Expert: James Gort - 7/12/2005

Question
-------------------------
Followup To
Question -
When we look at distant galaxies (or other distant objects), we see them as they were a very long time ago.
Today's powerful telescopes can see so far away (20 billion light years ?), how come we're not seeing the actual "big bang" ?

Answer -
Hello Mike,

I wanted to be rigorously correct, but just noticed an error!  Instead of "dark matter" in my answer, that should read "dark energy".

Cheers,

Prof. James Gort
     
 ========================================

I can't find the answer anywhere!
If the above is your answer, then it has nothing to do with my question.

Maybe you sent my answer to someone else, and vice-versa.

So, could you please answer my question correctly.

Thank you.  

Answer
Mike,

That message was a follow-up to the original message.  Perhaps you didn't receive it.  But I'll repeat both messages below:

Hello Mike,

That's a very good question!  I'm glad you asked it, but it's a little difficult to answer absolutely correctly.  I want to be as correct as possible, making no non-scientific assumptions.

First, the idea of a "big bang" is still only a suggestion of one possibility (although, admittedly, it is the most popular of the various cosmological models).  But let's assume that it happened, for the sake of this question.

It's pretty well established that the velocity of light in vacuum ("c") is an upper limit to the velocity that matter can attain.  In fact, it would take an infinite amount of energy to reach c.  So matter (even that blown out from the big bang) is pretty much restricted to velocities less than
c.

For the sake of our discussion, assume the big bang occurred 20 billion years ago.  Also assume we (the mass making up our galaxy) was expelled at 1/2 c.  Also assume that velocity has been fairly constant (we're not accelerating - although the recent discovery of dark matter seems to
indicate we may be accelerating, and we're not decelerating - although this makes some intuitive sense based on Newtonian gravitation).

We'll look at things classically (not relativistically) to make it easier.  It should give the same answer, though.  In 20 billion years (i.e., today) the light from the big bang will have reached 20 billion light-years from "ground zero".  Our slower moving galaxy would have reached only 10 billion light-years from "ground zero".

If we look back with a powerful telescope, we can't see the light from the initial "bang" because it's already passed us!  We can see the "area" of the big bang, but as it was 10 billion years ago (i.e., 10 billion years AFTER the big bang).

That's one reason we can't see the light from the actual "big bang".  The other reason is because the light itself gets more and more red-shifted (loses energy) as we see more distant objects.  Some (actually, most) people attribute this to a Doppler shift (although there are alternative
explanations).  At any rate, as we try to see more distant objects, the light is shifted to the red, then infra-red, then radio, then ...?  Eventually, our detectors can't see the photons!  So there's a limit to how far even the most powerful telescopes can see.

Hope that answers your question.  But I hope it also shows that there are a lot of unanswered questions!

Prof. James Gort



Hello Mike,

I wanted to be rigorously correct, but just noticed an error!  Instead of "dark matter" in my answer, that should read "dark energy".

Cheers,

Prof. James Gort