How Can Space Expand The Way It Does?

With all these popular science words flying around these days, it can be difficult to get a handle on what is actually being talked about. Question from twitter:

So, the expansion of space is quicker than light? […] A different level/type of speed? If so, and the fact that space itself can bend around an object then space itself can’t be a ‘pure’ vacuum; it would be made up of ‘things’, that could/would travel faster than light […] and if space itself was a vacuum why would it bend around an object like a planet? […] the expansion of space would still not mean the reversal of time as time is a speed of light concept. diff systems.

Let’s start with a few points of clarification:

  1. Space-time is a mathematical structure in which events occurring in the universe can be described – just like the grids on graph paper. Space-time is not made of ‘things’. Things happen at a point in space-time.
  2. Space-time doesn’t exist within some bigger space. We can’t observe it like some sort of rubber sheet twisting and contorting – we’re inside it. Actual things – like particles and humans – are not allowed to travel through space-time faster than the speed of light. (Einstein!) BUT space-time itself isn’t travelling through anything, and so it isn’t violating any speed limit by expanding the way it does.
  3. Space-time isn’t a vacuum, it is a mathematical construct. A vacuum is a region where there is no matter. (Thanks to quantum mechanics, we now know that a lot of things happen in such a ‘vacuum’ region.)
  4. According to general relativity, distances and times get squashed and stretched around massive objects – like planets and stars. This is a description of gravity. Sometimes we represent this as a bending of space-time. But space-time doesn’t ‘bend around’ these objects – there’s still ‘space-time inside the star’.

This might help you picture it:

Space-time is expanding – but not at a particular speed. Think of a circular rubber band. The rubber band is stretching, doubling its length every hour. The rubber band is not expanding at a definite speed. But, if we were standing on the rubber band, the number of steps between us would be getting bigger. I.e. there would be a definite speed between you and me. The further we stand apart, the faster our relative speed would become. In fact, there may be some distance at which our relative speed is faster than our maximum running speed. However, if we tried to run along the rubber band, we would still not exceed our maximum running speed relative to the rubber band. That is all Einstein requires – that actual things don’t travel through space faster than the speed of light.