Quote:
Originally Posted by sman
I'll keep trying - there's no deadline.
|
Fantastic outlook!
Believe me when I say, you are not alone in your confusion. The different ways of expressing distance have generated a lot of confusion. The following link shows one astronomer who appears more than a little irritated by the way the popular press expresses cosmic distances:
Light Travel Time Distance
You must imagine that this galaxy which you're looking at is moving away from you. By the time we absorb light from that galaxy it is no longer in the spot where it emitted the light which we just received. It has expanded away from us by the time we receive the light. So, how far is the galaxy really? There's more than one answer.
Let's say the galaxy has a redshift of 1.815. This will mean light traveling from that galaxy to the milky way took 10 billion years. When the light was emitted the universe was much younger than it is today (the universe was about 3.7 billion years old). So this is not a very sensible distance. Light travel time distance is comparing how far apart a young galaxy is to an old galaxy. We're looking at two different time periods.
The comoving distance looks at how far that other galaxy is *today* compared to the Milky Way today. This distance is necessarily going to be greater than the light travel time because these two galaxies have been moving apart during the time the light has been traversing the distance. In this case of redshift 1.815 and light travel time of 10 billion years the comoving distance is 16.165 billion light years.
Just as there is more than one measure of distance, so too there is more than one measure of velocity. The velocity that goes into Hubble's law is not the same as you would figure in special relativity. The link I gave yesterday gets into that.
But, yeah, this can get confusing quick—your puzzlement is quite understandable.
~modest
