Redshift of distant galaxies...

[Alistair Bingham]

Main Question or Discussion Point

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

 

[PeroK]

...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]

...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]

(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]

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

Yes, but due to the way the universe has been expanding, many galaxies are receding even faster today than they were when the light we see left them.

To see how this works, consider a constant rate of expansion: this is the state our universe is currently approaching, as it is the expansion rate if all you have is a cosmological constant.

The rate of expansion gives the ratio of recession velocity to distance: twice the distance, twice the recession velocity. If the rate of expansion is a constant, then as the distance between two objects increases, so does their recession velocity relative to one another.

Of course, if the rate of expansion slows down rapidly enough, the recession velocity will also slow. But this hasn't been the case in our universe for a few billion years.