This is Part 2 in a series on the age of the universe. Read Part 1.
But wait, wait wait wait.
Wait.
Hold up.
This is all based on the assumption that galaxies are receding away from us. And I actually cheated a little. The actual observation isn’t that galaxies appear to be flying away, it’s that the light from distant galaxies is REDSHIFTED.
This is a tiny distinction, but in science, details matter.
We see galaxies redshifting away from us. That’s the original observation (and, to be fair, the critical observation today). This result came to us from Edwin Hubble’s work in the late 1920’s. The common accepted explanation is that the universe is expanding: the light from distant galaxies gets redshifted because as that light travels the long lonely depths of space, it gets stretched out by cosmic expansion. But for quite a bit of time, it was debatable what exactly was causing this redshift.
The simplest interpretation, that the galaxies are well and truly flying away from us, doesn’t work out so well. That’s because it’s not just that the galaxies are redshifting. It’s that the amount of redshift is proportional to the distance. The greater the distance to a galaxy, the bigger the redshift effect. If the redshift is due to motion, then the galaxies twice as far away from us would have to all KNOW that, and conspire to move away from us twice as quickly.
Which is weird.
Another option was proposed by astronomer Fritz Zwicky. Not old Fritz was a bit of a character. He did a lot of important work – for example, he coined the term supernova, which deserves recognition just by itself, not to mention his early discovery of dark matter – but he also had a habit of going against the status quo. And you know I’m always a fan of cantankerous and crochety old scientists doing their best to hold back the tide of progress.
“I have read every paper you ever wrote, I have listened to every presentation you have ever given, and I can tell you quite categorically that I have never found a single original idea that you could honestly call your own,” he once said to a colleague, Robert Millikan. Robert Millikan was a Nobel prize winner. He was also Zwicky’s boss.
So yeah, that’s the kind of dude we’re dealing with.
Anyway, Zwicky wasn’t super fond of the whole “expanding universe” concept, so he worked to come up with alternatives, and his most promising idea came to be known as “tired light.”
Maybe there’s something funny about light, or something funny about the nature of space between the galaxies, that causes redshift without needing the galaxies to move. Now this is a tall order that will require remapping our understanding of physics…but hey so is the idea of an expanding universe so at least we’re in the same ballpark.
For tired light to work, you need a few things to line up. One, light has to somehow lose energy as it travels, but ONLY over extremely long distances. Second, it can’t involve some sort of scattering process where light filters through a substance or interacts with something, because that would cause the light to spread out on its way here – at the distances we’re talking about, it would cause our images of distant galaxies to get progressively blurrier the farther away they are.
And this mechanism would have to have the same effect on all wavelengths of light equally, hitting hard X-rays and wobbly radio waves in the exact same way.
Oh, AND it would have to reproduce the relationship between distance and reshift at extremely large scales, which isn’t exactly 1:1 as you go really far out.
Oh Oh AND it would have to keep the universe static, which goes against the natural inclinations of general relativity.
Oh Oh Oh AND it would have to be compatible with special relativity and quantum mechanics, neither of which admit a mechanism for just randomly sapping every out of traveling radiation without any sort of interaction.
So tired light is a tired idea. It just doesn’t work when you stack up all the hurdles it has to clear. It’s not that the whole concept has been proven wrong (that’s kind of hard to do in physics), but that every time someone puts a model together, it just fails and you have to throw it away. And so far, nobody’s been able to come up with a way to make light tired that agrees with all observations.
So case closed? Tired light doesn’t work, nor does any other explanation of galaxy redshifts. The only thing left standing is an expanding universe, which we use the FLRW metric to describe, which gives us a universal clock.
Well, what if the metric was wrong?
To be continued…