Astronomers observe that light from distant galaxies is red-shifted. Explain what red-shift tells us about the universe, and describe how the amount of red-shift changes with distance from Earth.

WJEC GCSE Physics (Wales) — 2.1 Light, lenses and the universe · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Red‑shift means the wavelength of light from a galaxy is longer, i.e. shifted towards the red end of the spectrum.

It shows that galaxies are moving away from us, which means the universe is expanding.

More distant galaxies show a greater red‑shift.

Thus more distant galaxies are moving away faster – there is a proportional relationship between recession speed and distance (Hubble’s law).

Examiner tips

  • Use the term ‘red‑shift’ and explain it as a longer wavelength. Mention that it indicates recession and expansion. State the distance–red‑shift correlation. Show the proportionality (Hubble’s law) to hit all marks.

Common mistakes

  • Confusing red‑shift with blue‑shift. Saying galaxies are moving towards us. Omitting the distance–speed relationship.

Mark scheme (4 marks)

  1. Red-shift means the wavelength of light from a galaxy is longer / shifted towards the red end of the spectrum
  2. Red-shift shows that galaxies are moving away from us / the universe is expanding
  3. More distant galaxies show a greater red-shift
  4. More distant galaxies are moving away faster / there is a relationship between recession speed and distance

Key terms in this question

red-shift · universe

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