Lots of people have been talking about expanding space in the enormous thread
Message 315. This thread is to explore a simple puzzle about expanding spaces. The solution may help folks understand what is going on a bit better.
(Percy, you are going to
love this one!)
As background, be aware that astronomers observe that all galaxies are moving away from us, and the further they are away the faster they move.
Modern physics explains this as expanding space. For every MegaParsec of separation distance between object in deep space, we get 70 new kilometres of space every second. (Actually it is 71, but let’s use 70 to keep calculations easy.) Thus galaxies that are 30 MegaParsecs away from each other are receding from each other, on average, at 2100 km/sec. A MegaParsec is about 3*10
19 kilometres.
On top of that, galaxies can be moving locally through space. A galaxy moving at 300 km/sec with respect to nearby galaxies will have this velocity added on to the recession velocity due to expanding space.
People sometimes think that space pulls things along with it, somehow. Let’s see.
Take two particles, and hold them at exactly the same (large) separation, while space is expanding. For simplicity, let particle "A" be at rest in the frame of background radiation, and let particle "B" be one MegaParsec away. Assume that these two particles are out in deep space, in one of the great voids. That is, there are no other galaxies anywhere near these particles to push or pull them with local gravitational attractions.
Space between these two particles is expanding at 70 new kilometers every second. To remain at the same separation distance, therefore, particle "B" must have a local motion of 70 km/sec towards particle "A". That is, the particles have a local velocity towards each other that exactly matches and cancels out the rate at which space is stretching between them.
Now release the particles, so that they are in free fall. Will the distance between them increase, or decrease, or say the same?
I’ll give the solution, but I want to give people the chance to think about it first. This is a good way to get a feel for whether your intuitions about expanding spaces are correct, or not.
Cheers — Sylas
This message has been edited by Sylas, 01-28-2005 18:07 AM
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