# Question on redshift/expanding universe

1. Feb 11, 2010

### gamblej

I am not a physicist so I was wondering if it is possible to explain the following question to a layman.

My information on this is subject to the whims of pop-sci journalists so if the answer will go over my head, that's OK. I'm still curious.

The further a galaxy is from us, the more distant back in time its light was sent. So those near the visible horizon of our universe that are truly hauling *** were hauling *** away from us 10 billion years ago. What about today? Couldn't a distant galaxy's relative velocity be a function of time rather than distance? How do cosmologists decouple these factors to arrive at the correct result (that our universe's expansion is accelerating)?

Thanks for any insight you can provide! :)

2. Feb 12, 2010

### Chalnoth

Yes, the relative velocity is both a function of time and distance. The way we model this in cosmology is we consider the universe to be homogeneous (the same everywhere) and isotropic (the same in every direction). If the universe is the same everywhere, then we can define a global expansion rate that depends upon time: H(t). If we take any two objects in the universe separated by some distance d, their relative velocity will be, on average, H(t) * d.

Over time what has happened is that early on, H(t) decreased very very rapidly, so that objects were originally moving away from one another at very high speed, but slowed down. More recently, H(t) has been approaching a constant value. This means that objects are now moving away from one another faster and faster with time (if H(t) approaches a constant, then as objects get further away, H(t) * d becomes larger, which means they move faster).