Thread: Edge of the Universe, Age/Size of the Universe

1. Newbie
Join Date
Oct 2003
Posts
4

Edge of the Universe, Age/Size of the Universe

If the most distant galaxies are receeding from us at close to the speed of light, and are 12 billion light years distant, wouldn't it mean that the light from those galaxies left 12 billion years ago when they were at the positions we see them today..?

And since they've been receeding away from us for the same amount of time as their light has taken to reach us, doesn't it actually mean that they should be anywhere from 20-24 billion light years away at this moment..?

Why hasn't anyone taken this into account when talking about the size/age of the universe..?

2. Yes, they have moved significantly from where they appear to be today. But you are assuming that the galaxies are moving at the speed of light...that in 12 billion light years they have physically moved 12 billion light years further away. In reality they are not moving nearly as fast as light, so while their position is different then it appears to be the difference is not as drastic as you think it is.

Scientists do indeed take this difference into account, btw.

Also, welcome to the boards!

3. Have a look at this site. And you might also try here.

4. Member
Join Date
Apr 2004
Posts
32
The age issue (inasmuch as you can refer to it as such) for YEC woowoos is argued away by:

1- The speed of light is changing, it was much faster 10k years ago
2- The universe is not expanding, its an illusion caused by mumble mumble mumble
3- The universe was created in motion, such that light was created to LOOK like it had travelled dozens of LYs and God is a tricksy one.
4- uh.. turtles all the way down.

YEC: The speed of light is slowing
SCI: No, look. We can see here in this distant galaxy that light from this pulsar reaches this point exactly when it would if travelling at c. If the speed were changing, the light would not reach this point when it does, due to the angle we're viewing from and if we do the math..
YEC: Nope, the speed of light is changing [-(

5. Psychonaut: If the most distant galaxies are receeding from us at close to the speed of light ...

Actually, they are probably moving, with respect to us, at many times the speed of light. Although it is commonly believed that recession velocities cannot exceed that of light, it is not correct. See, for instance, Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe, T.M. Davis &amp; C.H. Lineweaver, Publications of the Astronomical Society of Australia 21(1): 97-109, 2004; Fundamental Aspects of the Expansion of the Universe and Cosmic Horizons, Tamara M. Davis, PhD Thesis, University of New South Wales, Australia. The misconception is common even amongst physicists.

Psychonaut: Why hasn't anyone taken this into account when talking about the size/age of the universe..?

One of the problems is that neither the "size" nor "age" of the universe probably means what you think it means. The concept of "size" in particular is very dodgy, since there is not just one kind of distance in general relativity. So "size" can mean whatever you want it to mean, simply by using your favorite version of distance. I prefer to think of the light travel time as a measure of the "distance". Ned Wright's cosmology tutorial, linked above by Spiff is offline at the moment. But when it comes back, if you poke around, you will find his javascript cosmology calculator, which will calculate the various sizes of the universe, depending on your interactive choice of cosmological variables.

As for "age", it is by definition the proper time interval between us (here &amp; now) and the "bang", where proper time is a time interval measured by a clock, moving along a geodesic (the shortest path between events in general relativity). That makes the motion of the other galaxies irrelevant, as they should be. Age is a measured quantity, so it only makes sense to whoever does the measuring (in other words, the universe does not have one global age, but rather the age depends entirely on the observer).

Clear as a bell, right?

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