Redshift of distant galaxies...

Main Question or Discussion Point

...does it not simply indicate that the galaxies WERE receding faster IN THE PAST?

PeroK
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...does it not simply indicate that the galaxies WERE receding faster IN THE PAST?
Essentially, yes. What you would have is essentially the following data:

7 billion years ago objects that were 7 units away were receding at 7 units of speed
6 billion years ago objects that were 6 units away were receding at 6 units of speed
5 billion years ago objects that were 5 units away were receding at 5 units of speed.
...
2 billion years ago objects that were 2 units away were receding at 2 units of speed
1 billion years ago objects that were 1 unit away were receding at 1 unit of speed.

Let's say for the sake of argument we have no data more recent than this. And, leave aside all other corroborating data and theoretical explanations. And, with apologies to the cosmologists, as the data and the picture are obviously not quite as simple as this!

You might say: "aha, so the expansion of the universe might have stopped 1 billion years ago?!"

Well, it might. But, if you were to take your best guess what an object 1-7 units away is doing today, would you say: it's probably receding at some rate proportion to its distance?

PeterDonis
Mentor
...does it not simply indicate that the galaxies WERE receding faster IN THE PAST?
Essentially, yes.
Careful. The expansion of the universe has been accelerating for the past few billion years, so there is a range of redshifts where the galaxies were receding slower when the light we see was emitted than they are now. The linear Hubble law only holds for small redshifts (i.e., much less than 1).

The simplest way of describing what the redshift indicates is that $1 + z$ (where $z$ is the redshift) is the factor by which the universe has expanded from the emission of the light to now. So a redshift of 1 means the universe has doubled in size from the emission of the light to now. (If the word "size" bothers you because our best current model has the universe being spatially infinite, substitute "observable universe" for "universe".)

mfb
Mentor
(If the word "size" bothers you because our best current model has the universe being spatially infinite, substitute "observable universe" for "universe".)
The observable universe grows faster than the scale factor, as light gets more time to reach us - we see more distant objects than we could see 5 billion years ago. "Size of the region that is today's observable universe" works.

Chalnoth