[eggn00dles]

Matter is very much pinned to the space its in. If the space between two galaxies expands, the distance between the two galaxies grows. If matter wasnt pinned the distance between the two galaxies would remain the same despite the expansion.

Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.

[spwa4]

I think the point is that the force of gravity is so much stronger than the expansion that the (equivalent) force a normal-sized star exerts on your body from the other side of our galaxy is greater than the force expanding space between you and that star. One of them, not all of them together.

All of them together are so much stronger it's not even funny. And that's for the "underdense" region that we are in. Not a void, but about half of our galaxy's environment does count as a void.

[spauldo]

You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in.

Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter isn't pinned to space - it just slides through it.

Gravity is too weak to affect distant objects, so we see the effects of the universe's expansion when we look at them.

[eggn00dles]

i walked around and it involved force and energy.

i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in.

you talk about gravity as if its something distinct from what is driving the expansion of the universe.

we have that model, its what newton proposed.

[spauldo]

I believe all of the current theories being seriously considered hold that whatever is causing expansion isn't gravity, so yes I do talk as if gravity is distinct from what is driving the expansion of the universe.

Regarding being "pinned," that still fails to account for inertia. The idea that there's a specific piece of space that we're stuck to implies there's a rest state at which there is no motion independent of any observer. We know that's not the case.

My original point was that gravity and the other forces that hold us together are so much stronger than whatever is causing expansion that the expansion of space doesn't affect us at small scales. The space we're occupying is expanding. We're not dragged along with it. The Triangulum galaxy doesn't move away from us because gravity keeps the Local Group together. We do see the expansion of space between us and distant objects, but that's because there's no force strong enough to hold those distant objects to us. That's not because we're "pinned" to our location, but because the space between us is getting larger.

[eggn00dles]

Not so long ago, noone knew about the weak force, and even the most brilliant minds would never think electricity and magnetism are two sides of the same coin. Now we have a theory that unites all three under the same umbrella.

In many interpretations of GR it is believed that mass deforms spacetime, but is also influenced by the spacetime it moves through. In the absence of mass, spacetime expands. Carrying along any mass with it. When mass is present this default expansion is overpowered by its influence which causes a contraction. Hence black holes.

Gravity is still colored by classical definitions so most assume its power is unidirectional. However there is a quantitative factor for expansion in the very same Einstein equation which defines GR.

We need to stop thinking of gravity as a force acting on objects, and rather something that acts on spacetime. Many people advocate this but stop at the rubber sheet analogy, which is completely devoid of the idea that the sheet actually moves as well.

[rantallion]

> Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it.

Gravity wells aren't pulling surrounding space toward their centre. They're only pulling other masses that occupy the surrounding space.