A student claims that the universe has always existed and has always been the same size. Explain why observations of red shift from distant galaxies do not support this claim.

Eduqas GCSE Physics — 10.2 Red shift and the Big Bang · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Light from distant galaxies is red‑shifted – the wavelength is longer than expected for nearby objects.

Red shift shows that galaxies are moving away from us; the galaxies are receding.

This means the universe is expanding – the space between galaxies is increasing.

An expanding universe suggests it began from a single point, supporting the Big Bang theory and contradicting the claim that the universe has always been the same size.

Examiner tips

  • Use the term ‘red‑shift’ and explain it as longer wavelengths. Show the causal chain: red‑shift → recession → expansion → Big Bang. Keep each point concise to fit the 4 marks.

Common mistakes

  • Confusing red‑shift with blue‑shift. Saying galaxies are moving towards us. Failing to link expansion to the Big Bang.

Mark scheme (4 marks)

  1. Light from distant galaxies is red-shifted / the wavelength of light from distant galaxies is longer than expected
  2. Red shift shows that galaxies are moving away from us / the galaxies are receding
  3. This means the universe is expanding / the space between galaxies is increasing
  4. An expanding universe suggests it began from a single point / supports the Big Bang theory, contradicting the claim that the universe has always been the same size

Key terms in this question

red shift

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