Question: if space is a vacuum then what is expanding?

Jarrod Hart answered on 18 Jun 2016:

Ha!
Maybe it’s not that space is expanding but all the stuff inside is shrinking!

But seriously, it is all relative. If you have two points in an empty space with no edges, then how far apart are they? There is nothing to compare with, so you can’t say…. you can’t even say if they are stationary or moving until you add a third point.

All we really know is that the galaxies out there all look like that are moving away from us – how do we know? Because the light we see from them is ‘stretched’ as if each successive wave was coming from further away (google ‘red-shift’)…

And the further away they are the faster that are receding – that means they are moving apart from each other too – now it *could* be they are all flying into open ‘space’, but there does not seem to be an edge and the ones further out would be breaking the speed limit of nature (the speed of light) – so according to relativity (Einstein’s baby) it makes more sense to think of all the galaxies as being on a rubber sheet that is being stretched out – or better yet a sphere (balloon?) as there is no obvious centre….

There is actually a lot of guesswork in all this, don’t take it as fact – and never trust a physicist who says it is – you yourself could find out the deeper truth!