➖ Source moving away
The waves are stretched out. The observed wavelength is longer and the observed frequency is lower.
Cosmology » Red-shift of Light from Galaxies
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 8.15P, 8.17P, 8.18P
- Describe how a moving wave source changes the wavelength and frequency an observer receives
- Describe what red-shift is and what we see in the light from galaxies
- Describe how red-shift changes with the distance of a galaxy
- Explain why red-shift is evidence that the universe is expanding
This lesson is examined in Paper 2 only.
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Unlock This CourseMoving sources change the waves we receive
In the lesson The Doppler Effect you saw this for sound. The idea works for any wave, including light. If a wave source is moving relative to an observer, the observer receives a different frequency and wavelength from the one the source gives out.
➕ Source moving towards
The waves are squashed together. The observed wavelength is shorter and the observed frequency is higher.
It does not matter which one is really moving. What matters is that the source and the observer are moving relative to each other.
Light from galaxies
Galaxies are far too distant to visit, so astronomers study the light that reaches Earth. Light from a galaxy can be split into a spectrum, a band of colours from red to violet. Red light has the longest wavelength of visible light and violet the shortest.
The spectrum of a galaxy is not a smooth band of colour. It has thin dark lines across it, at particular wavelengths. When we measure the same lines here on Earth, they sit at known places in the spectrum.
Key terms:
- Red-shift: the shift of the dark lines in the spectrum of a galaxy towards the red end, compared with the same lines measured on Earth.
What red-shift looks like
When astronomers look at the light from galaxies, they find that the dark lines are not where they are on Earth. They have moved towards the red end of the spectrum. The pattern of lines is the same, but every line has moved to a longer wavelength.
Drawing it in words
Spectrum measured on Earth: violet on the left, red on the right, with dark lines at certain places.
Spectrum of a galaxy: the same pattern of dark lines, but every line is further to the right, towards the red end.
A longer wavelength is exactly what the Doppler effect gives for a source moving away. So the red-shift tells us the galaxy is moving away from us.
The galaxy does not look red. Its light is still the same mix of colours, and only the positions of the dark lines move.
Red-shift and distance
Astronomers have measured the red-shift of many galaxies at different distances from the Earth. They find a clear pattern.
| Galaxy | Red-shift of the dark lines |
|---|---|
| Closer to the Earth | Small shift towards the red end |
| Further from the Earth | Larger shift towards the red end |
Learn this
The further away a galaxy is, the greater its red-shift.
A greater red-shift means a bigger change in wavelength, which means the galaxy is moving away faster. So the more distant galaxies are moving away from us faster.
Why red-shift shows the universe is expanding
Put the two findings together.
The explanation, step by step
- The light from galaxies is red-shifted.
- Red-shift shows the galaxies are moving away from us.
- The further away a galaxy is, the greater its red-shift, so the faster it moves away.
- This is exactly what we would see if the whole universe is expanding.
Think of a lump of dough with raisins in it, rising in an oven. As the dough expands, every raisin moves away from every other raisin. Raisins that start far apart move apart faster than raisins that start close together. Galaxies behave like the raisins.
🔭 What we measure
The red-shift of light from galaxies at different distances.
🌈 What it tells us
The galaxies are moving away, and the more distant ones are moving away faster.
Worked examples
Worked example 1: describe the shift
Describe how the dark lines in the spectrum of a distant galaxy differ from the same lines measured on Earth.
Step 1: the lines are in the same pattern.
Step 2: they are shifted towards the red end of the spectrum.
Answer: the dark lines are shifted towards the red end, to longer wavelengths.
Worked example 2: compare two galaxies
Galaxy P has a small red-shift and galaxy Q has a large red-shift. Which is further away, and which is moving away faster?
Step 1: the greater the red-shift, the further away the galaxy.
Step 2: the greater the red-shift, the faster it is moving away.
Answer: galaxy Q is further away and is moving away faster.
Worked example 3: the wave change
Light from a galaxy is received on Earth. How do the wavelength and frequency compare with the light that was given out?
Step 1: the galaxy is moving away, so the waves are stretched.
Step 2: stretched waves have a longer wavelength and a lower frequency.
Answer: the observed wavelength is longer and the observed frequency is lower.
Common mistakes
Common mistakes
- Saying the galaxy looks red. Red-shift means the dark lines have moved towards the red end, not that the whole galaxy is red.
- Mixing up the direction. Moving away gives a longer wavelength and a lower frequency. Moving towards gives a shorter wavelength and a higher frequency.
- Saying red-shift shows a galaxy is getting hotter. It shows the galaxy is moving away.
- Forgetting the distance link. The explanation needs "the further away, the greater the red-shift" as well as "galaxies are moving away".
- Saying the shift is towards the blue end for galaxies. The shift is towards the red end.
Exam-style question
Astronomers measure the spectrum of light from two galaxies, X and Y. Dark lines are seen in both spectra. The lines for galaxy Y are shifted much further towards the red end than the lines for galaxy X.
(a) Describe how the dark lines in the spectrum of galaxy X differ from the same lines measured on Earth. [1 mark]
(b) State what the red-shift shows about the movement of the galaxies. [1 mark]
(c) Galaxy Y is further from the Earth than galaxy X. Explain how the results provide evidence that the universe is expanding. [3 marks]
Model answer
(a) The lines are shifted towards the red end of the spectrum (1).
(b) The galaxies are moving away from us (1).
(c) The more distant galaxy Y has the greater red-shift (1), so it is moving away faster than galaxy X (1). Galaxies moving away, with the more distant ones moving faster, is what we would expect if the universe is expanding (1).
Exam tip
In an "explain" question on expansion, always include both parts: the galaxies are moving away, and the further away they are, the faster they move. Giving only one part loses marks.