Astronomers observing the spectral lines of hydrogen from a distant galaxy cluster notice that the characteristic Hα\text{H}\alphaHα line (measured at 656.3 nm656.3\text{ nm}656.3 nm in the laboratory) is shifted to a longer wavelength of 682.1 nm682.1\text{ nm}682.1 nm in the light received from the cluster.
Explain how this spectroscopic observation, when observed for distant galaxies across the cosmos, provides key observational evidence supporting the Big Bang theory.