system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that the universe is expanding and light is not just losing momentum?

We know that the universe is expanding since light coming from distant galaxies are redshifted. How do we know that the redshift isn't the result of light losing momentum over incredibly long periods of time? (As momentum decreases, wavelength increases (p=h/λ))

Are there any other methods to verify that the universe is expanding other than observing redshifts of light?
gpt
This question is extremely common, there are likely many good threads with further discussion if you wish to search for it.

Much of the observational evidence for the big bang comes from studying its 'echo', the cosmic microwave background (CMBR). There are many measurements you can do with a quality map of the CMBR and each one we do matches our predictions for an expanding universe.

Another good piece of evidence is the relative ratios of primordial elements. We know roughly how much of each stuff was in the universe, 75% H 24%He and some lithium. We know how long the universe was at a certain temperature and we know fusion rates and decay rates of unstable isotopes so we should be able to predict this ratio. Our predictions match our observations.