Explain why astronomers observe that light from distant galaxies appears shifted towards the red end of the electromagnetic spectrum, and describe what this observation tells us about the universe.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When astronomers study the light from distant galaxies, they find that the frequencies of known spectral lines are lower than expected.
Model answer (5 marks)
The light from distant galaxies is red‑shifted because the galaxies are moving away from us. As a source recedes, the wavelength of its light is stretched, so the observed frequency is lower – this is the Doppler redshift. The further a galaxy is, the larger its redshift, indicating it is receding faster. This systematic increase in redshift with distance shows that the universe is expanding. The observation supports the Big Bang model, in which the universe began from a single, dense point and has been expanding ever since.
Examiner tips
- Use the word ‘red‑shift’ and explain it as a Doppler effect; link distance to redshift; state that this proves expansion; mention the Big Bang. Keep each point brief and use exam terminology.
Common mistakes
- Confusing redshift with blueshift; not linking redshift to distance; omitting the Big Bang conclusion.
Mark scheme (5 marks)
- The galaxies are moving away from us / the Earth / our galaxy
- As a source moves away, the observed wavelength of its light increases / frequency decreases (redshift)
- The more distant the galaxy, the greater the redshift / the faster it is moving away
- This tells us the universe is expanding
- This supports the idea that the universe began from a single point / the Big Bang
Key terms in this question
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