Explain why red-shift from distant galaxies and cosmic microwave background radiation (CMBR) together provide strong evidence for the Big Bang Theory.

Edexcel A-Level Physics (9PH0) — 7.1 Electric fields · Explain · 5 marks · View as Markdown

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

Astronomers observe that light from distant galaxies has a longer wavelength than expected. They also detect low-level microwave radiation coming from all directions in space.

Model answer (5 marks)

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

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

The cosmic microwave background radiation (CMBR) is electromagnetic radiation detected in all directions throughout the universe.

The CMBR is the stretched, red‑shifted remnant of the high‑energy radiation that was released by the hot young universe shortly after the Big Bang.

The Steady State Theory cannot explain the existence of the CMBR, so the Big Bang Theory is the more accepted model as it accounts for both the red‑shift and the CMBR.

Examiner tips

  • Use the exact phrases from the mark scheme – e.g. ‘red‑shift shows that distant galaxies are moving away’, ‘CMBR is the stretched remnant’, ‘Steady State Theory cannot explain CMBR’. Show the logical link: expansion → Big Bang → CMBR. Keep each point short and to the point, matching the 5‑mark structure.
  • common_mistakes
  • :
  • Mixing up red‑shift with blueshift; writing ‘galaxies are moving towards us’. Forgetting to mention that CMBR is detected in all directions. Claiming the Steady State Theory explains CMBR, or omitting the comparison with it.

Mark scheme (5 marks)

  1. Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
  2. If the universe is currently expanding, it must have originated from a single small, hot and dense point (the Big Bang)
  3. CMBR is electromagnetic radiation detected in all directions throughout the universe
  4. CMBR is the stretched / red-shifted remnant of the high-energy radiation released by the hot young universe shortly after the Big Bang
  5. The Steady State Theory cannot explain CMBR, so the Big Bang Theory is the more accepted model as it accounts for both red-shift and CMBR

Key terms in this question

red-shift · cosmic microwave background radiation (CMBR) · Big Bang Theory

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