Cosmic Acceleration and Redshift

Im trying to understand this and would appreciate your help. My understanding is that the further away a galaxy is the faster its moving away from us. Considering that the further away a galaxy is the further back in time we are seeing how do we know from such "old" measurements that its accelerating? Couldnt the argument be made that since the closer (newer) observations are slower than the distant(older) observations that its slowing down?
- Chad (age 38)
Asheboro NC

Figuring out the acceleration requires a careful comparison of the redshifts and the apparent distances, based on brightness. Say that there was no acceleration. Then for smallish distances, the redshift and the distance would be exactly proportional. For larger distances, there's a slightly more complicated relativistic formula, but still just a calculation of how distance should depend on redshift. So to judge acceleration people look at the differences from that calculated curve. For smallish redshifts, objects seem to be too close, as if they weren't moving away that quickly until recently. That means the expansion has been accelerating. For big redshifts, the effect reverses, indicating that the expansion was decelerating a few billion years ago. That's just what's expected if there's a constant background acceleration. Recently, with the matter whose gravity causes deceleration being all spread out, the acceleration wins. When the matter was more concentrated, the deceleration was winning.

Here's a figure showing how the aparent distance depends on redshift, and then showing the small deviation from what would be expected in a no-acceleration picture.

Mike W.

(published on 08/14/2013)

Question: We think the expansion of the universe is increasing in speed because galaxies farther away from us appear to be moving faster and faster away the father we look. When we look deeper into space we are also looking back in time. So aren't we seeing the speed these galaxies were traveling in the distant past? So not necessarily how fast they are moving at present? In which case they may actually be currently slowing down at least galaxies far out, even starting to collapse back in?
- Joseph Pezzolla (age 31)
Albany, NY, USA

The increase of distance proportional to speed is what says that the universe is expanding. Acceleration is estimated from small deviations from the distance-speed relation you'd get for constant expansion. Those deviations indicate that the expansion has been accelerating. A few billion years ago, when the density of matter was higher, it was decelerating.

Mike W.

(published on 08/21/2013)