Explain how astronomers use red-shift to provide evidence that the universe is expanding.

WJEC A-Level Physics (Wales) — 4.3 Orbits and the wider universe · Explain · 5 marks · View as Markdown

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

When astronomers observe light from distant galaxies, they notice that the wavelengths of known spectral lines are shifted compared with the same lines measured in a laboratory on Earth.

Model answer (5 marks)

Light from distant galaxies is observed to be red‑shifted – the spectral lines are displaced to longer wavelengths.

Red‑shift indicates that the galaxies are moving away from us, as the Doppler effect stretches the light.

The larger the red‑shift, the higher the recession velocity of the galaxy.

Because galaxies are receding in every direction, the whole universe is increasing in size – it is expanding.

This observation supports the view that the universe started from a single, dense point – the Big Bang.

Examiner tips

Use the word ‘red‑shift’ and explain it as a shift to longer wavelengths.
Show the link between larger red‑shift and higher recession speed.
Mention that recession occurs in all directions to justify expansion.
Connect expansion to the Big Bang idea.

Common mistakes

Confusing blue‑shift with red‑shift; stating that galaxies are moving towards us.
Failing to mention that red‑shift is a Doppler effect.

Mark scheme (5 marks)

Light from distant galaxies is red-shifted (wavelengths are longer / shifted towards the red end of the spectrum)
Red-shift means the galaxies are moving away from us
The greater the red-shift, the greater the speed at which the galaxy is moving away
Galaxies are moving away in all directions, so the universe must be getting bigger / expanding
This supports the idea that the universe began from a single point / the Big Bang

Key terms in this question

red-shift · universe