“You can’t have a light without a dark to stick it in.” –Arlo Guthrie
You know the story. Every galaxy we look out at, with hardly any exceptions, appears to be moving away from us. Not only that, but the farther away a galaxy appears to be, the faster it appears to move away from us!
What do we observe that leads us to believe this? Well, thanks to Edwin Hubble, who looked at the spectra of distant galaxies, we discovered that, compared to Earth, where we make atoms emit and absorb light at very specific wavelengths (or frequencies):
for distant galaxies, these emission/absorption lines are shifted towards the red! This means that they lose energy, somehow.
The standard interpretation is that these very distant galaxies are caught up in the expansion of the Universe! As the space between these galaxies increases, the wavelengths of the light emitted from them gets stretched, shifting them towards lower (redder) energies and wavelengths.
But we do make a large leap to go from that observation — that the more distant an object is, the more redshifted it is — to state that the Universe is expanding. Sure, it makes a wonderful picture that’s very consistent with Hubble’s Law, even when we look at galaxies and quasars a billion light years away!
But that doesn’t mean it’s right.
One alternative is known as “tired light.” In a Universe with tired light, each wave (or particle) of light loses energy, a little at a time, as it travels through space. The more space you travel through, the more energy you lose.
For light in our own galaxy, this is negligible, but for galaxies million (or billions) of light years away, this energy loss could add up, and could be an equally good explanation for why the Universe appears to have this distance-redshift relation.
But there’s a huge problem with the idea of “tired light” for the Universe, observationally.
This is a map of the temperature fluctuations in the microwave background: the leftover glow from the Big Bang. It isn’t coming from anywhere near us, so the only known explanation is that the Universe was once very, very hot — so hot that you can’t form neutral atoms — and that it then expanded and cooled.
When we look out at the microwave sky, we see this microwave background nearly equally in all directions, at a temperature of 2.725 Kelvin.
Perhaps, you think to yourself, this is not a problem for tired light! After all, if these photons come from a long distance away, they’ll cool down too. And by time they reach you, they could be at the same exact uniform temperature!
But although the temperature is right, the spectrum would be all wrong! Allow me to explain.
In the Big Bang, the photons cool down, but they also dilute as the Universe expands. In other words, as the Universe gets bigger (more volume), the number of light particles in a given region of space decreases (less density).
This type of expansion, in technical terms, is called either adiabatic (no heat loss) or isentropic (constant entropy). But what would we get in a tired-light Universe?
You can plainly see that if you start with a blackbody spectrum, and cool it down without expanding it, you end up with the wrong spectrum for your observed radiation!
So that’s the simplest explanation of why tired-light doesn’t work. Remember this when you look for alternatives to a scientific theory: it isn’t enough to make a better explanation of some of the observations; you have to account for them all.