The further out in space you look, the faster objects are moving away from each other. This is explained by the accelerating expansion of space, correct?
However, the further out in space you look, the further back in time you’re looking.
So it would seem to follow that the faster objects are moving away from each other, the further back in time they are relative to us.
And that, in turn, would seem to imply that the expansion of space is decelerating.
Very early in the morning, four hours of sleep only, etc etc…
No, the further out in space we look, the faster objects are moving away from us. This is explained by the expansion of space, with no acceleration necessary.
Various differences in our measurements from those expected with a constant rate of expansion are explained by accelerated expansion.
That’s part of my problem, then. I thought the speed differential was in need of explanation beyond just the expansion of space.
I’m still not clear on something here:
If everything were moving away from us at the same rate, would that suggest that space is expanding?
If so, then why doesn’t the actual speed differential call for further explanation in addition to the expansion of space?
And if not, then why not?
That makes sense–I now recall that we’ve known about the speed differential (and the expansion) for much longer than we’ve known about the accelerated rate of expansion. From that I should have realized that the accelerated rate of expansion was arrived at as a conclusion from data other than those which let us know things are moving away faster when they are farther away.
But what about the argument from the OP:
Things farther away from us are observed to move away from us faster.
Our observations of things farther away from us are observations of things further back in time relative to us.
So our observations of things moving away from us faster are observations of things moving away from us faster further in the past.
So the more recently something is observed to be moving away from us, the slower it is observed to be moving away from us.
So the observed rate at which things are moving away from us is decelerating.
So the expansion of space is decelerating.
Even if the final conclusion doesn’t follow from the penultimate one, (I don’t know if it does or not), I’m still curious to know what’s wrong with the penultimate conclusion itself.
Assuming your question is why things further away are moving faster, I think the following description may help:
Imagine a 10x10 grid of cells, each cell 1cm sq. Now make every cell double in size. The sides of any given cell are now 1cm further apart. But the sides of the whole grid have got 10cm further apart. So the further away, the larger the increase in distance with expansion.
Here’s the details of expansion in a universe of three dots separated by a distance of a dash at time t1. The rate of expansion is one dash per dash per unit of time. This is a constant expansion rate.
t1 .-.-.
t2 .–.--.
t3 .----.----.
Taking the first dot as the reference, dot two (d2) has an instant speed away fron d1 at t3 of four dashes per unit of time, while d3, which is further away, has a speed of eight dashes per unit of time. Both d2 and d3 are also becoming distant at a faster rate than they did at t1 and t2 when they were closer.
If everything moved away at the same constant rate it’d look like this:
t1 .-.-.
t2 .–.-.
t3 .—.-.
Which isn’t symmetrical, so obviously isn’t what’s happening.
Real world observations are of course complicated by us having to wait for the light of the dots to reach us.
With a uniform expansion, you would have “Things farther away from us are observed to move away from us faster, with a linear relation between distance and velocity.”
This can be true whether expansion is uniform, accelerating, or decelerating, so the next step does not follow:
You’d need to look at how the variation of speed with distance compares with linear variation to say that the expansion is accelerating or decelerating.