Explain how observations of red-shift from distant galaxies provide evidence for the Big Bang theory.

Pearson Edexcel International GCSE Physics (4PH1) — 8.3 Cosmology · Explain · 4 marks · View as Markdown

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

Astronomers observe that the light reaching Earth from distant galaxies has a longer wavelength than the same light produced in a laboratory on Earth. Galaxies that are further away show a greater increase in wavelength than those that are closer.

Model answer (4 marks)

Red‑shift means the wavelength of light from distant galaxies is longer than expected.
Red‑shift shows that galaxies are moving away from us (Doppler effect).
The further the galaxy, the greater the red‑shift, so it is moving away faster.
This pattern matches an expanding universe from a single point, supporting the Big Bang theory.

Examiner tips

  • Use the word ‘red‑shift’ and explain it as increased wavelength. Show the link to Doppler effect and motion away. State the distance–red‑shift relationship. Finish by connecting to the Big Bang expansion.
  • common_mistakes
  • :
  • Confusing red‑shift with blue‑shift. Saying galaxies are moving towards us. Missing the distance–red‑shift correlation. Not linking the observations to the Big Bang theory.

Mark scheme (4 marks)

  1. Red-shift means the wavelength of light from distant galaxies is increased / longer than expected
  2. Red-shift shows that galaxies are moving away from us / Earth (due to the Doppler effect)
  3. The further the galaxy, the greater the red-shift / the faster it is moving away
  4. This pattern is consistent with the universe expanding from a single point / supports the Big Bang theory

Key terms in this question

red-shift · Big Bang theory

Related

More Cosmology questions

▶ Try answering this question with AI marking (free) →