The universe contains billions of galaxies, each made up of billions of stars. Astronomers use the red-shift of light from distant galaxies to learn about the universe. Explain what red-shift tells astronomers about the movement of distant galaxies and about the origin of the universe.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Light from distant galaxies shows that the wavelengths of known spectral lines are shifted towards the red end of the spectrum. The further away a galaxy is, the greater the shift observed.
Model answer (4 marks)
Red‑shift means the wavelength of light from distant galaxies is increased, i.e. shifted towards the red end of the spectrum.
It shows that those galaxies are moving away from us.
The larger the red‑shift, the faster (and therefore further) the galaxy is receding.
This observation supports the Big Bang theory, which states that the universe began from a single point and has been expanding ever since.
It shows that those galaxies are moving away from us.
The larger the red‑shift, the faster (and therefore further) the galaxy is receding.
This observation supports the Big Bang theory, which states that the universe began from a single point and has been expanding ever since.
Examiner tips
- Use the word ‘red‑shift’ and explain it as a wavelength increase; link it to motion away from us; mention the speed–distance relation; finish with the Big Bang theory. 4 points total.
- Keep each point brief and use the exact terminology the mark scheme rewards.
- Show the causal chain: shift → motion away → expansion → Big Bang.
- Avoid unnecessary detail or repetition.
Common mistakes
- Confusing red‑shift with blue‑shift or saying the galaxy is moving towards us.
- Using vague terms like ‘moving away’ without linking to wavelength increase.
- Failing to mention the Big Bang theory as the explanation for the expansion.
Mark scheme (4 marks)
- Red-shift means the wavelength of light from distant galaxies is increased (shifted towards longer wavelengths / towards the red end of the spectrum)
- Red-shift shows that distant galaxies are moving away from us / the Earth
- The greater the red-shift, the faster / the further away the galaxy is moving away
- This evidence 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 WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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