# Series of GR questions about universe expansion

1. Jun 13, 2012

### lobro2002

The relationship between space (or spacetime) and matter is giving me some trouble. If the universe was created (all its matter) in a planktime explosion then the force of said matter must be driving the expansion of space itself. I view the matter as pressing against the walls of space and exerting force on them, thusly driving its expansion. Or does space expand without the aid of the matter contained in it? At what rate is spacetime expanding relative to the the matter contained inside? I can't imagine any matter could move faster than this initial spatial expansion. How could matter exist outside of spacetime?

I'll save my questions about matter on larger scales (galaxies) moving faster than the speed of light relative to one another for later; if someone can answer the exact rate of expansion of spacetime for me.

2. Jun 13, 2012

### twofish-quant

It's not an explosion.

The way that you start thinking about cosmology is to start with the assumptions of isotropic and homogenity. What that means is that any part of the universe is going to be the same as any other part of the universe.

Once you start with that assumption and you run the numbers, you find out that the universe can't be static for very long. Either the entire universe is expanding or the entire universe is contracting. It happens to be expanding.

If you imagine the universe expanding into something, then it's going to get confusing very quickly.

I don't like thinking in terms of a "rubber sheet" in which "space time" expands. I think it's more confusing than anything else.

3. Jun 20, 2012

### clamtrox

No, the expansion and matter content are related through the equations governing gravity. If the matter is "normal stuff" (not dark energy or anything like that) then the gravitational pull of the matter causes the expansion to slow down.

Matter is on average moving along with the expansion. It may be easier for you to visualize this if you stop thinking about spacetime as a thing of it's own, and just think abstractly about the distances between dust particles. Don't think that there's something which is expanding, but instead just say that the distances are growing.

That would indeed not make much sense.

It might be prudent to use the search tool in the forums for this one; this question is quite popular.