How is it that the farther we see into the universe, the younger the objects we see?

SDStaff Ian, greatly assisted by UndeadDude replies:

SDStaff Ian replies:

I can see why Cecil decided to let the Straight Dope Science Advisory Board have a crack at this one. It’s such a softball.

Let’s deal with the last part of the question first. No matter how far-reaching our telescopes, we’ll never be able to see the beginning of the universe, for the same reason we can’t see the broadcast of the last Seinfeld again — we were there when it happened. The light that came from our TV sets left us then, and it isn’t going to come back. In the universe, unlike TV land, there are no reruns.

Similarly, while you are looking back into the past when you look into the night sky, you see only a very specific past which is the intersection of light from each celestial object and the path of the earth. In other words, the light from the sun may actually be light from eight minutes in the past, but you can’t see the sun 20 minutes ago, or 20 million years ago, because that light has already past us and is gone forever.

As for the Big Bang, current theory contends (as always, this is simplified) that the universe was all condensed into a “singularity” (like space collapsed into a point) all those billions of years ago. During the initial explosion of the Big Bang that we were a part of — or at least that our corner of the universe was a part of — the light that originated in our vicinity left us and is gone, and it won’t come back. That’s why no matter how far we look with our telescopes, we’re never going to see the first moment of the cosmos. It’s gone, Jack. It ain’t comin’ back.

We can and do see remnants of the Big Bang, however. The universe has gotten a lot bigger since that first moment, and what was once an incredible explosion is very diluted now. We can detect residual radiation from the Big Bang. Since the universe originated in a single point (don’t forget, I’m simplifying), and that point has now expanded into a huge universe, the residue of that explosion appears to come from everywhere. This radiation now has the lofty name “cosmic blackbody microwave radiation.” It was first detected accidentally by researchers at Bell Labs in 1964. It is a smooth constant signal, coming from every direction in space. So while we can’t see the Big Bang per se, we can still detect evidence of it. As the universe gets older, this radiation will appear to be weaker and weaker as it is diluted. Not much of a souvenir for so momentous an event, but it’s all we’ve got.

SDStaff Ian, Straight Dope Science Advisory Board, greatly assisted by UndeadDude

Send questions to Cecil via cecil@straightdope.com.

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