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rubix
2008-Feb-16, 04:13 PM
I read the Universe is 13.7b years old. When I attempt to understand this, there always appears to be a problem with that number. Can't put my finger on it but it just doesn't sit right. Then I start thinking about some galaxy recently viewed through Hubble that may be the farthest distant galaxy seen to date. This galaxy is called A1689-zD1. They say this galaxy is 13b light years away. OK. Now I start thinking about the BIG BAng and that everything we see in our univerise originated from the same point and began to expand with the help of the big bang. This means that A1689-zD1 and earth were once close together and now are 13b light years away from each other.

This is the problem I have. If it took light from A1689-zD1 13b light years to travel to earth, how long did it take earth to travel to where it is now from where the big bang occured. It must be longer than 13b light years because the earth cannot travel at the speed of light. It must be much longer.

antoniseb
2008-Feb-16, 04:48 PM
....This is the problem I have. If it took light from A1689-zD1 13b light years to travel to earth, how long did it take earth to travel to where it is now from where the big bang occured...

Yes, the problem you have is that you are misunderstanding some things about the Big Bang as science currently sees it. For starters, the Big Bang didn't start at a place, unless you want to say that everyplace in the universe was once 'where' the Big Bang happened. Our non-technical language doesn't really provide words or allegories that accurately reflect what happened, since nothing like it has happened since.

The universe is expanding. It is doing it by new space appearing everywhere, not by everything in it energetically exploding away from some center point.

Jeff Root
2008-Feb-16, 05:13 PM
This is the problem I have. If it took light from A1689-zD1 13b light years
to travel to earth,
It took the light 13 billion years to travel to Earth; the distance it
traveled is 13 billion light-years.

...how long did it take earth to travel to where it is now from where
the big bang occured.
It took 13.7 billion years -- the time since the Big Bang occurred.

It isn't possible to know "where" the matter that now composes the
Earth was located 13.7 billion years ago, since positions of things in
space can only be determined relative to other things, but all the
matter in the portion of the Universe that we can see was packed
tightly together 13.7 billion years ago. Space expanded rapidly.
Which is to say that galaxy clusters, or the clouds of gas that
eventually became galaxy clusters, moved apart rapidly. The
farther apart two particles were, right after the Big Bang, the faster
they moved apart, and the faster they are moving apart now.

Essentially, the Earth is not far from where its constituent particles
were 13.7 billion years ago, and A1689-zD1 is not far from where its
constituent particles were 13.7 billion years ago, but the distance
between the two has increased a lot.

That's the expansion of space for ya.

-- Jeff, in Minneapolis

Cougar
2008-Feb-16, 11:36 PM
They say this galaxy is 13b light years away.

Another problem is, you're just thinking of distances, like from here to Chicago. Distance in space cannot be thought of separately from time. They are inextricably linked, so we talk about spacetime.

So that galaxy's light that has been traveling for 13 billion years left that galaxy just 700 million years after the big bang.