further and imagine that things close to each other are holding hands while the balloon is being blown up that might help too. Local gravity effects can over-ride the expansion of space as a whole. I'm very fuzzy on the details though, most of the physics I do is in boring old flat space.

[Teddio]

Consider a spiral galaxy say 100 000 light years on diameter. Does this mean that its diameter will increase as a function of time as a result of the space expanding between its constituent stars ? If so, old galaxies should be larger than newer galaxies.

[Graham-W]

Because the size of a galaxy is expressed in Very Large Numbers a large expansion of a galaxy of say, a couple of billion miles, is such a small percentage it would be unmeasurable on such a small thing as our Earth.

[ToraToraTora]

The holding hands analogy is a good one. It is also that the size of interstellar space is so miniscule compared to interglactice space that any expansion, even if it was not negated by gravity on a local level, would be so small to be immeasurable.

[jim360]

Thanks Tora -- credit where it's due, I thought of it after reading your post.