A student says: 'Red-shift proves that all galaxies are moving away from us, which means the Milky Way must be at the centre of the universe.' Evaluate this statement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Red‑shift is observed in the light from almost all distant galaxies, not just a few.
2. The shift to longer wavelengths shows that these galaxies are receding from us – the universe is expanding.
3. The fact that galaxies move away does not mean the Milky Way is at the centre; expansion is uniform and every galaxy sees the same.
4. The observation of red‑shift is a key piece of evidence for the Big Bang theory, which posits a single origin point from which the universe has been expanding.
2. The shift to longer wavelengths shows that these galaxies are receding from us – the universe is expanding.
3. The fact that galaxies move away does not mean the Milky Way is at the centre; expansion is uniform and every galaxy sees the same.
4. The observation of red‑shift is a key piece of evidence for the Big Bang theory, which posits a single origin point from which the universe has been expanding.
Examiner tips
- Use the exact terms ‘red‑shift’, ‘receding’, ‘expanding universe’, ‘Big Bang theory’.
- Show the logical flow: observation → interpretation → critique of conclusion.
- Mention that expansion is homogeneous – no centre.
- Keep answer concise to fit 4 marks.
Common mistakes
- Claiming red‑shift is only seen in a few galaxies.
- Assuming the Milky Way is the centre because we observe recession.
- Confusing red‑shift with Doppler shift only, ignoring cosmological expansion.
Mark scheme (4 marks)
- Red-shift is observed from all/most distant galaxies (not just some)
- Red-shift indicates that galaxies are moving away from us / the universe is expanding
- The student is correct that galaxies are moving away, but the conclusion about the Milky Way being at the centre is wrong
- Red-shift supports the Big Bang theory — the universe began from a single point and has been expanding ever since
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
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