Red-shift and Expansion | GCSE Physics

Space

Lesson overview

This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.

FocusRed-shift, expansion and Big Bang evidence

Time45-60 minutes

EquipmentSpectrum diagram practice and calculator for simple proportional reasoning.

Practical linkNo separate practical method focus

Maths tagsM1 units and equation sense

What you will learn

Describe red-shift as an increase in observed wavelength.
Link greater red-shift to greater recession speed.
Explain how galaxy red-shift supports an expanding universe.
Use evidence language when discussing the Big Bang model.

Cosmology evidence prompts supplied on this page

Use the spectrum and galaxy prompts to practise explaining red-shift without saying the galaxy itself turns red.

Red-shift, the expanding universe and Big Bang evidence infographic

Infographic explaining GCSE Physics red-shift, including spectral lines shifting to longer wavelength, galaxy recession, expanding space, Big Bang evidence and common exam misconceptions. — Use this visual to compare spectral lines, link red-shift to recession speed and explain how expansion supports the Big Bang model.Download visual

Clear explanation

Red-shift means light from a distant galaxy is shifted towards longer wavelengths compared with light from the same elements measured in a laboratory.

If a galaxy is moving away, its observed wavelengths are stretched. A larger red-shift suggests a greater recession speed.

The pattern that distant galaxies show red-shift is evidence that space is expanding. Running that expansion backwards supports the idea that the universe began in a much hotter, denser state.

Key diagram

Diagram: the matching line pattern moves towards longer wavelength, showing why red-shift is evidence of recession.

Worked examples

Interpreting a shifted spectral line

A hydrogen line appears at a longer wavelength in light from a distant galaxy than in a laboratory spectrum.

Longer observed wavelength means red-shift.

Red-shift suggests the galaxy is moving away from us.

Answer: The galaxy's light is red-shifted, so the galaxy is receding.

Quick checks

Choose an answer, then check your thinking.

1. What does red-shift mean in GCSE Physics?

The galaxy is painted red Observed wavelength has increased The light has stopped moving

2. What does a greater red-shift usually suggest?

Lower mass only No movement Greater recession speed

Practice questions

Question 1

A spectral line from a distant galaxy is shifted towards longer wavelength. Name the effect.

Reveal answer and marking guidance

Answer: Red-shift.

Marking: Credit red-shift or redshift.

Question 2

Explain why red-shift supports the idea that galaxies are moving away.

Reveal answer and marking guidance

Answer: The observed wavelengths are stretched compared with laboratory values, which suggests recession.

Marking: Credit longer wavelength and moving away.

Question 3

Why is 'the galaxy is red' a weak explanation?

Reveal answer and marking guidance

Answer: Because red-shift is about the wavelength of light increasing, not the actual colour of the galaxy.

Marking: Credit distinction between spectrum shift and object colour.

Question 4

How does red-shift evidence support the Big Bang model?

Reveal answer and marking guidance

Answer: If space is expanding now, then in the past the universe was smaller, hotter and denser.

Marking: Credit expansion backwards and hot dense early universe.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.

ApplicationApply red-shift, expansion and big bang evidence to an unfamiliar device, practical setup or data description.

Practical interpretationUse evidence, graph features, uncertainty, method quality or conclusion wording where the question asks you to evaluate.

Maths skillM1 units and equation sense

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. For red-shift, marks come from comparing observed spectral lines with laboratory wavelengths, stating that wavelength has increased, linking larger red-shift to greater recession speed, and using the expanding-universe evidence carefully rather than saying galaxies simply turn red.

Common mistakes

Saying red-shift means the galaxy is red.
Forgetting that wavelength increases.
Treating one galaxy as enough evidence for the whole model.
Saying expansion means galaxies move through space like ordinary objects only.

Extension challenge

Compare red-shift evidence with one other piece of evidence for the Big Bang model, such as cosmic microwave background radiation if your course covers it.

Reveal answer

Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.

Exam-board guidance

Short board notes only. Learn the core physics above first.

AQA GCSE Physics

AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for red-shift, expansion and big bang evidence.

OCR GCSE Physics

OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.

Eduqas GCSE Physics

Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.

WJEC Wales

WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.

Next lesson

Next, continue with Atomic Models and Evidence.

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