2.2.2Space
The Big Bang Theory — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 12
Part 1 of 4The Idea
The theory proposes that the universe has not always looked the way it does today.
Beginning
- About 13.8 billion years ago all matter and energy were packed into an extremely hot, dense state.
Expansion
- From that start space itself has been expanding and cooling, and the expansion continues now.
Structure
- As the universe cooled, matter clumped together under gravity to form stars, galaxies and eventually planets.
Not an explosion in space
- It was not a blast in an existing space; space and time themselves grew from the early state.
Part 2 of 4Stages of the Universe
The history can be told as an ordered sequence of changes.
1
The universe begins extremely hot and dense, and it starts expanding rapidly in all directions.
2
As it expands, the temperature falls and the first simple particles and atoms are able to form.
3
Gravity pulls clouds of mostly hydrogen and helium together into the first stars and galaxies.
4
Stars fuse lighter elements into heavier ones and later scatter them when they die.
5
New stars and planetary systems, including our own, form from this recycled material.
Part 3 of 4Evidence for the Theory
Scientists accept a theory when observations match its predictions.
Evidence for the Theory
Red-shift
- Light from distant galaxies is shifted towards the red end of the spectrum, showing that they move away from us.
Distance link
- Galaxies that are further away show a larger red-shift, so they are receding faster.
Cosmic microwave background
- Faint microwave radiation arrives from every direction and is the cooled remnant of the early hot universe.
Consistency
- Both pieces of evidence fit an expanding universe that began from a hot dense state.
Prediction
- Because the early universe was hot, the theory predicted a leftover glow, and finding it strongly supported the idea.
| Observation | What it shows |
|---|---|
| Red-shifted galaxies | The universe is expanding |
| Greater shift for distant galaxies | Expansion is happening everywhere |
| Microwave background | The early universe was hot and has since cooled |
Part 4 of 4Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Big Bang theory The theory that the universe began about 13.8 billion years ago and has been expanding since.
- Red-shift A shift of light from receding objects towards longer wavelengths.
- Cosmic microwave background Microwave radiation reaching Earth from all directions, left over from the early universe.
- Galaxy A huge collection of stars, gas and dust held together by gravity.
Common Mistakes
- Describing the Big Bang as an explosion into empty space; space itself expanded.
- Saying the red-shift proves the galaxies are getting redder in colour rather than moving away.
- Claiming that galaxies are expanding in size instead of moving apart from one another.
- Forgetting to link the evidence (red-shift, background radiation) back to the theory in a written answer.
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