Explain why the Big Bang Theory is the currently accepted model for the origin of the universe, whilst the Steady State Theory has been rejected. Refer to red-shift and cosmic microwave background radiation (CMBR) in your answer.

Edexcel A-Level Physics (9PH0) — 7.3 Magnetic fields and electromagnetic induction · Explain · 5 marks · View as Markdown

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

Scientists have proposed different theories to explain the origin and evolution of the universe. Two such theories are the Big Bang Theory and the Steady State Theory.

Model answer (5 marks)

1. Red‑shift shows that distant galaxies are moving away from Earth, indicating the universe is expanding.
2. If the universe is expanding, it must have started from a single small, hot, dense point – the Big Bang.
3. The cosmic microwave background radiation (CMBR) is low‑level microwave radiation detected in all directions.
4. The Big Bang explains the CMBR as high‑energy radiation from the early hot universe that has been stretched (red‑shifted) to microwave wavelengths as the universe expanded.
5. The Steady State Theory cannot explain the CMBR and predicts the universe looks the same at all times, yet observations show it has evolved (different types of stars at different eras).

Examiner tips

  • Use the exact terms ‘red‑shift’, ‘CMBR’, ‘expanding universe’, ‘hot dense point’. Show the logical chain: red‑shift → expansion → Big Bang. Mention the CMBR as evidence that the universe was once hot and dense. Contrast with Steady State’s failure to explain CMBR and evolution.
  • common_mistakes
  • :
  • Mixing up red‑shift with Doppler shift; forgetting to link it to expansion. Forgetting to state that CMBR is detected in all directions. Claiming the Steady State can explain CMBR or ignoring the evidence of evolution.

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 originally have started from a single small, hot, dense point (consistent with the Big Bang Theory)
  3. CMBR is low-level microwave radiation detected in all directions / throughout the universe
  4. CMBR is explained by the Big Bang Theory as high-energy radiation from the early hot universe that has been stretched / red-shifted to microwave wavelengths as the universe expanded
  5. The Steady State Theory cannot explain CMBR / is rejected because it predicts the universe looks the same at all times, but evidence shows the universe has evolved, for example different types of stars existed at different eras

Key terms in this question

Big Bang Theory · Steady State Theory · red-shift · CMBR

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