Explain why the Big Bang Theory is accepted as the model for the origin of the universe, whilst the Steady State Theory has been rejected. In your answer, refer to red-shift and cosmic microwave background radiation (CMBR).
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 an expanding universe.
2. Both theories can account for red‑shift, but the Steady State Theory explains it by continuous expansion.
3. CMBR is electromagnetic radiation found in small quantities all around the universe, detected in every direction.
4. The Big Bang Theory explains CMBR: the very hot, dense early universe emitted high‑energy radiation which has been stretched (red‑shifted) to microwave wavelengths as the universe expanded.
5. The Steady State Theory cannot explain CMBR, so it is rejected; the Big Bang Theory accounts for all experimental evidence and is the accepted model.
2. Both theories can account for red‑shift, but the Steady State Theory explains it by continuous expansion.
3. CMBR is electromagnetic radiation found in small quantities all around the universe, detected in every direction.
4. The Big Bang Theory explains CMBR: the very hot, dense early universe emitted high‑energy radiation which has been stretched (red‑shifted) to microwave wavelengths as the universe expanded.
5. The Steady State Theory cannot explain CMBR, so it is rejected; the Big Bang Theory accounts for all experimental evidence and is the accepted model.
Examiner tips
- Use the exact terms ‘red‑shift’, ‘CMBR’, ‘expanding universe’.
- Show the logical flow: red‑shift → expansion, CMBR → Big Bang explanation, lack of CMBR explanation in Steady State → rejection.
- Keep each point concise and directly linked to the mark scheme.
- Use bullet‑style numbering to match the 5 marks.
Common mistakes
- Confusing red‑shift with Doppler shift terminology; use ‘red‑shift’. Failing to mention that CMBR is detected in every direction. Claiming the Steady State Theory explains CMBR, which it does not.
Mark scheme (5 marks)
- Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
- Both theories can account for red-shift / Steady State Theory explains red-shift by the continuous expansion of the universe
- CMBR is electromagnetic radiation found in small quantities all around the universe / detected in every direction
- CMBR is explained by the Big Bang Theory — the very hot, dense early universe emitted high-energy radiation which has been stretched / red-shifted to microwave wavelengths as the universe expanded
- The Steady State Theory cannot explain CMBR, so it is rejected / the Big Bang Theory accounts for all experimental evidence so it is the currently accepted model
Key terms in this question
Big Bang Theory · Steady State Theory · red-shift · CMBR
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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