Red-shift — AQA GCSE Physics

Module 8 · Lesson 8.04 · Spec 4.8.2

Red-shift and the Big Bang

The single most important piece of evidence in modern cosmology — and the lowest-scoring Paper 2 topic. Get this right and you score what most students miss.

Physics-only · Paper 2

What you'll learn

Spec 4.8.2Paper 2Physics-only

By the end you'll be able to

Define red-shift — the increase in wavelength of light from distant galaxies.

State the relationship: more distant galaxies recede faster and show greater red-shift.

Conclude that the universe is expanding.

State the Big Bang theory: universe began from a very small region that was extremely hot and dense.

Use red-shift as evidence for the Big Bang using a 4-step structured answer.

Mention 1998 supernova observations and the open questions of dark mass and dark energy.

Why it matters most

This is the lowest-scoring Paper 2 topic. 17% reached full marks in Jun 2024. 6% scored full marks in Jun 2023. Master the four-step chain in this lesson and you score what almost everyone else misses.

The observation — spectral lines

Every chemical element absorbs and emits light at specific wavelengths. These appear as dark or bright "lines" in the spectrum from a star or galaxy — like a fingerprint.

Setup

The same element on Earth always shows lines at the same wavelengths.

So when astronomers see those lines in light from a distant galaxy, they know which elements are there.

And — crucially — they can see if those lines have shifted.

Why this matters

Because the reference wavelengths are fixed (set by atomic physics, identical everywhere), any shift means something is happening to the light on its journey. That "something" turns out to be the expansion of space itself.

What is red-shift?

There is an observed increase in the wavelength of light from most distant galaxies. — AQA spec 4.8.2

Red-shift

The observed increase in wavelength of light from distant galaxies — their spectral lines appear shifted toward the red end of the spectrum (longer wavelengths) compared with the same lines observed on Earth.

Why the lines shift — mechanism

The qualitative explanation

The galaxy is moving away from us (receding).

As the galaxy moves away, the light waves it emits get stretched — their wavelength increases.

Longer wavelength = shifted toward the red end of the spectrum.

The faster the galaxy is moving away, the more the wavelength increases, and the bigger the red-shift.

Exam wording — important

"Light waves are stretched" is the intuitive explanation, but Jun 2022 Q6.2 mark scheme explicitly IGNORES this phrase ("ignore: light waves are stretched"). For marks, write: "the (observed) increase in wavelength" and "galaxies are moving away from us".

Terminology note

This is similar to the Doppler effect for sound (ambulance sirens drop in pitch as they pass) — but the AQA spec uses "red-shift", not "Doppler effect". Use "red-shift" in your answers; "Doppler" is fine as background context but not required.

Distance and recession — the link

The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength. — AQA spec 4.8.2

Two linked relationships

Greater red-shift → faster recession

More distant → greater red-shift

Combined: more distant galaxies are moving away faster

Conclusion — the universe is expanding

The observed red-shift provides evidence that space itself (the universe) is expanding. — AQA spec 4.8.2

The leap of logic

Light from distant galaxies is red-shifted.

So distant galaxies are moving away from us.

The further away, the faster they're moving away.

This isn't because the galaxies are moving through space — it's because space itself is expanding between us.

The whole universe is expanding.

Common misconception

It's not "we're the centre of the universe and everything is moving away from us specifically". From every galaxy's perspective, every other galaxy is moving away. The expansion is happening everywhere — like raisins in rising bread dough.

The Big Bang theory

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense. — AQA spec 4.8.2

The model

The universe began from a state that was very small, extremely hot, and extremely dense.

It has been expanding and cooling ever since.

If you "play the expansion backwards" you arrive at this initial state.

Enrichment

The universe is estimated to be about 13.8 billion years old — but this figure is NOT in the AQA spec and not required by mark schemes. Useful context only.

Don't write "explosion"

The Big Bang is not an explosion of matter into pre-existing space. It is the rapid expansion of space itself from a tiny, hot, dense beginning. Examiners may not credit "the Big Bang was an explosion".

Red-shift as evidence for the Big Bang

The observed red-shift provides evidence that space itself (the universe) is expanding and supports the Big Bang theory. — AQA spec 4.8.2

The logic chain

Red-shift shows the universe is expanding now.

If we run that expansion backwards in time, the universe was smaller, hotter, denser in the past.

Extrapolated to its earliest point: a very small, extremely hot and dense beginning — exactly what the Big Bang theory predicts.

Red-shift = directly observable evidence that matches the Big Bang model.

The crucial connection

Red-shift on its own doesn't prove the Big Bang. It proves expansion. The Big Bang theory is the model that best explains what an expanding universe implies about the past — and red-shift gives us the observation that the model must explain.

The 4-step chain — for 6-mark answers

This is the structure you need for every "how does red-shift provide evidence for the Big Bang?" question.

SPECTRAL LINES shifted to longer wavelength

GALAXIES MOVING AWAY from us

MORE DISTANT = faster recession

UNIVERSE EXPANDING supports Big Bang

The 4 reasoning points (synthesised from mark schemes 2022–2024)

"Spectral lines are shifted to longer wavelength" (i.e. red-shifted) — Jun 2022 Q6.2 / Jun 2024 Q3.4

"Galaxies are moving away from us" — Jun 2022 Q6.2 / Jun 2024 Q3.4 verbatim

"The furthest galaxies are moving away the fastest" — Jun 2022 Q6.3 verbatim

"The universe is expanding" (supports Big Bang) — general MS pattern

Memorise this chain

This is the consistent reasoning structure mark schemes reward on red-shift questions across all years. Each point must be explicitly stated in your answer to score the corresponding mark. Don't skip steps — examiners need to see each link.

The 1998 wrinkle — accelerating expansion

Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding ever faster. — AQA spec 4.8.2

What the spec wants you to know

Until the 1990s: scientists thought the expansion might be slowing down (gravity from galaxies pulling everything back).

1998 onwards: observations of distant supernovae showed they were further away than expected.

Interpretation: distant galaxies are receding ever faster — the expansion is accelerating.

This was unexpected and required new physics to explain.

Why supernovae? — enrichment

The spec just says "observations of supernovae". Beyond the spec but useful context: a specific kind of supernova (Type Ia) has a known, fixed brightness. By measuring how bright it appears, scientists can calculate the distance very accurately — so they could tell distant galaxies are further than the simple-expansion model predicted. Not required for exam — but answers the "how do they know?" question.

Open questions — dark mass and dark energy

Students should be able to explain that there is still much about the universe that is not understood, for example dark mass and dark energy. — AQA spec 4.8.2

Dark mass (dark matter)

Invisible matter — doesn't emit, absorb, or reflect light

Detected only through its gravity on visible matter

Holds galaxies together; without it, they would fly apart

Makes up most of the matter in the universe

Dark energy

The "thing" causing the accelerating expansion

Properties largely unknown

Acts like a repulsive force, pushing space apart

Makes up most of the energy in the universe

Exam-ready statement

"There is still much about the universe that is not understood — for example, dark mass (an unseen form of matter detected only by its gravity) and dark energy (the unknown cause of the accelerating expansion)."

How science works — observation → theory

The spec also requires you to explain how scientists arrive at theories like the Big Bang:

The scientific method here

Observation: distant galaxies show red-shift

Pattern: red-shift increases with distance

Inference: the universe must be expanding

Theory: the Big Bang model explains an expanding universe arising from a hot dense start

Refinement: as new evidence appears (e.g. 1998 supernovae), the theory adapts (e.g. dark energy added)

Why theories change

"Sometimes scientists have to change theories about the universe" — this was a real Jun 2022 question. The answer: because new evidence is observed that doesn't fit the existing theory. Theories must always be tested against observation.

Examiner alert — Red-shift questions

Pattern: this is THE hardest Paper 2 topic

Red-shift questions appear on every Paper 2 — usually 2-3 mark questions on definition or comparing two galaxies. The 4-6 mark "explain how red-shift provides evidence for the Big Bang" appears less frequently but is heavily weighted when it does. Performance is consistently low across both types.

17%

All 3 marks (Jun 2024 Q3.4 — compare A vs B)

6%

All 3 marks (Jun 2023 Q6.4 — compare A vs B)

~50%

Scored ZERO on Jun 2022 Q6.3 (Big Bang evidence)

What went wrong (per examiner reports)

Skipped the "red-shift" word — just said "spectra were shifted"

Failed to state "moving away from us" explicitly

Missed the distance-recession link

Didn't conclude "universe is expanding"

Said galaxies "emit radiation with greater wavelength than the Sun" — misunderstanding of mechanism (Jun 2023)

Mark scheme — credited phrases (verbatim)

Use these phrases verbatim in your answers. Each is verified from a specific AQA mark scheme.

TopicVerbatim credited phraseSource

Red-shift definition"The (observed) increase in wavelength (of light from galaxies)"Jun 2022 Q6.2 MS

Alternative definition"The wavelength of the (absorption) lines has increased "Jun 2024 Q3.4 MS

Interpretation" Galaxies are moving away from us "Jun 2022 Q6.2 / Jun 2024 Q3.4 MS

Distance-speed link" The furthest galaxies are moving away (from the Milky Way) the fastest "Jun 2022 Q6.3 MS

Big Bang implication" At some time all galaxies / matter started at the same point "Jun 2022 Q6.3 MS

Comparison (A > B)"A shows a greater red-shift · so A is travelling faster · because A is further away "Jun 2023 Q6.4 / Jun 2024 Q3.4 MS

Conclusion"The universe is expanding "General MS pattern + spec

Big Bang theory"Universe began from a very small region that was extremely hot and dense "AQA spec 4.8.2 verbatim

Memorise verbatim

These phrases are taken directly from mark schemes and the AQA spec. Using these exact phrases guarantees the marks.

Mark scheme — REJECTED phrases

These are phrases verified as not credited by AQA mark schemes. Avoid them.

Don't writeWhy rejectedSource

"Light is moving to the red end of the light spectrum" (alone)Insufficient — doesn't explain the mechanism or what it tells usJun 2022 Q6.2

"Galaxies emit radiation with greater wavelength than the Sun"Misunderstands red-shift mechanism — galaxies don't emit at different wavelengths to the SunJun 2023 Q6.4

"Galaxies are getting bigger"Wrong physics — galaxies are getting further apart , not biggerGeneral teaching observation

"New observations were made" (without specifics)Vague — doesn't link the observation to the change in theoryJun 2022 Q6.4

"The Big Bang was an explosion"Wrong concept — Big Bang is rapid expansion of space itself, not an explosion into spaceGeneral mark-scheme guidance

Weak vs strong — 6-mark answer

Question: Explain how red-shift provides evidence for the Big Bang theory.6 marks

Weak (Level 1)

"The light from galaxies is moving to the red part of the spectrum so the universe is expanding which means the Big Bang happened."

"Red part" — not "wavelength shift"

No mention of distance-recession link

No mention of "hot and dense" Big Bang state

Likely 1–2 marks

Strong (Level 3)

"Light from distant galaxies shows spectral lines shifted to longer wavelengths (red-shift). This means the galaxies are moving away from us. More distant galaxies recede faster, showing greater red-shift. So the universe is expanding. The Big Bang theory states the universe began from a very small, extremely hot, dense state and has been expanding since — red-shift directly supports this."

All 4 chain points hit

Big Bang state described per spec wording

Likely 5–6 marks

Common errors to avoid

ErrorCorrect versionWhy

"The galaxies are red"The galaxies' spectral lines are shifted toward longer (red) wavelengthsRed-shift = wavelength shift of specific spectral lines, not the colour of the galaxy

"Galaxies are getting bigger"Galaxies are getting further apart (the space between them is expanding)Different physics entirely

"Big Bang was an explosion in space"Big Bang is the rapid expansion of space itself from a hot dense stateNo "outside" to explode into — space and time both begin at the Big Bang

"We are at the centre of the universe"Expansion happens everywhere — every galaxy sees others receding from itThe universe has no centre

"Red-shift proves the Big Bang"Red-shift is evidence for / supports the Big Bang — not absolute proofScientific theories are supported by evidence, never "proven"

"Galaxies emit light at different wavelengths to the Sun"Galaxies emit at the same wavelengths ; the light we receive is shifted because the source is recedingSpec mechanism: source motion causes the shift, not different emission

Worked example

Q1. Describe what is meant by red-shift. 2 marks

Model answer

The observed increase in wavelength of light from distant galaxies [1]

Spectral lines are shifted toward the red end of the spectrum (longer wavelengths) compared with the same lines on Earth [1]

Q2. Two galaxies (A and B) both show red-shift. Galaxy A's lines are shifted by less than Galaxy B's. What can you conclude about the two galaxies? 3 marks

Model answer

Both galaxies are moving away from us (because both show red-shift) [1]

Galaxy B has the bigger red-shift, so Galaxy B is receding faster than Galaxy A [1]

Galaxy B is further away than Galaxy A (more distant galaxies recede faster) [1]

Checkpoint

You can now:

Define red-shift as the observed increase in wavelength of light from distant galaxies.

State that more distant galaxies recede faster and show greater red-shift.

Conclude that the universe is expanding .

State the Big Bang theory: universe began from a very small, extremely hot and dense region.

Use the 4-step chain (lines shifted → moving away → distance-recession → universe expanding) for 6-mark answers.

Mention 1998 supernova observations + dark mass and dark energy as open questions.

Reproduce the mark scheme credited phrases verbatim.

Module 8 complete · what's next

Module 8 — Space Physics — recap

8.01: solar system contents, Sun formation, equilibrium

8.02: star life cycles, element formation in stars and supernovae

8.03: orbital motion, geostationary vs polar orbits

8.04: red-shift, expanding universe, Big Bang theory

Coming next: Module 9

Synoptic Masterclasses — three lessons that consolidate cross-topic exam patterns: (9.01) motor vs generator effect, (9.02) energy approaches across topics, (9.03) required practical mastery.

Final word on red-shift

This is the highest-value topic in Module 8. The four-step chain is your insurance against losing easy marks. Practise saying it aloud until it's automatic.