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I read several questions/answers about red shift on this web site, but there is something I still don't understand.

Why is red-shift considered to be the effect of expanding space-time, why it could not be a change in the speed of light. After all, if the speed slowly increases, it could affect the wavelength of the light. And perhaps something else we call "constant" could be changing and affect the wavelength of light. I don't understand why space-time would change but no other physics constants or even laws.

Xoff
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  • (1) It's not meaningful to talk about whether a dimensionful constant (i.e., one that has units) changes over time. This is discussed in more detail here: http://physics.stackexchange.com/questions/34874/has-the-speed-of-light-changed-over-time . (2) It's not mandatory to describe cosmological red-shifts as being due to cosmological expansion. It's equally valid to call it a kinematic redshift because the universe is expanding and therefore distant galaxies are moving away from us. This is a verbal distinction, not a distinction that is meaningful in the actual mathematical theory. –  Sep 30 '13 at 22:27
  • (3) Cosmological redshifts are not the only evidence we have for cosmological expansion. For example, we have models of big bang nucleosynthesis that agree with observation, and also observations of anisotropy in the cosmic microwave background. –  Sep 30 '13 at 22:32

2 Answers2

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Variable speed of light theories go all the way back to Einstein himself in 1907. The trouble is that allowing the speed of light to vary affects so many other things that no-one has so far come up with a plausible VSL theory that doesn't contradict experiment. On the other hand, GR and in particular the FLRW solution to GR, matches experiment very well. Unless some unexpected experimental evidence shows up, Occam's razor suggests we interpret the red shift as due to the expansion of spacetime.

John Rennie
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I don't understand why space-time would change but no other physics constants or even laws.

First, it's the metric expansion of space, not spacetime.

Second, this expansion is a consequence of physical law, as modelled by General Relativity and, in fact, could have been predicted before it was first observed.

In other words, the geometry of spacetime is not fixed, like a physics constant or law, but is dynamic and evolves according to physical law.

Alfred Centauri
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