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

Edexcel A-Level Physics (9PH0) — 7.2 Capacitors · Explain · 5 marks · View as Markdown

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

Scientists have proposed different theories to explain how the universe began. Two of these are the Big Bang Theory and the Steady State Theory.

Model answer (5 marks)

The Steady State Theory cannot explain the cosmic microwave background radiation (CMBR). CMBR is electromagnetic radiation found everywhere in the universe, detected in all directions in the sky. The Big Bang Theory explains CMBR as the stretched, red‑shifted high‑energy radiation left over from the hot, early universe. Red‑shift shows that galaxies are moving away from us, indicating that the universe is expanding. Because the universe is expanding, it must have started from a single small, hot, dense point, which supports the Big Bang Theory.

Examiner tips

  • Use the exact terms ‘cosmic microwave background radiation’, ‘red‑shift’, and ‘expanding universe’.
  • Show the logical sequence: CMBR evidence → expansion → origin point.
  • Keep each point brief and directly linked to the mark scheme points.

Common mistakes

  • Confusing the Steady State Theory with the Big Bang Theory, or vice versa.
  • Failing to mention that CMBR is detected in all directions.
  • Using vague terms like ‘old universe’ instead of ‘hot, early universe’.

Mark scheme (5 marks)

  1. The Steady State Theory cannot account for / does not explain cosmic microwave background radiation (CMBR)
  2. CMBR is electromagnetic radiation found everywhere in the universe / detected in all directions in the sky
  3. CMBR is explained by the Big Bang Theory as the stretched / red-shifted high-energy radiation left over from the hot, early universe
  4. Red-shift shows that galaxies are moving away from us / the universe is expanding
  5. Because the universe is expanding, it must have started from a single small, hot, dense point / this supports the Big Bang Theory

Key terms in this question

Big Bang Theory · Steady State Theory

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