The universe is thought to have begun with the Big Bang approximately 13.8 billion years ago. Cosmic Microwave Background (CMB) radiation is detected coming from all directions in space. Explain what CMB radiation is and why it supports the Big Bang theory.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
CMB radiation is low‑frequency microwave electromagnetic radiation that fills the whole universe and is detected from all directions.
It is the thermal energy left over from the Big Bang, released shortly after the Big Bang.
At the time of the Big Bang the universe was extremely hot and dense.
As the universe expanded it cooled, causing the radiation to red‑shift to longer wavelengths, i.e. microwave wavelengths.
The fact that CMB radiation is uniform and detected from all directions supports the idea that the Big Bang happened everywhere and that the universe expanded from a single point, which is consistent with the Big Bang model.
It is the thermal energy left over from the Big Bang, released shortly after the Big Bang.
At the time of the Big Bang the universe was extremely hot and dense.
As the universe expanded it cooled, causing the radiation to red‑shift to longer wavelengths, i.e. microwave wavelengths.
The fact that CMB radiation is uniform and detected from all directions supports the idea that the Big Bang happened everywhere and that the universe expanded from a single point, which is consistent with the Big Bang model.
Examiner tips
- Use the exact terms ‘microwave radiation’, ‘thermal energy’, ‘red‑shift’, and ‘uniform’ as the mark scheme rewards precise language.
- Show the causal chain: hot dense → expansion → cooling → red‑shift to microwave. Mention that uniformity implies a single origin everywhere.
Common mistakes
- Confusing CMB with other background radiation or using ‘visible light’ instead of ‘microwave’. Failing to link the uniformity of CMB to the Big Bang’s single‑point origin.
Mark scheme (5 marks)
- CMB radiation is (low-frequency / microwave) electromagnetic radiation that fills the whole universe / detected from all directions
- CMB radiation is the (thermal / heat) energy left over from the Big Bang / released shortly after the Big Bang
- At the time of the Big Bang / early universe, temperatures were extremely high / the universe was very hot and dense
- As the universe expanded it cooled, causing the radiation to (redshift to / stretch to) longer wavelengths / lower frequencies / microwave wavelengths
- The fact that CMB radiation is uniform / detected from all directions supports the idea that the Big Bang happened everywhere / the universe expanded from a single point / it is consistent with the Big Bang model
Key terms in this question
Cosmic Microwave Background (CMB) radiation · Big Bang · universe
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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