Free GCSE Physics lesson: Red-shift and Expansion
Aplailasain by Static Carnival

Free Lessons -> GCSE / Key Stage 4 -> Physics -> Red-shift and Expansion

Lesson 46 · GCSE / Key Stage 4 · Physics

Red-shift, the expanding universe and Big Bang evidence

Use red-shift evidence to explain galaxy recession, the expanding universe and the Big Bang model.

Qualification: GCSE Subject: Physics Physics-only stretch Separate Physics and higher-grade Space Physics routes where cosmology is taught

Space

This lesson builds red-shift, expansion and big bang evidence for GCSE Physics.

Use the core lesson first, then match the exam-board guidance to your school route. Many pupils meet this content through Combined Science as well as Separate Physics.

Good forSeparate Physics and higher-grade Space Physics routes where cosmology is taught
FocusRed-shift, expansion and Big Bang evidence
Time45-60 minutes
EquipmentSpectrum diagram practice and calculator for simple proportional reasoning.
Paper fitPaper 2 focus on most GCSE Physics routes
TierFoundation core with Higher stretch
Practical linkNo required practical 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.

Exam-board fit

RouteSeparate Physics and higher-grade Space Physics routes where cosmology is taught
PaperPaper 2 focus on most GCSE Physics routes
TierFoundation core with Higher stretch
Specification fitSpace: Red-shift, expansion and Big Bang evidence
Practical linkNo required practical focus
Maths ladderM1 units and equation sense

Exact paper labels and specification-point numbering vary by board and cohort, so match this lesson to your school route before using past-paper questions.

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.

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

Red-shift shown by spectral lines moving to longer wavelength Two simple spectra show the same dark absorption lines shifted towards the red, longer-wavelength end for a distant galaxy. laboratory spectrum distant galaxy: lines shifted to longer wavelength
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?

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

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.

Exam practice ladder

AO1 fluencyRecall the key definition, unit, equation or model before using the lesson questions.
AO2 applicationApply red-shift, expansion and big bang evidence to an unfamiliar device, practical setup or data description.
AO3 analysisUse 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 this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.

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.

Exam-board guidance

All supported routes assess the core physics idea, but they may group topics, practicals and paper wording differently.

AQA GCSE Physics

AQA GCSE Physics: use this lesson for red-shift, expansion and big bang evidence, then check whether your class is taking Separate Physics or Combined Science.

OCR GCSE Physics

OCR GCSE Physics: the core physics idea is shared, but Gateway and Twenty First Century may organise questions differently.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: practise the concept, the equation use and the practical language because questions often connect them.

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: check whether your class is using the current GCSE Physics route or a newer science route, then use this lesson for the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your unit and remember that practical skills are assessed directly.

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.

Next lesson

Next, continue with Atomic Models and Evidence.

Previous lesson: Reflection and Refraction PracticalBack to GCSE Physics lessonsNext lesson: Atomic Models and Evidence