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.
53 exam-style questions on Edexcel GCSE Physics 7.2 The Universe and red-shift, covering 7.2.1 Big Bang and Steady State theories, 7.2.2 Evidence for the Big Bang, 7.2.3 The Doppler effect and red-shift, 7.2.4 Red-shift as evidence for an expanding Universe, and 7.2.5 The CMB and the accepted model. Each one has a worked solution and a mark scheme showing where the marks go.