Use red shift as evidence
Spectral lines from distant galaxies are shifted towards longer wavelengths. This shows recession and, with the relationship between red shift and distance, supports the model that the universe is expanding.
GCSE Physics resources
AQA GCSE Space physics
Red Shift, the Expanding Universe and the Big Bang
Spectral lines from distant galaxies are shifted towards longer wavelengths. This shows recession and, with the relationship between red shift and distance, supports the model that the universe is expanding.
Start here
The quick revision snapshot
Spectral lines from distant galaxies are shifted towards longer wavelengths. This shows recession and, with the relationship between red shift and distance, supports the model that the universe is expanding.
By the end, you should be able to…
- Explain use red shift as evidence using precise physics language.
- Apply the relevant equations, diagrams or practical method to an exam-style situation.
- Use the mini quizzes and practice answers to identify and correct a misconception.
Core learning content
Learn the idea before you practise it
Start with this short teaching sequence for Red Shift, the Expanding Universe and the Big Bang. Read the model, use the visual, then test your recall before moving into examples and exam practice.
Build the picture
Light from an element has characteristic spectral lines. If the observed lines are at longer wavelengths than laboratory lines, the source is moving away: this is red shift. The tendency for more distant galaxies to show greater red shift supports an expanding-universe model.
Use the right relationship
Choose a relationship because of the physical change in the question, not because it is familiar. Red-shift idea: red shift = observed wavelength greater than emitted wavelength (qualitative). Orbital model: gravity provides the centripetal force (qualitative). Keep the unit next to each value while you substitute.
What a strong answer does
Compare emitted and observed wavelengths, state the direction of motion and link the evidence to expansion rather than saying red shift is a colour. Before writing, underline the command word and identify the information that matters. Keep a calculation as equation, substitution and answer with unit; keep an explanation as a linked chain using because, so or therefore. Finish by checking that the final sentence answers the exact context in the question and that the number, direction or conclusion agrees with the evidence given.
Retrieval practice
Quick checks
Answer without looking back first. The feedback explains the model, so a wrong answer still helps you learn.
Mini quiz 1
Check the core idea
Mini quiz 2
Apply the model
Mini quiz 3
Use exam precision
Try it
Red-shift evidence model
How does a wavelength shift support an expanding universe?
What to notice: the readout and written explanation remain available if you do not use the controls.
Worked examples
See the method being built
Cover the answer, attempt the method and then compare your physics language as well as the number.
Example 1 · Interpret spectrum
A galaxy’s hydrogen line is observed at a longer wavelength than in a lab. State conclusion.
- Compare wavelengths.
- Name red shift.
- State direction of motion.
Show the answer
The light is red-shifted, so the galaxy is moving away.
Example 2 · Expansion evidence
More distant galaxies show greater red shift. Explain how this supports expansion.
- State greater recession effect.
- Link to distance pattern.
- Conclude universe expanding.
Show the answer
The trend shows more distant galaxies are generally receding faster, supporting an expanding-universe model.
Example 3 · Big Bang evidence
Explain how red shift relates to the Big Bang model.
- Use expansion evidence.
- Run expansion back in time.
- State denser/hotter past.
Show the answer
If the universe is expanding, it was smaller, denser and hotter in the past, consistent with the Big Bang model.
Misconception clinic
Common wrong turns
Each of these is tempting because it sounds almost right. Replace it with the precise physics before you meet it in a question.
- Calling red shift a change in the galaxy’s actual colour only.
- Saying red shift means a galaxy is moving towards us.
- Forgetting the wavelength comparison.
Independent practice
Exam-style questions
Use the working space first. Reveal the guidance only after you have committed to an answer, then improve the exact part that would gain the next mark.
1. Define red shift.
[2 marks]Show answer and marking guidance
An increase in observed wavelength of light from a receding source.
2. State what blue shift indicates.
[1 mark]Show answer and marking guidance
The source is moving towards the observer.
3. State what spectral lines provide.
[1 mark]Show answer and marking guidance
A known reference wavelength to compare with observed light.
4. Explain why red shift is evidence, not absolute proof, of expansion.
[3 marks]Show answer and marking guidance
It supports a model through an observed pattern; science evaluates evidence and alternative explanations.
5. State the trend between galaxy distance and red shift.
[1 mark]Show answer and marking guidance
More distant galaxies generally show greater red shift.
6. State one other Big Bang evidence line commonly taught.
[1 mark]Show answer and marking guidance
Cosmic microwave background radiation.
Questions pupils ask
GCSE Physics FAQs
What is the key idea in Red Shift, the Expanding Universe and the Big Bang?
Space questions reward scale, sequence and evidence. Link observations such as red shift to the model they support, and keep Separate Physics-only content clearly labelled.
How should I practise Red Shift, the Expanding Universe and the Big Bang?
Read the snapshot, attempt the worked examples without looking, complete the mini quizzes and then use the practice questions to check your wording and method.
Next step
Keep your revision moving
Physics sticks when you move between explanation, retrieval and application.