Describe what scientists observe when they analyse light from distant galaxies, and explain what this tells us about how the universe is changing.

AQA GCSE Physics (8463) — 4.8.2 Red-shift (Phys only) · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Scientists observe that light from distant galaxies is red‑shifted – its wavelengths are longer and frequencies lower than expected.
The red‑shift increases with distance; the more distant the galaxy, the greater the shift.
This indicates that galaxies are moving away from us.
Hence the universe is expanding.

Examiner tips

  • Use the phrase "red‑shifted" and mention longer wavelengths/ lower frequencies. Show the distance–red‑shift relationship. Explain the motion away from us. State that this implies an expanding universe.

Common mistakes

  • Saying the light is blue‑shifted or ignoring the distance relationship. Failing to link red‑shift to galaxies moving away. Using vague terms like "moving" without specifying direction or expansion.

Mark scheme (4 marks)

  1. Light from distant galaxies is red-shifted (wavelengths are longer / frequencies are lower than expected)
  2. The more distant the galaxy, the greater the red-shift observed
  3. This shows that galaxies are moving away from us / the Earth
  4. This means the universe is expanding

Key terms in this question

universe

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