Explain why observations of red-shift from distant galaxies support the Big Bang Theory as the origin of the universe.

Edexcel GCSE Physics (1PH0) — 7.1 Astronomy (Phys only) · Explain · 4 marks · View as Markdown

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

Astronomers studying light from distant galaxies notice that the wavelengths of light received are longer than expected. The further away the galaxy, the greater this effect.

Model answer (4 marks)

Red‑shift is an increase in the wavelength of light received from distant galaxies.

It shows that those galaxies are moving away from us – the universe is expanding.

The further away a galaxy, the greater its red‑shift, meaning it is moving away faster.

If the universe is now expanding, it must have started from a single small, hot, dense point – the Big Bang.

Examiner tips

  • Use the word ‘red‑shift’ and explain it as a wavelength increase. Show the link between red‑shift and galaxies moving away. Mention the distance–red‑shift relationship. Finish with the implication for the Big Bang.

Common mistakes

  • Confusing red‑shift with blue‑shift. Saying galaxies are moving towards us. Omitting the distance–red‑shift correlation. Not linking expansion to a Big Bang origin.

Mark scheme (4 marks)

  1. Red-shift is an increase in the wavelength of light received from distant galaxies
  2. Red-shift shows that distant galaxies are moving away from us / the universe is expanding
  3. The further away the galaxy, the greater the red-shift / the faster it is moving away
  4. If the universe is currently expanding, it must originally have started from a single small, hot, dense point / the Big Bang

Key terms in this question

red-shift · Big Bang Theory · universe

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