Ask Ethan #39: Why does light stretch as the Universe expands? (Synopsis)

“They say the universe is expanding. That should help with the traffic.” –Steven Wright

When you think of the expanding Universe, you very likely think of galaxies moving away from one another, of space getting stretched, and quite possibly, of light redshifting as it travels through this expanding, stretching space.

Image credit: James Imamura of University of Oregon, via http://hendrix2.uoregon.edu/~imamura/123cs/lecture-5/lecture-5.html.
Image credit: James Imamura of University of Oregon, via http://hendrix2.uoregon.edu/~imamura/123cs/lecture-5/lecture-5.html.

But does it have to be this way? In other words, these galaxies are moving away from us; are we sure that the light is getting redshifted because space is expanding and not, for instance, because of the rapid recession of these individual objects? After all, Doppler shifts can redshift light, too.

Image credit: Larry McNish of RASC Calgary Center, via http://calgary.rasc.ca/redshift.htm.
Image credit: Larry McNish of RASC Calgary Center, via http://calgary.rasc.ca/redshift.htm.

We do know the answer. Go read the whole thing and find out for yourself.