Explain why scientists believe the universe began with a Big Bang. In your answer, refer to the motion of galaxies and the nature of the cosmic microwave background radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers observe that distant galaxies are moving away from us. They also detect microwave radiation arriving at Earth from all directions in space.
Model answer (4 marks)
Galaxies are observed to be moving away from us, showing that the universe is expanding.
The further a galaxy is, the faster it moves away (greater red‑shift), which indicates that all matter was once concentrated at a single point.
Microwave radiation is detected from every direction in space – the cosmic microwave background (CMB).
The CMB is the remnant afterglow of the intense heat produced when the universe was very hot and dense, matching the conditions expected from a Big Bang.
The further a galaxy is, the faster it moves away (greater red‑shift), which indicates that all matter was once concentrated at a single point.
Microwave radiation is detected from every direction in space – the cosmic microwave background (CMB).
The CMB is the remnant afterglow of the intense heat produced when the universe was very hot and dense, matching the conditions expected from a Big Bang.
Examiner tips
- Use the words ‘expanding’, ‘red‑shift’, ‘remnant’ and ‘afterglow’ as these are key terms in the mark scheme.
- Show the link between distance, speed and red‑shift to justify a single origin point.
- Mention that the CMB comes from all directions to support a uniform early universe.
Common mistakes
- Confusing the CMB with ordinary starlight or radio waves.
- Failing to state that more distant galaxies move faster (missing the red‑shift point).
- Using vague phrases like ‘the universe is old’ instead of specific evidence such as expansion and CMB.
Mark scheme (4 marks)
- Galaxies are moving away from us / the universe is expanding
- More distant galaxies move away faster (greater red-shift), supporting the idea that everything originated from a single point
- Cosmic microwave background (CMB) radiation exists / is detected from all directions in space
- The CMB is the remnant / afterglow of the intense heat from the Big Bang, consistent with a very hot, dense origin
Key terms in this question
Big Bang · cosmic microwave background radiation
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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