Explain why scientists believe the universe began with a Big Bang. Your answer should refer to evidence from the motion of distant galaxies and the nature of background radiation detected throughout space.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers observe that light from distant galaxies is shifted towards the red end of the spectrum. Scientists have also detected a faint microwave radiation coming from all directions in space.
Model answer (4 marks)
1. Light from distant galaxies is red‑shifted, meaning the wavelength of light is stretched.
2. Red‑shift shows that distant galaxies are moving away from us, indicating the universe is expanding.
3. Cosmic microwave background (CMB) radiation is detected uniformly from all directions in space.
4. CMB radiation is the remnant energy/heat from the Big Bang, supporting the idea that the universe started from a single, very hot, dense point.
2. Red‑shift shows that distant galaxies are moving away from us, indicating the universe is expanding.
3. Cosmic microwave background (CMB) radiation is detected uniformly from all directions in space.
4. CMB radiation is the remnant energy/heat from the Big Bang, supporting the idea that the universe started from a single, very hot, dense point.
Examiner tips
- Use the exact terms ‘red‑shift’, ‘expanding universe’, ‘CMB’, and ‘remnant energy’. Explain each point in one sentence to match the 4‑mark structure. Show the link between red‑shift and expansion, and between CMB and the Big Bang.
Common mistakes
- Confusing red‑shift with blue‑shift. Failing to mention that CMB is uniform. Using vague phrases like ‘light is moving’ instead of ‘galaxies are moving away’.
Mark scheme (4 marks)
- Light from distant galaxies is red-shifted, meaning the wavelength of light is stretched/increased
- Red-shift shows that distant galaxies are moving away from us, indicating the universe is expanding
- Cosmic microwave background (CMB) radiation is detected uniformly from all directions in space
- CMB radiation is the remnant energy/heat from the Big Bang, supporting the idea that the universe started from a single, very hot, dense point
Key terms in this question
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