What other evidence do we have of the expanding universe?

25 June 2011

Question

Thanks for the podcasts, they're really really great!

I've got an astronomy question which has been bugging me for a while.  I'm sure I am missing details and am wrong.  I was hoping you could explain and make me less wrong.  :-)

As far as I know, the current best theory is that the universe is expanding, and the rate of expansion is increasing.
And if you ask how we know it, astronomers point to the red-shift of the light from distant stars, and how it gets more and more red-shifted the farther out the star is in a pattern that is consistent with an accelerating expansion.
(Am I close?).  But my question is:  is this the only way we know that the rate of expansion is increasing?

The reason I ask, is that I have not heard a good explanation for a mechanism that would cause that sort of accelerating expansion.
Its easy to accept that the universe is expanding, but the idea that the rate of expansion is increasing is harder.

So, how do we know that there is not some other phenominon at work that makes "nice normal explainable expansion" look like "accelerating expansion"?

I've been pondering this and have a few alternatives, and I am wondering how scientists rule them (or some combination of them) out.
I'm sure I am not the first to suggest such things, but I can't seem to find information about why these ideas were rejected.

From my college physics classes I remember there being some president for time-space being somewhat malleable.
For example, is it possible that time is accelerating?  I.e. time was moving more slowly in the past compared to today?
If time were accelerating, when we looked at very distant objects we would effectively be seeing light that started out its journey in a slower time-frame than the one it arrives in.   It would be like looking at objects running in slow motion... and we should see some sort of red-shifting from that, right?  (In addition to any red shift from motion)  How do we know that this isn't the case?  (I realize we would need to come up with an explanation for time speeding up... but that doesn't seem any messier than explaining why expansion is accelerating.)

Another possibility might be red-shifting due to the light starting in an area with stronger gravity, and getting to us now in an area with comparatively weak gravity.  In the past, the entire universe was overall more dense, so had more gravity, and now it is less dense so we have less.  Doesn't gravity cause red-shifting like this?  Wouldn't this also cause further objects to be more shifted?

I'm sure these ideas are not new... I just don't know what makes astronomers think they are wrong.
It seems to me that one, or the other, or some combination of both, or some combination of them and other ideas might fit observations.

I suspect I am very wrong here in several ways, and want to be less wrong.
Help me Naked Astronomers!  You're my only hope!

Charlie

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