Cosmology » The Big Bang Theory
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.13P, 8.14P
- Describe how the universe has changed since the Big Bang
- Explain how red-shift supports the Big Bang theory
- Describe cosmic microwave background (CMB) radiation and why it supports the theory
- Write clear 4 to 6 mark answers about the evidence
This lesson is examined in Paper 2 only.
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Unlock This CourseThe Big Bang theory
Astronomers have found that the universe is expanding. If it is getting bigger now, then in the past it must have been smaller. Follow that idea backwards and you arrive at the Big Bang theory.
Key terms:
- Big Bang theory: the theory that the universe began about 13.8 billion years ago from a tiny, extremely hot, dense region, and has been expanding and cooling ever since.
- Cosmic microwave background (CMB) radiation: microwave radiation that reaches the Earth from every direction in space. It is left over from the hot early universe.
How the universe has changed
The theory describes the past of the universe as a story in stages:
- The start. About 13.8 billion years ago, all the matter and energy in the universe was concentrated in a tiny, extremely hot, dense region.
- Expansion. Space began to expand. It has been expanding ever since.
- Cooling. As space expanded, the universe cooled down.
- Atoms. Once it was cool enough, atoms formed.
- Stars and galaxies. Gravity pulled matter together to form stars, and the stars were gathered into galaxies.
🍰 Think of dough
Picture raisins in a lump of dough rising in an oven. As the dough expands, every raisin moves away from every other raisin. Run the film backwards and the raisins all squash together. The raisins are like galaxies. The dough is like space.
Evidence 1: red-shift
In the lesson Red-shift of Light from Galaxies you saw that the light from distant galaxies is red-shifted. That means the galaxies are moving away from us, and the more distant the galaxy, the bigger the red-shift. So the galaxies are moving apart.
This is the first main argument for the Big Bang theory:
- Galaxies are moving apart, so the universe is expanding.
- If they are moving apart now, they must have been closer together in the past.
- Going back far enough, everything was close together in one tiny, hot, dense region.
Evidence 2: cosmic microwave background radiation
The second piece of evidence is CMB radiation. In the early universe, the radiation filling the tiny, extremely hot, dense region was very energetic, with a very short wavelength. As space expanded, that radiation was stretched. Its wavelength grew until it became microwaves.
We still detect this radiation today. It has these features:
- It is microwave radiation.
- It comes from every direction in space, and it is detected almost equally from every direction.
- It is left over from the hot early universe.
The Big Bang theory predicts that this radiation should exist. Finding it fits the theory, which is why it counts as strong evidence.
| Evidence | What we observe | What it tells us |
|---|---|---|
| Red-shift | Light from distant galaxies is shifted towards the red end | Galaxies are moving apart, so they were once close together |
| CMB radiation | Microwaves arriving almost equally from every direction | Radiation left over from the hot early universe, stretched by expansion |
Worked examples
Worked example 1
Question: Explain how red-shift supports the Big Bang theory. [3 marks]
Answer: The light from distant galaxies is red-shifted (1). This shows the galaxies are moving away from each other, so the universe is expanding (1). If they are moving apart now, they must have been close together in the past, which fits the idea of a tiny region at the start (1).
Worked example 2
Question: Describe what has happened to the universe since the Big Bang. [4 marks]
Answer: All the matter and energy started in a tiny, extremely hot, dense region (1). Space has been expanding since then (1) and the universe has been cooling (1). Atoms formed first, and later stars and galaxies formed (1).
Common mistakes
Common mistakes
- Thinking the Big Bang was an explosion in a place in space. It was the start of the expansion of space itself.
- Saying the CMB comes from one place in the sky. It comes almost equally from every direction.
- Writing "red-shift proves it" with no explanation. Say that galaxies are moving apart, so they were once close together.
- Forgetting that CMB radiation is microwaves now because expansion stretched the wavelength.
Exam-style question
Scientists use the Big Bang theory to describe the past of the universe.
Describe the evidence that supports the Big Bang theory. [6 marks]
Model answer
Red-shift: light from distant galaxies is red-shifted (1). This shows the galaxies are moving apart, so the universe is expanding (1). If they are moving apart now, they must once have been close together (1), which fits all the matter and energy starting in a tiny, extremely hot, dense region (1).
CMB radiation: microwaves are detected coming almost equally from every direction (1). They are radiation left over from the hot early universe, stretched to microwave wavelengths by the expansion of space (1).
Exam tip
For "describe the evidence" questions, give each piece of evidence its own sentences: what is observed, then what it shows about the early universe.