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.

Edexcel A-Level Physics (9PH0) — 7.2 Capacitors · 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 of these theories are the Big Bang Theory and the Steady State Theory.

Model answer (5 marks)

1. Red‑shift of light from distant galaxies shows they are moving away, proving the universe is expanding.
2. An expanding universe must have started from a single, small, hot, dense point – the Big Bang.
3. The cosmic microwave background radiation (CMBR) is observed uniformly in all directions.
4. CMBR is the remnant of the hot early universe, now stretched to microwave wavelengths by the expansion.
5. The Steady State Theory is rejected because it cannot explain the CMBR and because observations show the universe has evolved, contradicting the idea that it is always the same.

Examiner tips

  • Use the exact terms ‘red‑shift’, ‘expanding universe’, ‘hot, dense point’, ‘CMBR’, ‘uniform’, ‘stretched’, ‘evolved’.
  • Show the logical sequence: red‑shift → expansion → Big Bang origin → CMBR evidence → rejection of Steady State.

Common mistakes

  • Mixing up red‑shift with blue‑shift; not linking expansion to a Big Bang origin.
  • Claiming the Steady State theory explains CMBR; forgetting that it predicts no evolution of the universe.

Mark scheme (5 marks)

  1. Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
  2. An expanding universe means it must have originated from a single small, hot, dense point (Big Bang)
  3. Cosmic microwave background radiation (CMBR) is detected uniformly in all directions across the universe
  4. CMBR is explained as high-energy radiation from the hot early universe that has been stretched / red-shifted to microwave wavelengths as the universe expanded
  5. The Steady State Theory is rejected because it cannot account for / explain CMBR, and/or because observations show the universe has evolved (different stars existed at different eras), contradicting the idea that the universe always looks the same

Key terms in this question

Big Bang Theory · Steady State Theory

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