query_ious said:
I think I get it... basically everything (including space itself?) was given an initial velocity during the big bang and started moving away from each other.
That's my understanding as well. The "including space itself" is the tricky bit, but yes.
When you look at small enough regions of space (which are still humongous) that are large enough to show up the expansion of the universe but small enough that the cosmological recession velocity of any two galaxies moving away from each other is << c, then what you have looks very like a large cloud of stuff that is dispersing. Everything moves away from everything else and the mean distance between things increases.
There are two ways in which this breaks down.
- Over smaller scales, the local movements of galaxies become significant, and in particular for a cluster of galaxies that is gravitationally bound there's no expansion at all within the cluster. It's not that gravity is enough to "hold it against the dispersal". Rather, gravity HAS held it together and altered the movements of galaxies so that as a group they are not moving apart. There's nothing to make them move apart either** and so it is not a case of gravity holding it against dispersal. Gravity has stopped dispersal of the cluster and it simply isn't dispersing.
- Over larger scales, the notions of distance we are used to start to fall apart. You start to need GR to describe space and distance and you start to find different notions of distance diverging from each other. You also find, on large scales, that there's no limit or edge to the galaxies or clusters of galaxies, and so the recession velocities end up being as large as you like, even much greater than the speed of light. And we can still see such galaxies, which is commonly a stumbling block for people first meeting up with this.
**(Caveat. Dark energy works as a kind of pressure that pushes things apart, a bit like gravity acts as a force to pull things together. The former tends to accelerate expansion, and the latter tends to decelerate it. Without the effects of such forces, expansion would just continue at the same rate forever. These are forces that slow things down or speed them up. A cluster bound together by gravity is not expanding, except that dark energy may still be kind of "force" to push things apart. But note it is not "expansion" that is forcing things apart, but some energy effect that is forcing things apart, and which therefore accelerates expansion.)
And if so, what exactly is 'empty space'?.. because once we claim that it can expand or collapse we make it 'real', give it 'substance' and yet to the best of my knowledge 'empty space' is a vacuum... and saying that a vacuum can expand or contract of its own accord without some additional medium for it to expand or contract into sounds, well, wrong... (and also forces the creation of an 'ether' in which 'space' exists... deja vu anyone?...)
Anyways I'm sure I misunderstood something..
I'm pretty sure I misunderstand something too. Compared with some of the people here, I am an egg. But I still think I can say a few useful things at a comprehensible level, secure that if I go wrong there are folks here who can put me right.
I very much like John Wheeler's famous two line summary of relativity.
Matter tells space how to curve.
Space tells matter how to move.
This is also often repeated a bit more precisely, using "spacetime" rather than just "space".
So if people think of space like the skin of a balloon dragging attached buttons along for the ride, they are missing something pretty crucial. "Spacetime" just gives the path along which things fall. Matter also determines what the "spacetime" looks like. So one should probably think just as well as of the skin of the balloon inflating because it is being dragged along by the expansion of the buttons.
But you're right. Space isn't a material thing. It does have geometric properties, and it gets mixed up with time so that mathematically its better to describe the geometry of spacetime. And on small enough scales you can still get all the same results to very good accuracy using the older Newtonian ideas of gravity as a force between particles moving around in a space described with a Euclidean co-ordinate system. Even the expansion of the universe can be described like that, at least with regions where distance and velocity still behaves itself.
Cheers -- sylas
