Explain how red-shift and cosmic microwave background radiation (CMBR) together provide evidence for the Big Bang Theory, and why the Steady State Theory cannot fully account for this evidence.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers studying distant galaxies observe that the light reaching Earth has a longer wavelength than the light originally emitted. Separately, sensitive radio telescopes detect low-level microwave radiation coming from all directions in the sky.
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, tracing the motion of galaxies back in time means that all matter was once concentrated in a single, very small, hot and dense point – a state that is exactly what the Big Bang Theory predicts.
The cosmic microwave background radiation (CMBR) is a low‑level microwave glow that is detected uniformly in all directions across the sky.
The CMBR is the stretched, red‑shifted remnant of the high‑energy radiation that was emitted when the universe was very young and hot, shortly after the Big Bang.
The Steady State Theory cannot explain the existence of this uniform background radiation, so it is rejected in favour of the Big Bang Theory.
If the universe is expanding, tracing the motion of galaxies back in time means that all matter was once concentrated in a single, very small, hot and dense point – a state that is exactly what the Big Bang Theory predicts.
The cosmic microwave background radiation (CMBR) is a low‑level microwave glow that is detected uniformly in all directions across the sky.
The CMBR is the stretched, red‑shifted remnant of the high‑energy radiation that was emitted when the universe was very young and hot, shortly after the Big Bang.
The Steady State Theory cannot explain the existence of this uniform background radiation, so it is rejected in favour of the Big Bang Theory.
Examiner tips
- Use the word "shows" or "indicates" for red‑shift evidence. Mention that the CMBR is uniform and its origin is the early hot universe. Explain why the Steady State model cannot produce the CMBR. Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Confusing red‑shift with Doppler shift only, ignoring cosmological expansion. Failing to state that the CMBR is uniform and a remnant of the early universe. Claiming the Steady State Theory explains the CMBR or omitting this point.
Mark scheme (5 marks)
- Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
- If the universe is currently expanding, tracing back in time means it must have originated from a single, very small, hot and dense point — consistent with the Big Bang Theory
- CMBR is low-level microwave radiation detected uniformly in all directions across the sky
- CMBR is the stretched / red-shifted remnant of the high-energy radiation emitted by the very hot, young universe shortly after the Big Bang
- The Steady State Theory cannot explain CMBR, so it is rejected in favour of the Big Bang Theory
Key terms in this question
red-shift · cosmic microwave background radiation (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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