Explain why the Big Bang Theory is the currently accepted model for the origin of the universe, while the Steady State Theory has been rejected.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists have proposed different theories to explain the origin and evolution of the universe. Two of these theories are the Big Bang Theory and the Steady State Theory.
Model answer (5 marks)
The Big Bang Theory states that the universe began as a very small, hot and dense point and has been expanding ever since.
Red‑shift of distant galaxies shows that they are moving away from us, which is evidence that the universe is expanding.
The cosmic microwave background radiation (CMBR) is detected uniformly across the sky; it is the remnant heat from the early hot universe and supports the Big Bang.
The Steady State Theory cannot explain the existence of the CMBR, because it predicts a universe that is always the same.
Observations of different types of stars and galaxies at different epochs show that the universe has evolved, contradicting the Steady State claim that the universe is unchanged in time and space.
Red‑shift of distant galaxies shows that they are moving away from us, which is evidence that the universe is expanding.
The cosmic microwave background radiation (CMBR) is detected uniformly across the sky; it is the remnant heat from the early hot universe and supports the Big Bang.
The Steady State Theory cannot explain the existence of the CMBR, because it predicts a universe that is always the same.
Observations of different types of stars and galaxies at different epochs show that the universe has evolved, contradicting the Steady State claim that the universe is unchanged in time and space.
Examiner tips
- Use the exact phrases ‘Big Bang Theory’, ‘red‑shift’, ‘CMBR’, and ‘Steady State Theory’.
- Show the causal link: expansion → red‑shift; hot dense start → CMBR.
- Mention the evolution of stars/galaxies as evidence against steady state.
Common mistakes
- Confusing the Big Bang with the Steady State as the same theory.
- Failing to mention the CMBR as key evidence.
- Using vague terms like ‘old stars’ without linking to evolution.
Mark scheme (5 marks)
- The Big Bang Theory states that the universe expanded from an initially very small, hot and dense point
- Red-shift provides evidence for the Big Bang Theory because galaxies are moving away from us, showing the universe is expanding
- Cosmic microwave background radiation (CMBR) is found all around the universe and is evidence for the Big Bang Theory
- The Steady State Theory cannot explain CMBR / does not account for cosmic microwave background radiation
- Observations show the universe has evolved over time (e.g. different types of stars existed at different eras), which contradicts the Steady State Theory's claim that the universe remains the same at all times and places
Key terms in this question
Big Bang Theory · Steady State Theory
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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