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

Eduqas GCSE Physics — 10.2 Red shift and the Big Bang · Explain · 4 marks · View as Markdown

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

Astronomers observe that light from almost all distant galaxies is shifted towards the red end of the spectrum. The further away a galaxy is, the greater the red shift observed.

Model answer (4 marks)

Red shift shows that galaxies are moving away from us, indicating the universe is expanding.

The greater the red shift, the faster a galaxy is receding, so more distant galaxies move away more quickly.

If the universe is expanding, it must have started from a state where all matter was concentrated in one place.

The Big Bang is the event that marks the beginning of this expansion, i.e. the start of space and time.

Examiner tips

  • Use the word "expanding" to link red shift to the Big Bang.
  • Show the causal chain: red shift → expansion → origin in one point → Big Bang.

Common mistakes

  • Confusing red shift with blueshift.
  • Failing to mention that the Big Bang is the start of space and time.

Mark scheme (4 marks)

  1. Red shift indicates that galaxies are moving away from us / the universe is expanding
  2. Greater red shift from more distant galaxies shows they are moving away faster
  3. This expansion implies that all matter was originally in one place / at one point
  4. The Big Bang was the event at the start of the universe / the start of space and time

Key terms in this question

red shift · Big Bang

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