An astrophysicist analyzing the absorption spectra of a distant galaxy, designated SG-409, observes that the prominent Hydrogen-alpha spectral line, which has a laboratory wavelength of λ0=656.3 nm\lambda_0 = 656.3\text{ nm}λ0=656.3 nm, is detected at a longer wavelength of λ=698.5 nm\lambda = 698.5\text{ nm}λ=698.5 nm.
Explain how this phenomenon of wavelength shifting provides evidence that the Universe is expanding.