16.4 The Expanding Universe

A free GCSE Physics lesson for AQA (8463) from OBA School: galaxies and light years, absorption spectra, red-shift and blue-shift, and the evidence that the universe is expanding (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe galaxies, including the Milky Way, and use the light year as a unit of distance
• Explain what red-shift and blue-shift are, using the absorption lines in a spectrum
• State that the faster a galaxy moves away, the bigger its red-shift
• Describe the observation that the further away a galaxy is, the bigger its red-shift, and analyse data showing this
• Explain how red-shift shows that the universe is expanding

Key words

• Galaxy: An enormous group of billions of stars held together by gravity. The Sun is one star in the Milky Way galaxy.
• Light year: The distance light travels in one year, about 9.5 × 10¹⁵ m. It is a distance, not a time.
• Spectrum: The band of colours made when light is split up, from violet / blue (short wavelength) to red (long wavelength).
• Absorption lines: Dark lines in a spectrum at particular wavelengths, caused by atoms such as hydrogen absorbing light.
• Red-shift: An increase in the wavelength of light from a galaxy moving away from us: the lines move towards the red end.
• Blue-shift: A decrease in the wavelength of light from a galaxy moving towards us: the lines move towards the blue end.
• Recede: To move away. The speed at which a galaxy moves away from us is its speed of recession.

Galaxies and the light year

The Sun is one of a few hundred billion stars in the Milky Way galaxy, a spiral of stars about 100 000 light years across. The universe contains billions of galaxies, separated by huge distances. Distances in space are so large that astronomers measure them in light years.

Worked example: how far is one light year?

distance = speed × time; speed of light = 3.0 × 10⁸ m/s
one year = 365 × 24 × 60 × 60 = 3.15 × 10⁷ s
distance = 3.0 × 10⁸ × 3.15 × 10⁷ = 9.5 × 10¹⁵ m

Answer: 1 light year ≈ 9.5 × 10¹⁵ m

• The Sun; Distance from Earth about 8 light minutes
• Nearest star after the Sun; Distance from Earth about 4 light years
• Centre of the Milky Way; Distance from Earth about 26 000 light years
• Andromeda, the nearest large galaxy; Distance from Earth about 2.5 million light years
• Most distant galaxies seen; Distance from Earth more than 13 billion light years

Spectra and absorption lines

A prism splits light into a spectrum. The wavelength increases from the violet / blue end to the red end. Light from a star or galaxy has dark absorption lines: atoms in the star absorb light of particular wavelengths. Each element gives its own pattern of lines, like a barcode, so the pattern for hydrogen can be recognised in any galaxy.

Red-shift and blue-shift

• If a galaxy is moving away, its light waves are stretched out. The wavelength increases (and the frequency decreases), so the absorption lines shift towards the red end: red-shift.
• If a galaxy is moving towards us, its light waves are squashed together. The wavelength decreases, so the lines shift towards the blue end: blue-shift.
• The faster the galaxy is moving, the bigger the shift.

You can hear a similar effect with sound: the pitch of a siren drops as an ambulance drives away from you.

Worked example: comparing two galaxies

A hydrogen line has a wavelength of 486 nm in the laboratory.
In galaxy R it is at 494 nm: shift = 494 − 486 = 8 nm longer, so R is moving away.
In galaxy S it is at 503 nm: shift = 503 − 486 = 17 nm, a bigger red-shift.

Answer: both are receding; S is moving away faster than R

Exam tip: Red-shift does not mean the galaxy looks red. It means the whole pattern of lines has moved to longer wavelengths.

The further away, the faster

In the 1920s astronomers measured the red-shifts and distances of many galaxies. They found that:

• light from almost all distant galaxies is red-shifted, so they are all moving away from us;
• the further away a galaxy is, the bigger its red-shift, so the faster it is moving away.

A few nearby galaxies, such as Andromeda, show a blue-shift: gravity is pulling them towards the Milky Way. This is a local effect; all the distant galaxies are red-shifted.

The universe is expanding

We have no special place in the universe, so observers in any other galaxy would see the same thing: all the distant galaxies moving away from them. The explanation is that space itself is stretching – the universe is expanding.

Exam tip: Evidence chain for exam answers: distant galaxies show red-shift → they are moving away → more distant galaxies have bigger red-shift → they move away faster → the universe is expanding.

Common mistakes

• Saying red-shift means the galaxy is red, or that its light gets brighter. The wavelength increases.
• Mixing up the directions: red-shift = moving away, wavelength longer; blue-shift = moving towards, wavelength shorter.
• Saying the Earth must be at the centre of the universe. Every galaxy sees all the others moving away.
• Treating a light year as a time. It is a distance.

Revision questions

Try these, then check your answers below each one.

Question 1. What does it mean if the light from a galaxy is red-shifted? (Choose from: the wavelength has increased because the galaxy is moving away, the wavelength has decreased because the galaxy is moving away, the galaxy is made of red stars, the wavelength has increased because the galaxy is moving towards us.)

Answer: the wavelength has increased because the galaxy is moving away

Working: Red-shift: the absorption lines move to longer wavelengths (towards red) because the galaxy is moving away. It does not mean the galaxy is red.

Question 2. True or false? The further away a distant galaxy is, the bigger its red-shift.

Answer: True

Working: True. More distant galaxies have bigger red-shifts, so they are moving away faster.

Question 3. Light from a nearby galaxy shows a blue-shift. What does this tell you? (Choose from: the galaxy is moving towards us, the galaxy is moving away from us, the galaxy is not moving, the galaxy is very far away.)

Answer: the galaxy is moving towards us

Working: Blue-shift means shorter wavelengths: the waves are squashed because the galaxy is approaching.

Related lessons

• Previous lesson: 16.3 Planets, Satellites and Orbits
• Next lesson: 16.5 The Beginning and Future of the Universe
• In this chapter: 16.1 Formation of the Solar System
• In this chapter: 16.2 The Life History of a Star

Continue in the full course

This lesson is section 16.4 (AGCPH1604) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.