A-Level Physics October/November 2024 Q10(a): Explain how redshift leads to the idea that the Universe is expanding.

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  Redshift is the observed increase in the wavelength (or decrease in frequency) of electromagnetic radiation from a source, often explained by the Doppler effect.

  Astronomical observations show that the light from distant galaxies is redshifted.

  This redshift implies that these distant galaxies are moving away from us (receding). Since this observation holds for galaxies in all directions, it leads to the conclusion that the entire Universe is expanding.

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  How the marks are awarded

  • B1 — Correctly defining redshift as the increase in observed wavelength or decrease in frequency.
  • B1 — Stating the key observation that radiation from distant galaxies is redshifted.
  • B1 — Linking redshift to the conclusion that galaxies are moving apart, which means the Universe is expanding.

  Common mistakes

  • Simply stating 'light becomes redder' without mentioning the scientific reason (increase in wavelength or decrease in frequency).
  • Confusing cosmological redshift with gravitational redshift or the Doppler effect for sound waves without context.
  • Failing to make the logical link between galaxies moving apart and the Universe itself expanding.
  • Forgetting to mention that this is observed for distant galaxies, which is the crucial evidence.

  Examiner tip: For 'explain' questions, structure your answer as a logical sequence: state the key principle, link it to the observed evidence, and then draw the final conclusion required by the question.

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