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.

AQA 8463 GCSE Physics FoundationHigher

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The 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:

  • Use red shift as evidence.
  • Apply the space physics model, evidence or method to an unfamiliar exam context.
  • Recognise and correct this wrong turn: Calling red shift a change in the galaxy’s actual colour only.

Before you revise

Diagnostic question

Choose an answer from memory. Your result tells you what to watch for in the guide.

Which statement correctly summarises red shift, expanding universe and the Big Bang?

Core revision guide

Learn the model, then use it

Red Shift, the Expanding Universe and the Big Bang questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.

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.

Build the physics 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.

Matching absorption lines from a galaxy are shifted to longer wavelength compared with laboratory lines.
The line pattern stays the same but moves towards longer wavelength; more distant galaxies generally recede faster.Open the full-size exam diagram

Misconception clinic

Replace the tempting answer

Read each belief, say what is wrong with it, then compare your correction.

Tempting idea: Calling red shift a change in the galaxy’s actual colour only.

Use instead: 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.

Tempting idea: Saying red shift means a galaxy is moving towards us.

Use instead: 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.

Tempting idea: Forgetting the wavelength comparison.

Use instead: Compare emitted and observed wavelengths, state the direction of motion and link the evidence to expansion rather than saying red shift is a colour.

Retrieval practice

Quick checks

1. Which exam method is most reliable for red shift, expanding universe and the Big Bang?

2. Which statement correctly applies red shift, expanding universe and the Big Bang to a question?

3. A pupil writes: “Calling red shift a change in the galaxy’s actual colour only.” Which replacement is accurate?

Explore the relationship

Red-shift evidence model

Compare an observed red shift with the red shift predicted by Hubble’s law for the selected distance.

What to notice: use the readout to describe how one variable changes when the other is controlled.

Worked examples

See the method being built

These examples expose the difference between a secure use red shift as evidence method and the common error “Calling red shift a change in the galaxy’s actual colour only.”.

Example 1 · Interpret spectrum

A galaxy’s hydrogen line is observed at a longer wavelength than in a lab. State conclusion.

  1. Compare wavelengths.
  2. Name red shift.
  3. 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.

  1. State greater recession effect.
  2. Link to distance pattern.
  3. 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.

  1. Use expansion evidence.
  2. Run expansion back in time.
  3. 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.

Exam precision

Exam technique: Use red shift as evidence

Do this: Compare emitted and observed wavelengths, state the direction of motion and link the evidence to expansion rather than saying red shift is a colour.

Independent practice

Exam-style questions

Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.

1. Define red shift.

[2 marks]
Show marking guidance and model answer

Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.

Model answer: An increase in observed wavelength of light from a receding source.

2. State what blue shift indicates.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: The source is moving towards the observer.

3. State what spectral lines provide.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: A known reference wavelength to compare with observed light.

4. Explain why red shift is evidence, not absolute proof, of expansion.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.

Model answer: 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 marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: More distant galaxies generally show greater red shift.

6. State one other Big Bang evidence line commonly taught.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: Cosmic microwave background radiation.

Questions pupils ask

Red Shift, the Expanding Universe and the Big Bang FAQs

What is the main idea in 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.

What mistake should I avoid in Red Shift, the Expanding Universe and the Big Bang?

Calling red shift a change in the galaxy’s actual colour only. 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.

Useful next steps

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