Submission + - Why does light stretch as the Universe expands?
StartsWithABang writes: On the one hand, galaxies are definitely redshifted, and they're redshifted more severely the farther they are; that's been indisputable since Hubble's data from the 1920s. But spacetime's expansion — the idea that photons get redshfited because expanding space stretches their wavelength — is just one possibility. Sure, it's the possibility predicted by General Relativity, but a fast-moving, receding galaxy could cause a redshift, too. How do we know what the cause is? Here's how.