Explain why the Big Bang Theory is the currently accepted model for the origin of the universe, whilst the Steady State Theory has been rejected.

Edexcel GCSE Physics (1PH0) — 7.1 Astronomy (Phys only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Red‑shift of light from distant galaxies shows they are moving away from us, indicating the universe is expanding.
2. An expanding universe must have started from a single, extremely dense point – the Big Bang.
3. The cosmic microwave background radiation is detected uniformly in all directions and is a remnant of the early hot universe, supporting the Big Bang.
4. The Steady State Theory cannot explain the CMBR and therefore has been rejected.

Examiner tips

  • Use the exact terms ‘red‑shift’, ‘expanding universe’, ‘cosmic microwave background radiation’.
  • Show the logical link: expansion → Big Bang; CMBR → evidence for Big Bang, lack of CMBR in Steady State.
  • Keep each point concise and directly tied to a mark scheme point.

Common mistakes

  • Confusing red‑shift with Doppler shift; not linking expansion to a single origin.
  • Mentioning other evidence (e.g., element abundances) that is not required for the 4 marks.
  • Failing to state that the Steady State Theory cannot explain the CMBR.

Mark scheme (4 marks)

  1. Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
  2. An expanding universe means it must have originated from a single point, supporting the Big Bang Theory
  3. Cosmic microwave background radiation (CMBR) is detected in all directions and supports the Big Bang Theory
  4. The Steady State Theory cannot explain CMBR / is rejected because it does not account for CMBR

Key terms in this question

Big Bang Theory · Steady State Theory

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