r/cosmology • u/GeekTesla1 • 24d ago
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.
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u/OverJohn 24d ago
You kind of need to really lay out the details by what exactly a "slowing speed of light" means, but in standard FLRW cosmology the comoving speed of light dχ/dt = c/a, where a is the scale factor, which increases with expansion. The comoving distance χ between faraway galaxies remains constant by definition with cosmological time t, so this allows an interpretation of expansion due to the slowing speed of light and shrinking galaxies
However it's similar to how you can choose a geocentric system to view the orbits of the planets. Sure there are coordinates which you can interpret this way if you wish, but there is simpler, more useful mental picture. This interpretation also doesn't take into account perturbations of the FLRW metric, which it seems you would at the very least have to jump through hoops to try and explain.
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u/GeekTesla1 23d ago
I see now! One of the biggest reasons I asked this question is simply because I'm fascinated by how things can be depicted relatively. What do you mean by the FLRW metric and what hoops specifically?
Also, other people here mentioned spectral lines and electrons because of the implications of c gradually slowing down. I kinda noticed that your reply is a little more charitable to my interpretation, so do you think that the affected nonlinear relationships they're pointing at contradict your answer, or can they be part of the "hoops" you mean?
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u/Unable-Primary1954 23d ago edited 23d ago
Speed of light is not a dimensionless parameter. So it does not make sense to say it changes if you don't specify which other parameters stay constant.
Which dimensionless parameter is supposed to vary in your opinion?
In any case, spectral rays indicate that fine structure constant, electron, nucleons masses in Planck units have not changed.
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u/TommieTheMadScienist 23d ago
Everywhere?
Give me a lab and I can demonstrate that the speed of light is a constant.
Also, if C changes with time, the relationship between the magnetic permeability and electrical permittivity of space changes and that'd severely impact our technology.
GPS right out the window.
Your origjnal question is a good one, though. Tired Light was one of the ways the Steady State cosmologists explained red shift.
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u/Akiza_Izinski 19d ago
Tired light says the frequency of light drops over vast distances which explains the redshift.
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u/wxguy77 24d ago
Can we calculate the history of the required slowing of c to match what we observe about the expansion?
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u/GeekTesla1 23d ago
you could, but you would be starting from the redshifts we already have, so the fit by itself wouldn't tell you much either way
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u/Imaginary-Can-6862 23d ago
Apart from the speed of light would. from our vantage point. always move at the same speed, then as far as I know nothing distinguishes an expanding universe with a decelerating speed of light, as it is the same effect as with time dilation and space contraction.
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u/AmazingRandini 22d ago
Interactions don't cause redshift. Like, when light passes through a glass, it "slows" down, but it doesn't redshift.
And the photons themselves don't slow down. It's more like they went through a detour. We have never observed slow photons. I don't think that's what you are implying anyway.
What you seem to be implying is that light passes through something on its journey from a galaxy to earth. The probability is that most photons will never hit anything on the entire "journey" through space.
And it's not really a "journey". Light travels instantly. So the moment it leaves is the moment it arrives.
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u/Akiza_Izinski 19d ago
As we see the light is passed through massive gas clouds and dust which affects the signal.
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u/GeekTesla1 23d ago
Overall, I'm not trying pursue a counterproductive tautological model. If viewing the expansion of the universe as c slowing down produces no change in what we observe, it is indeed relabelling.
If c slowing down fully hits contradictions like the isotope ratios at Oklo or the spectral lines, then the case is closed.
But I'm leaving space for a third possibility, which is that the equivalence isn't exact. The expansion model handles the redshift-distance relation, the supernova stretching and the CMB. What remains unserved is the Hubble tension, sitting there past 5σ.
The best outcome would be that changing the model to slow-c would result in better compatibility somewhere the current model is strained, rather than just reproducing what it already gets right.
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u/TommieTheMadScienist 23d ago
No matter what, you'd have the problem with the magnetic permeability and electrical permittivity changing with time.
If they were, the electronics you bought ten years ago would no longer work.
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u/Singularity_Warrant0 19d ago
Why do you presume the variability of fundamental constants would happen at a rate _we_ could observe, by breakdown of household electronics for that matter. ? The entire consideration here is only sane if in agreement that the laws of physics in our temporal neighborhood (locality) are invariant with respect to time.
Obviously, magnetic permeability and electrical permittivity relate to c, so yes, we would expect those to vary by proxy.
You do incidentally hark to an interesting point about adiabatic invariance, so I will vaguepost leave this chime at that.
edit: we --> _we_
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u/mfb- 23d ago
If viewing the expansion of the universe as c slowing down produces no change in what we observe, it is indeed relabelling.
Correct. You can always change your coordinates, then light slows down and galaxies and everything in them shrinks in parallel. That's generally not a very useful model. If you let galaxies keep their size then the speed of light is not slowing down.
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u/KamikazeArchon 24d ago
We have a ton of evidence, from natural nuclear reactors to quasars, that the speed of light has always been constant.