Explain why red-shift and cosmic microwave background radiation (CMBR) are used as evidence to support the Big Bang Theory rather than the Steady State Theory.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists have proposed two competing theories to explain the origin and nature of the universe: the Big Bang Theory and the Steady State Theory. Only one of these theories is currently accepted by the scientific community.
Model answer (5 marks)
Red‑shift shows that distant galaxies are moving away from Earth, indicating that the universe is expanding.
The expansion implies that the universe must have started from a single, small, hot, dense point – a condition that is satisfied by the Big Bang but not by the Steady State model.
CMBR is a low‑energy microwave radiation that is detected uniformly throughout the universe.
The Big Bang explains CMBR as the high‑energy radiation emitted when the early universe was hot, which has been stretched (red‑shifted) to microwave wavelengths as the universe expanded.
The Steady State Theory cannot account for this uniform background radiation, so it is rejected in favour of the Big Bang Theory.
The expansion implies that the universe must have started from a single, small, hot, dense point – a condition that is satisfied by the Big Bang but not by the Steady State model.
CMBR is a low‑energy microwave radiation that is detected uniformly throughout the universe.
The Big Bang explains CMBR as the high‑energy radiation emitted when the early universe was hot, which has been stretched (red‑shifted) to microwave wavelengths as the universe expanded.
The Steady State Theory cannot account for this uniform background radiation, so it is rejected in favour of the Big Bang Theory.
Examiner tips
- Use the exact phrases ‘red‑shift’, ‘expanding universe’, ‘CMBR’, ‘uniform’, ‘high‑energy radiation’, ‘stretched to microwave wavelengths’, and ‘cannot account for’ to match the mark scheme.
- Structure the answer in five clear points, each addressing one of the five scheme points.
- Show the causal link: red‑shift → expansion → Big Bang origin; CMBR → evidence of early hot universe.
- Mention the failure of Steady State to explain CMBR to secure the final mark.
Common mistakes
- Confusing red‑shift with Doppler shift only, ignoring its cosmological significance.
- Failing to state that CMBR is uniform and low‑energy, or that it is a remnant of the early hot universe.
- Claiming the Steady State theory explains CMBR, which is incorrect.
Mark scheme (5 marks)
- Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
- The expansion of the universe implies it originated from a single small, hot, dense point (consistent with the Big Bang)
- CMBR is low-energy microwave radiation detected uniformly throughout the universe
- CMBR is explained as high-energy radiation from the early hot universe that has been stretched / red-shifted to microwave wavelengths as the universe expanded
- The Steady State Theory cannot account for CMBR, so it is rejected in favour of the Big Bang Theory
Key terms in this question
red-shift · CMBR · Big Bang Theory · Steady State Theory
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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