Explain how red-shift provides evidence that the universe is expanding, and describe what this suggests about how the universe began.

OCR GCSE Physics A: Gateway Science (J249) — P6.3 Astronomy · Explain · 4 marks · View as Markdown

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

Astronomers studying distant galaxies have observed that the light reaching us from these galaxies has a longer wavelength than expected.

Model answer (4 marks)

1. Light from distant galaxies is red‑shifted – its wavelength is longer than expected.
2. Red‑shift occurs because the galaxies are moving away from us; the further a galaxy, the faster it recedes, giving a larger shift.
3. Red‑shift is observed in galaxies in every direction, showing that the whole universe is expanding.
4. If the universe is expanding now, it must have started from a single, very small, hot and dense point – the Big Bang.

Examiner tips

  • Use the exact terms ‘red‑shift’, ‘moving away’, ‘expanding universe’, ‘Big Bang’
  • Show the causal link: red‑shift → recession → expansion → Big Bang
  • Keep each point brief and to the point

Common mistakes

  • Confusing red‑shift with blue‑shift; describing only one direction; omitting the link to the Big Bang

Mark scheme (4 marks)

  1. Light from distant galaxies is red-shifted, meaning its wavelength is observed to be longer (shifted towards the red end of the spectrum).
  2. Red-shift occurs because distant galaxies are moving away from Earth, and the further away a galaxy is, the faster it is moving away and the greater the red-shift observed.
  3. Red-shift from galaxies in all directions shows that the universe itself is expanding.
  4. If the universe is currently expanding, it must have originally started from a single, very small, hot and dense point — consistent with the Big Bang Theory.

Key terms in this question

red-shift · universe

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