Hey, where's the fun in that? Let's try our hand at a simple answer (and assume we all agree that the universe
is expanding, the question is just whether that expansion is steady or speeding up/slowing down):
Suppose the expansion doubles all distances over a given time. So the galaxy that is 1 unit away at the start will wind up 2 units away at the end. The one that's 10 units away, 20 units and so on.
So:
The further an object is away, the faster it will be receding from us. 1 unit vs. 10 units in the same amount of time in the above example, with correspondingly stronger redshift. So the further away the more redshift, even if the expansion
does not change in speed.
That alone would help explain why looking further away (and necessarily back in time) results in greater redshift being observed.
Except:
The above would result in a
linear distance/redshift chart, but that's not what's being observed. Instead,
an additional amount of distance travelled is being seen* (for a certain amount of redshift), and the more distant the object is, the stronger the deviation from the straight line. That's because during the time the light has taken to reach us, acceleration of the expansion has had time to add more distance on top of what what you'd expect anyway from a non accelerated expansion. If it was decelerating, you'd end up with less distance than what a constant rate suggests.
It's worth noting that the light we see isn't just a snapshot of the object we are looking at,
or its (radial) speed, but also the product of the various effects that light has been subjected to on its way here. So we are not just seeing what the distant object looked like when the light was emitted, but also what happened to the space in between since then.
*All of that would not be possible without knowing how far away the object in question is. Luckily there are certain types of object that are a fairly predictable brightness at the source. By putting that against the brightness being seen from here, one can work out the distance. So the distance observed by redshift and the distance observed by apparent brightness don't agree in the way they should if there was no acceleration. Instead, their relationship is consistent with accelerated expansion.
I
could be wrong though
