Explain how the observed redshift of light from distant galaxies provides evidence that the universe is expanding.

AQA A-Level Physics (7408) — 3.9.3 Cosmology · Explain · 5 marks · View as Markdown

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

Astronomers study the light from distant galaxies and observe that the wavelengths of spectral lines are shifted compared to the same lines measured in a laboratory on Earth.

Model answer (5 marks)

The light from distant galaxies is shifted towards longer wavelengths (redshift). This redshift occurs because the galaxies are moving away from us. The further away a galaxy is, the greater its redshift, indicating it is moving away faster. Thus all galaxies are moving away from each other, showing that space itself is stretching. This observation supports the idea that the universe began from a single point, consistent with the Big Bang theory.

Examiner tips

Use the exact phrase "redshift" and "moving away" to match the mark scheme.
Show the logical chain: redshift → recession → expansion → Big Bang evidence.
Keep each point brief and to the point to fit the 5‑mark allocation.

Common mistakes

Confusing blueshift with redshift; the question is about redshift. Using vague terms like "moving apart" without linking to space stretching. Omitting the link to the Big Bang or the idea that all galaxies recede.

Mark scheme (5 marks)

The light from distant galaxies is shifted towards longer wavelengths (redshift)
This redshift occurs because the galaxies are moving away from us
The further away a galaxy is, the greater its redshift / the faster it is moving away
This means all galaxies are moving away from each other / space itself is stretching
This provides evidence that the universe began from a single point / supports the Big Bang theory

Key terms in this question

redshift · universe