What I am understanding on the articles I have read is that the conclusion "universe expands" depends on the assumption "the speed of light is constant". Of course it's not the major factor but what if the universe does not expand but the speed of light decreases. How Could we possibly figure it out since we are also the part of the system? What I mean by "the part of the system" is that our biological time also depends on time, hence we cannot understand that the speed decreases. For instance, as analogy (a weak and most probably inconsistent one) consider we are in a car and can only see the car in front. We are going at the same speed, if we drop our speed with the same deceleration simultaneously, can we understand that the speed decreased? We can't see neither outside nor the speed of car's and ignore the inertia. Basically we say that if $c$ is constant and the distance increases, the universe expands. What if the distance is constant and $c$ decreases?

  • $\begingroup$ Related, possible duplicate: physics.stackexchange.com/q/25328 $\endgroup$
    – user146020
    Feb 26, 2017 at 22:49
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    $\begingroup$ Cosmological redshift is independent of the value of c, it's a wavelength (and frequency) change. Redshift proves expansion. We see it, we measure it. So maybe you want to say that the frequency also decreases, as it is emitted from far away. Why would it go lower with distance (the freq): we see a little freq shift say a hundred million light years away, and more a billion ly, and more 5 billion ly. But spectra in nearby stars are almost the same as here. So, it just changes to satisfy your theory? Or do you just assume that atoms in other places are different than here, and why would they? $\endgroup$
    – Bob Bee
    Feb 26, 2017 at 23:02
  • $\begingroup$ Note that the accepted answer linked to by @Countto10 states that emission from galaxies will never reach us, if the space in beetween expands faster than light. That is wrong (see e.g. here for an explanation). $\endgroup$
    – pela
    Feb 26, 2017 at 23:03
  • $\begingroup$ A change in $c$ would change literally everything. This change doesn't happen, or either the current, or the world 4.5bln years ago had had too much different laws for the Solar System to be created. $\endgroup$
    – peterh
    Feb 26, 2017 at 23:23
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    $\begingroup$ related: physics.stackexchange.com/q/34874 $\endgroup$
    – user4552
    Aug 4, 2017 at 13:13

3 Answers 3


One of the ways we detect what is theorized to be the expansion of space time is by observing the red-shift of stars. We observe that objects which are far away from us show a red shift that indicates that they are traveling away from us. This red shift is surprisingly proportional to distance to the object. This either means that Earth is in a special location in the universe where this property holds true, or there is something which can be observed by all which yields this result. One solution is that space is expanding, which is the currently preferred scientific theory. You suggest an alternative that this may be caused by light decreasing in speed.

One major issue with this is that our units are fixed such that the speed of light is 299,792,458 m/s. This is because we defined the meter to be the distance a photon travels in 1/299792458 of a second. If light were slowing down, our meter would shrink with it because we defined it to do this. Accordingly, light can never slow down by our measurements, so the universe must be expanding by our measurements.

Now you could build a different model of the universe. You could make one that is structured such that the universe is not expanding and the speed of light is decreasing. However, this comes with many side effects. The speed of light is also tied to Maxwell's equations, $c=\frac{1}{\sqrt{\epsilon_0\mu_0}}$. You would have to figure out which of these two constants must also be variable (or perhaps both are variable). All sorts of side effects crop up when you do these things.

I harken back to Einstein's special relativity. When Einstein put his theory forth, we had already figured out the Lorentz transforms which make Maxwell's equations fit with the empirically gathered data. However, Einstein's formulation was unique because it made the model simpler. It made it easier to predict what would happen when we took the next step. Likewise the model of the universe where the universe is expanding creates a simple model which is easy to make predictions from. A theory where light changes speed is far more complicated. You'd have to work out all of those details, and then we could propose a way to test that hypothesis, or perhaps find existing empirical data to refute the suggestion.


Not going to go into great detail but perhaps this will help you:

The coordinate system of the Universe is expanding. Space is expanding. What that means is that as a photon at a particular wavelength moves through the universe, it is moving through a universe which is expanding over time. As such, the actual wavelength of the photon also expands over time and the photon becomes "red-shifted".

That being said, you are never going to see your desk at home expanding to anything other than the dimensions given by the metre stick you measure it with.

Space and time, wavelength and frequency of light are inextricably linked, but the speed of light always remains the same.

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    $\begingroup$ This doesn't really address the OP's concern, it just restates that space is expanding. $\endgroup$
    – pela
    Feb 26, 2017 at 23:18

Good question I think.

If speed of light was decreasing with distance (and suppose frequency/wavelength adjusted accordingly), then, you know, the speed of the light emitted from very far off galaxies should become slower and slower and we would have seen some of the light just crawling by the time it reaches here.

Here is the way to prove or disprove it. I am not saying it has been done, but I think if this was the case, someone would have noticed it.

  1. Take a long tube and point it to a single galaxy.

  2. close far end of the tube and put a detector on the rear end of the tube.

  3. Open the aperture on the front end and measure the time taken by the light to cross the tube (to reach the detector).

  4. Perform these steps by pointing the tube to various galaxies - by guessing far and near.

  5. If the speed was decreasing with distance and/or time, then we would see a time difference in the above experiment depending upon how far the galaxy was pointed at.

  6. If the time is always same, then there is no impact of distance/time on the speed of light.

I do not believe that the light speed changes. I do not think any one will even want to conduct this test.


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