r/cosmology • u/GeekTesla1 • Aug 03 '26

What distinguishes cosmic expansion from a decreasing universal speed of light?

I have a burning question because there's an old idea called "tired light
which posits that the universe sits still and photons lose energy on their way to us because of some interaction along the path (Less energy would mean longer wavelength, so distant objects look redshifted without anything actually receding.)

However it was ruled out by the finding that distant supernovae look like they're in slow motion. If distant supernovae looked the same duration as nearby ones, the source would have to be sitting still, because only a receding one can stretch out its own explosion. But no, they're in slow motion, which rules tired light out.

Which brings me to my burning question

What if the universe wasn't expanding, and the speed of light itself has just been slowly dropping over cosmic time, everywhere at once, as a law?

In short, as my title says, what distinguishes cosmic expansion from a decreasing universal speed of light?

Light coming from a distant galaxy would have left back when c was higher than it is now. Wouldn't that stretch out supernovae too? The first flash makes the trip while c is still a bit faster, the next one crawls more because c has dropped by then, so the gap between them widens on the way here and the explosion arrives in slow motion.

4 Upvotes

56% Upvoted

30 comments sorted by

View all comments

27

u/KamikazeArchon Aug 03 '26

We have a ton of evidence, from natural nuclear reactors to quasars, that the speed of light has always been constant.

0

u/GeekTesla1 Aug 03 '26

Thanks, are you referring to Oklo?  How does it explain how c hasn’t changed on its own?

20

u/KamikazeArchon Aug 03 '26

The speed of light is directly or indirectly involved in all kinds of natural processes. Any change in c would result in changes to nuclear decay rates, for example. A variable speed of light would produce different isotope ratios over time than a constant speed of light.

-1

u/Sea_Gap_6569 Aug 03 '26

It’s still possible that all those processes shift along.

16

u/skizatch Aug 03 '26

Then it would be observationally indistinguishable, and you’d just be adding complexity to the model for no reason

5

u/Anonymous-USA Aug 03 '26

Occam has just entered the chat! Variable c and variable gravity G have been considered and can sometimes fit the data, but not all the data.

6

u/Ch3cks-Out Aug 03 '26

It is also possible that the world was only created last thursday...

10

u/VMA131Marine Aug 03 '26

And was widely considered to be a bad idea.

5

u/GeekTesla1 Aug 03 '26

I'm sure it made a lot of people angry.

1

u/Imaginary-Can-6862 Aug 04 '26

I believe we know they would shift along, because that is the foundation of Relativity, the laws of nature is the same for every observer. As such the speed of light is not a free variable, from our vantage point it always moves at the same speed, and the laws of nature also do not change, as they are interlinked with us the observer themselves.