is the acceleration of expanding universe the same everywhere?

[sabianq]

when we look at red shifted galaxies, we see some that are moving away very quickly and some moving away slowly, apparently the further away they are the faster they are receding..

my question is, is the rate of acceleration that we observe (the observation that the universe appears to be expanding at an increasing rate.) the same for all observed galaxies?

i guess i am asking if further galaxies that are moving faster away are accelerating faster, or is the acceleration the same for all bodies regardless of distance?

[WayneFrancis]

It is a bit complicated.

The theory is as follows
For any random point in the universe at Time n from the big bang that point would observe the same expansion rate as any other random point in the universe for the same point in time.

Basically the Hubble Parameter (often referred to as the Hubble Constant) is not actually constant. It changes with time. But for any given time the expansion rate seems to be linear. IE the rate at which an object would be receding is a direct correlation with respect to its distance. But since when we look out through space we also look back in time and the rate changes over time the larger the distance we look at the more it will deviate from a linear rate.

I'm sure someone can put it in better terms then I.

[Ken G]

That sounds fine to me-- there is a dynamical equation that gives us the acceleration as a function of age, and as the universe ages, its acceleration changes (it was decelerating for most of its history, and has only more recently shifted to acceleration). So when we look farthest away, we see galaxies as they were during the decelerating phase of the universal expansion.

[mugaliens]

sabianq, first, it's not acceleration we're talking about, here. It's expansion. As WayneFrancis mentioned via his comment about the Hubble constant not being constant, the rate of expansion changes over time, as depicted in this graphic of the universe over time. Between the formation of the first stars, about 400 million years after the Big Bang, and major development of galaxies, planets, etc., about half way between the Big Bang and now, the rate of expansion was decreasing. Since that halfway point, however, the rate of expansion has been increasing.

[Nereid]

As far as we can tell, so far, there is no preferred direction of the expansion; in all directions, a plot of the estimated value of the Hubble constant, by redshift, would be the same.

[sabianq]

sabianq, first, it's not acceleration we're talking about, here. It's expansion. As WayneFrancis mentioned via his comment about the Hubble constant not being constant, the rate of expansion changes over time, as depicted in this graphic of the universe over time. Between the formation of the first stars, about 400 million years after the Big Bang, and major development of galaxies, planets, etc., about half way between the Big Bang and now, the rate of expansion was decreasing. Since that halfway point, however, the rate of expansion has been increasing.

fine..
so an increasing rate of expansion is not an acceleration.

i will revise my question.
is that "increasing rate of expansion" the same everywhere?

[Cougar]

my question is, is the rate of acceleration that we observe (the observation that the universe appears to be expanding at an increasing rate.) the same for all observed galaxies?

I'm sure someone can put it in better terms then I.

How's this:

It's the same everywhere, but not everywhen.

[speedfreek]

fine..
so an increasing rate of expansion is not an acceleration.

Actually an increasing rate of expansion is an accelerating rate of expansion, but as Wayne said, it is a bit complicated. The rate of expansion is usually generalised to be the same, across the universe, at any given time, but the rate has been changing, over time.

When we say that the further away a galaxy is, the faster it apparently recedes, this is not an accelerating expansion - it is the result of any type of expansion, be it accelerating, decelerating or constant. Basically, if everything (at the largest scales) moves apart equally at any given time, then the further the place you consider, the faster that place apparently recedes.

Imagine you are holding a long rubber band between your hands, with lines marked all the way across it at 1cm intervals. Lets say the band is 10cm long. Now you stretch it to twice its length so each line is now 2cm apart and the band is 20cm long.

Choose a hand - it doesn't matter which. In the time it took you to stretch the rubber band, the closest line to your hand moved from 1cm to 2cm away from your hand whilst the furthest line moved from 10cm to 20cm away. The furthest line from either hand receded at 10 times the speed of the closest line.

It doesn't matter whether you stretch the band at a constant rate, or start off stretching it quickly and then slow down, or start off slowly and then speed up - the furthest line from your hand will always be receding faster than a closer line!

[tommac]

Doesnt it depend if it is homogeneous and infinite?

[Nereid]

Doesnt it depend if it is homogeneous and infinite?

Indeed ... and I already addressed that (see post #5).

[WayneFrancis]

How's this:

It's the same everywhere, but not everywhen.

I like your answer!