[spauldo]

Matter isn't pinned to the space it's in (source: try walking around). As space expands, the other forces which are orders of magnitude stronger than the expansion of space slide matter along so that distances don't change. You can only detect the expansion of space by measuring the distance between things that are so spread apart that the other forces between them are essentially zero.

[maaaaattttt]

That's what I understood from the explanation on stackexchange. But given what you said, if we take the plank length as the shortest length unit, and we consider two theoretical "objects" placed at one plank length away from each other. Does the universe expanding for these two objects mean: 1. the plank length is becomming bigger, 2. more plank lengths are added in between the two objects, 3. Something else and I'm completely off

[spauldo]

The Plank length thing is just going to add confusion here. Quantum stuff is weird. That's where analogies go to die.

But generally speaking, the answer is 2. That's assuming there's no forces between the two objects.

Space doesn't expand like the outside of a balloon or a rubber sheet - I hate those analogies because they give you the wrong idea.

I'm not aware of any major current theories that say space is quantized, or any theories that have a way of pinpointing a "piece" of space, so the following analogy is flawed. But it should at least point you in the right direction.

Draw a line and put eight dots on it. Draw arrows pointing to the fourth, fifth, and eighth dot. We'll call those dots A, B, and C respectively.

We're going to measure distance by dots. Dots A and B have a distance of 1. Dots A and C have a distance of 4.

Now for each dot, add a new dot before and after it. Measure the distances. Dots A and B now have a distance of 3. Dots A and C now have a distance of 12.

Repeat. Measure the distances. Repeat again. Measure the distances. You'll notice that the distances between the dots you've marked is increasing faster with each repetition, and that the distance between A and C is increasing faster than A and B. An object at any of those points would not be experiencing any force - nothing is pushing or pulling on them - but an observer at any of those points would observe the objects at the other points to be accelerating away from them.

That's sort like how space expands. Of course, space doesn't have "points" as far as we can tell, so there's all kinds of problems with the above analogy, but hopefully it helps.

[mr_mitm]

It's 3), I'm afraid. There is no shortest length unit (as far as we know), the Planck length is a constant, and its only significance is that at these length scales, we need a theory of quantum gravity to describe what's happening.

[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.

[CGMthrowaway]

That's an interesting point. The expansion of space still impacts matter on an atomic level though. The space between atom core and electrons influences its bonding abilities and other properties

Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change

[spauldo]

The magnet analogy works here, so let's give it a try. Put one magnet on a table and then put one under the table so that it's held up by the magnet above it. Then observe what happens. This will take a while, so maybe go get a 2^128 cups of coffee.

The space between the magnets is expanding, just like space everywhere. Assuming the table and magnets are immune to deterioration over time, you can come back after several billion years and the distance between the magnets will have stayed the same. Space expanded, sure, but the stuff occupying the space didn't.

The forces that hold an atom together are significantly stronger than what's holding the two magnets in the example above.