I was reading an article on cosmology, as I am sometimes wont to do, and it brought back to me one of those questions that I’ve had for a while now, concerning the redshifting of light from distant galaxies as it relates to the history and expansion of the universe.
For those of you not familiar with this topic, the general idea here is that when we look at galaxies outside our own, the light they give off is shifted toward the redder end of the electromagnetic spectrum, which means the wavelengths are getting longer. According to our present understanding of physics, the simplest explanation for this observation that the further away a galaxy is, the faster it is receding from us—thus redshifting the light it gives off, thanks to the Doppler effect. It turns out that the amount of redshifting is directly and linearly proportional to the distance of the galaxy, a ratio named the Hubble constant in honor of Edwin Hubble, the man who first made this observation. (He’s also the namesake of the Hubble Space Telescope, of course.)
It seems to me that this explanation either overlooks or glosses over one kind of important point: we don’t see those galaxies as they are right now. In fact, we’re seeing them as they were in the past, and the further out we look, the further back in time we’re looking. If a galaxy is a five million light-years distant, then we see it as it was five million years ago. Double the distance, and double the amount of time involved, which would seem to mean that greater redshifts are as much a product of how far back in time we’re looking as they are distance.
So why is it that distance is regarded as the primary factor here? Why don’t we assume that the universe’s expansion is actually slowing down, given that the closer things are (and therefore the more recent they are), the less quickly they’re receding, whereas the really distant (and therefore much, much older) galaxies were receding more quickly back then?
I’ve no doubt this has been explained one way or another by people way smarter than me, but some Googling yielded no decent results—just about everything I came up with challenged the Hubble constant on various and sundry grounds, not all of them sensical (at least to me). Nothing I found addressed this specifically. Though I figure the explanation is straightforward enough, I don’t seem to be using the right search terms to find it. Anyone got any help for me here?