GCSE Physics revision

Red-shift (physics only)

AQA 4.8.2

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain how galactic red-shift supports an expanding universe and the Big Bang, how observations develop theories, and why dark matter and dark energy remain unresolved.

MOST 🎯🎯

Apply the scientific explanation of red-shift to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving red-shift and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

The Expanding Universe

From Earth, every direction we look into space, galaxies appear to be moving further away, meaning the universe is expanding.
The main evidence for the expanding universe is the red shifting of light observed from distant galaxies.
The explanation for why the universe is expanding is provided by the Big Bang theory.

Absorption Spectra Explained

Absorption spectra show the pattern of wavelengths of light emitted from a source, such as the Sun, as observed on Earth.
The Sun emits all visible wavelengths, but chemicals in its atmosphere (such as oxygen and hydrogen) absorb specific wavelengths.
The absorbed wavelengths appear as dark lines on the absorption spectrum, creating a unique pattern that acts like a chemical fingerprint.
Visible light ranges from longer red wavelengths on one end to shorter violet wavelengths on the other.
What is Red Shift?
When we analyse light from distant galaxies, their absorption spectra show the same pattern of dark lines as local spectra, but shifted towards the red end of the spectrum.
Red shift occurs because light waves are stretched as they travel towards Earth, increasing their wavelength.
For example, a dark line originally at 600 nm might appear at 650 nm in the observed spectrum due to this stretching effect.
Red shift is evidence that distant galaxies are moving away from us.

2. Absorption Spectra ExplainedAQA

Notes

Absorption spectra show the pattern of wavelengths of light emitted from a source, such as the Sun, as observed on Earth.
The Sun emits all visible wavelengths, but chemicals in its atmosphere (such as oxygen and hydrogen) absorb specific wavelengths.
The absorbed wavelengths appear as dark lines on the absorption spectrum, creating a unique pattern that acts like a chemical fingerprint.
Visible light ranges from longer red wavelengths on one end to shorter violet wavelengths on the other.
What is Red Shift?
When we analyse light from distant galaxies, their absorption spectra show the same pattern of dark lines as local spectra, but shifted towards the red end of the spectrum.
Red shift occurs because light waves are stretched as they travel towards Earth, increasing their wavelength.
For example, a dark line originally at 600 nm might appear at 650 nm in the observed spectrum due to this stretching effect.
Red shift is evidence that distant galaxies are moving away from us.

Why Galaxies Move Apart

Galaxies are not moving away by travelling through space; instead, the space itself between us and distant galaxies is expanding.
As space expands, the distance between galaxies increases, which is why light from them gets stretched and red-shifted.
The further away a galaxy is, the faster it appears to be moving away from us.

3. Why Galaxies Move ApartAQA

Notes

Galaxies are not moving away by travelling through space; instead, the space itself between us and distant galaxies is expanding.
As space expands, the distance between galaxies increases, which is why light from them gets stretched and red-shifted.
The further away a galaxy is, the faster it appears to be moving away from us.

The Balloon Model

The expanding universe can be modelled using a balloon covered in markers, where the balloon represents space and the markers represent galaxies.
As the balloon is inflated, all the markers move further apart because the space between them stretches, not because the markers themselves move across the surface.
From the perspective of any single galaxy (marker), all other galaxies appear to be moving away, just as we observe from Earth.

4. The Balloon ModelAQA

Notes

The expanding universe can be modelled using a balloon covered in markers, where the balloon represents space and the markers represent galaxies.
As the balloon is inflated, all the markers move further apart because the space between them stretches, not because the markers themselves move across the surface.
From the perspective of any single galaxy (marker), all other galaxies appear to be moving away, just as we observe from Earth.

Limitations of the Balloon Model

All models have limitations; for example, a balloon will eventually burst, whereas the real universe is not expected to do so.
In the balloon model, galaxies are shown on the surface, but in reality galaxies exist throughout the volume of the universe, not just on an outer boundary.
Despite its limitations, the balloon model is still useful because it helps visualise the difficult concept of expanding space.

5. Limitations of the Balloon ModelAQA

Notes

All models have limitations; for example, a balloon will eventually burst, whereas the real universe is not expected to do so.
In the balloon model, galaxies are shown on the surface, but in reality galaxies exist throughout the volume of the universe, not just on an outer boundary.
Despite its limitations, the balloon model is still useful because it helps visualise the difficult concept of expanding space.

The Big Bang Theory

If the universe is currently expanding, it follows that in the past the universe must have been much smaller.
The Big Bang theory proposes that initially all matter in the universe occupied an unimaginably tiny, dense, and hot space.
Suddenly, an explosion caused space itself to begin expanding, and this expansion is still continuing today.
The Big Bang theory is the generally accepted scientific explanation for the origin of the universe.

6. The Big Bang TheoryAQA

Notes

If the universe is currently expanding, it follows that in the past the universe must have been much smaller.
The Big Bang theory proposes that initially all matter in the universe occupied an unimaginably tiny, dense, and hot space.
Suddenly, an explosion caused space itself to begin expanding, and this expansion is still continuing today.
The Big Bang theory is the generally accepted scientific explanation for the origin of the universe.

Evidence Linking Red Shift to the Big Bang

The observation that all distant galaxies are red-shifted supports the idea that the universe has been expanding since the Big Bang.
The fact that more distant galaxies show greater red shift is consistent with the universe expanding uniformly from a single point in the past.

Key Terminology Recap

Red shift is the increase in wavelength (shift towards the red end) of light from a source moving away from the observer.
Absorption spectra are the unique patterns of dark lines produced when specific wavelengths of light are absorbed by chemicals in a star's atmosphere.
The Big Bang theory is the scientific model proposing that the universe began from an extremely dense, hot point and has been expanding ever since.

8. Key Terminology RecapAQA

Notes

Red shift is the increase in wavelength (shift towards the red end) of light from a source moving away from the observer.
Absorption spectra are the unique patterns of dark lines produced when specific wavelengths of light are absorbed by chemicals in a star's atmosphere.
The Big Bang theory is the scientific model proposing that the universe began from an extremely dense, hot point and has been expanding ever since.