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Disclaimer: I asked this at Astronomy.SE, but got no answer whatsoever, so I am trying my luck here.

As you probably know current state-of-the art physics (i.e. gravitational waves, cosmic expansion) basically states that space itself is subject to expansion or contraction. Since there is no moving matter or energy involved, this might even happen at a "speed" faster than light.

So far, so good and obscure. What strikes me is the principle that the speed of light as a fundamental constant can only be expressed as a function of space-time. Where do we know that the one is constant but the other can suddenly be variable?

Is there any reason why the point of view of an expanding or contracting space is preferred over, say, a reduction in the speed of light or an increase in the "speed" of time? Is there any objective difference, a mathmatical model being a better fit or is it just the good old rubber metaphor being stretched (pun intended) too far?

In case the answer is: Both are equal w.r.t. current observations: How do we know that not both are actually variable?

Community
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choeger
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2 Answers2

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When we observe very distant objects, the spectral lines of atoms seems to keep the same, and the physics (and the objects it permits) as well, despite c appears in many balances of the microphysics. So c does not seems to change in all the observable Universe. Beside, some pulsing phenomena seems to be slowered as predicted by time contraction in relativity. Therefore the red-shift really means expansion of space, and not change in the light.

Fabrice NEYRET
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Is there any reason why the point of view of an expanding or contracting space is preferred over, say, a reduction in the speed of light or an increase in the "speed" of time

The postulate that the speed of light is the same in different reference frame led to a whole theory that can explain observations well and solved problems you would see otherwise. There is no theoretical preference for it otherwise.

Jan Bos
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