Explain how observations of red-shift in light from distant galaxies support the Big Bang theory.

AQA GCSE Physics (8463) — 4.8.2 Red-shift (Phys only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Red‑shift is an increase in the observed wavelength (or decrease in frequency) of light from a galaxy.

It indicates that galaxies are moving away from us (the Earth).

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

This shows the universe is expanding, which is consistent with the idea that it began from a single point – the Big Bang.

Examiner tips

  • Use the exact definition of red‑shift; link it to wavelength. Show the distance–red‑shift relationship. Explain that expansion supports a single origin. Keep each point brief but complete.

Common mistakes

  • Confusing red‑shift with blue‑shift. Failing to mention the distance–red‑shift correlation. Using vague terms like "moving away" without linking to expansion.

Mark scheme (4 marks)

  1. Red-shift is an increase in the observed wavelength (/ decrease in frequency) of light from a galaxy
  2. Red-shift indicates that galaxies are moving away from us / the Earth
  3. The more distant the galaxy, the greater the red-shift / the faster it is moving away
  4. This shows the universe is expanding, which is consistent with / supports the idea that it began from a single point (the Big Bang)

Key terms in this question

red-shift · Big Bang

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