Is the Universe's Expansion Truly Slowing Down?

[Megalepton]

New member here, thanks for your patience. The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?

 

[PeterDonis]

The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?

Yes, it does. But we don't think the expansion of the universe is currently accelerating just because of light from those old galaxies. We think the expansion of the universe is currently accelerating because the expansion rate we infer from looking at light from galaxies at various distances (and therefore from various times) changes in a way that makes it clear that the expansion was slowing down until a few billion years ago, but since then has been speeding up.

 

[Chalnoth]

New member here, thanks for your patience. The farthest galaxies are the most redshifted, but doesn't that old light tell us more about the rate of expansion closer to the beginning of the Universe?

Yes and no.

First, a constant rate of expansion is an accelerated expansion. This is because expansion is speed per distance, so that an object twice as far away is receding twice as rapidly. This means that if the rate of expansion is a constant, then as objects move further away they speed up.

The current rate of expansion isn't quite a constant. It is still slowing down. But the rate of slowing is low enough that objects are still speeding up as they get further. Note that this is mostly only true for relatively nearby galaxies (as PeterDonis mentioned).