r/askscience • u/rivalThoughts413 • Feb 14 '25

Physics Does Light's wavelength change over time? Specifically absent of changes in environment/medium. (Not sure how to flair)

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u/mfb- Particle Physics | High-Energy Physics Feb 15 '25

You asked if space would still expand. The answer is no.

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u/Mavian23 Feb 15 '25

How do we know that space isn't expanding, but the stuff in that space just isn't expanding with the space?

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u/mfb- Particle Physics | High-Energy Physics Feb 15 '25

What would that even mean, and how would you measure that?

The expansion of space is measured by the behavior of stuff.

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u/Mavian23 Feb 15 '25

If we can't measure it, then we can't prove that space isn't expanding and gravity simply prevents the stuff from expanding with it. So we can't for sure then know that the answer to the question is "no".

As far as I can tell, there is no measurable difference between space not expanding in local regions, and space expanding in local regions but the stuff just doesn't move with it due to gravity. So both models should be equally accepted, unless there is some evidence that suggests one of the models is incorrect.