Astronomers observing the absorption spectra of distant galaxies notice that key spectral lines are shifted towards the longer wavelength (red) end of the spectrum compared to a laboratory reference source. Explain how the relationship between the size of this red-shift and the distance of galaxies from Earth provides evidence that the Universe is expanding.
53 exam-style questions on Edexcel GCSE Physics 7.2 The Universe and red-shift. Each one has a worked solution and a mark scheme showing where the marks go.