AQA GCSE Physics Foundation (8463), Foundation tier · Space physics
Practise Red-shift. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Red-shift is the observed increase in the wavelength of light from most distant galaxies. You need to explain what it shows about how galaxies are moving, why it is evidence for an expanding universe and the Big Bang theory, and what is still not understood, such as dark mass and dark energy. GCSE Physics only, not in Combined Science.
Key facts
Red-shift: an observed increase in the wavelength of light from most distant galaxies
Red-shift → the galaxy is moving away from us
Further away → moving away faster → bigger red-shift
Red-shift is evidence that space itself (the universe) is expanding
Big Bang theory: the universe began from a very small region that was extremely hot and dense
Red-shift supports the Big Bang theory
Since 1998: supernova observations suggest distant galaxies are receding ever faster
Not yet understood: dark mass and dark energy
Notes
What red-shift is
diagram
Light from most distant galaxies is observed to have longer wavelengths than expected. This observed increase in wavelength is called red-shift.
In the spectrum of a galaxy, the pattern of dark lines is the same as in the Sun's spectrum, but every line is shifted towards the red (longer wavelength) end.
Same pattern of lines, shifted towards the red end: red-shift.
Red-shift happens because the galaxy is moving away from us: the light waves are stretched out, so the wavelength increases and the frequency decreases.
The further away a galaxy is, the faster it is moving away and the bigger its red-shift.
Further away → moving away faster → bigger red-shift.
Evidence for an expanding universe and the Big Bang
Distant galaxies in every direction are moving away from us, and the most distant ones are moving fastest. This is evidence that space itself (the universe) is expanding.
If the universe is expanding, it must have been smaller in the past. Tracing the expansion back suggests the universe began from a very small region that was extremely hot and dense.
This is the Big Bang theory. The observed red-shift supports it.
Scientists arrived at this theory from observations: they measured red-shifts, proposed an explanation that fits them, and checked it against more data. Theories can change if new evidence does not fit.
What is still not understood
Since 1998, observations of supernovae suggest that distant galaxies are moving away ever faster: the expansion of the universe is speeding up.
Scientists do not know what causes this. The unknown cause is called dark energy.
Galaxies also seem to contain much more mass than we can see. This unseen mass is called dark mass (dark matter). It is detected only by the effect of its gravity.
So there is still much about the universe that is not understood.
How to answer each type of question
State what red-shift is and what it shows
1 to 2 marksGrade 4
Define it: an observed increase in the wavelength of the light from a distant galaxy.
Say what it shows: the galaxy is moving away from us.
Example. The light from a distant galaxy shows red-shift. Describe what is meant by red-shift and what it tells you about the galaxy.
Show the model answerHide the model answer
The wavelength of the light we observe from the galaxy is increased (shifted towards the red end of the spectrum) (1). This shows that the galaxy is moving away from us (1).
Compare the red-shifts of two galaxies
2 marksGrade 5
Check the line pattern: the same pattern moved to longer wavelengths means red-shift.
The galaxy with the bigger shift is moving away faster.
It is also further away.
Example. Astronomers compare the spectra of two galaxies, X and Y, with the spectrum of the Sun. Both galaxies show the same pattern of dark lines as the Sun. The lines for galaxy Y are shifted further towards the red end of the spectrum than the lines for galaxy X.
What can you conclude about galaxy Y compared with galaxy X?
Show the model answerHide the model answer
Galaxy Y is moving away from us faster (1). Galaxy Y is further away (1).
Don’t lose marks
Saying the galaxy or its light turns red. Red-shift means the wavelength increases, so the lines in the spectrum move towards the red end.
Saying the frequency increases. When the wavelength increases, the frequency decreases.
Describing the Big Bang as an explosion of matter into empty space from one point. The theory says the universe began from a very small, extremely hot and dense region, and space itself is expanding.
Thinking that because distant galaxies in every direction are moving away from us, we must be at the centre. Every galaxy would see the others moving away.
Mixing up dark mass and dark energy: dark mass is unseen mass; dark energy is the unknown cause of the expansion speeding up.
More tips
Memory tricks
Red light has the longest wavelength of visible light, so 'red-shift' means 'shifted to a longer wavelength'.
Further, faster, more red: all three go together.
Picture dots drawn on a balloon as it is blown up: every dot moves away from every other dot, dots that are further apart separate faster, and no dot is the centre.
Spectrum check: same pattern of lines, all moved towards the red end, means the galaxy is moving away.
Exam technique
Use the phrase 'observed increase in wavelength' when you define red-shift.
For Big Bang evidence, build a chain with 'so': red-shift → moving away → further away means faster → space is expanding → the universe began very small, hot and dense.
When describing data, say whether the relationship is directly proportional, not just 'as one goes up, the other goes up'.
For 'not understood' questions, name dark mass and dark energy and say that scientists do not yet know what they are.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Describe what red-shift isRed-shift is an observed increase in the wavelength of light from most distant galaxies.
Grade 5
Explain what red-shift shows about galaxiesA red-shifted galaxy is moving away from us, and a bigger red-shift means it is further away and moving faster.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Astronomers study the light from distant galaxies. What is meant by red-shift?
An increase in the observed wavelength of the light from a galaxy.
Describe what the Big Bang theory suggests about the beginning of the universe.
The universe began from a very small region that was extremely hot and dense.
State whether the wavelength of light from a red-shifted galaxy is longer or shorter than it would be from a stationary source.
Longer.
Scientists think that there is still much about the universe that is not understood. Give one example of something about the universe that is not yet understood.
Dark mass (dark matter) or dark energy.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Astronomers study the light from distant galaxies.
(a) What is meant by red-shift?[1]
(b) What does the red-shift of the light from a galaxy show about the motion of the galaxy?[1]
(c) How does the red-shift of very distant galaxies compare with the red-shift of nearer galaxies? Tick (✓) one box.[1]
Very distant galaxies show a bigger red-shift.
Very distant galaxies show a smaller red-shift.
All galaxies show the same red-shift.
(d) Name the theory about the origin of the universe that is supported by red-shift.[1]
Show the answer and mark scheme
(a)Answer: An increase in the observed wavelength of the light from a galaxy.
an observed increase in the wavelength of light from (distant) galaxies
(b)Answer: It is moving away from us.
the galaxy is moving away from us / from the Earth
(c)Answer: Very distant galaxies show a bigger red-shift.
(d)Answer: The Big Bang theory
the Big Bang theory
Question 2Medium6 marks
Most scientists accept the Big Bang theory of the origin of the universe.
(a) Describe what the Big Bang theory suggests about the beginning of the universe.[2]
(b) Observations show that the further away a galaxy is, the faster it is moving away from us. Explain how this supports the idea that the universe is expanding.[2]
(c) Nobody observed the beginning of the universe. Suggest why scientists accept the Big Bang theory.[2]
Show the answer and mark scheme
(a)Answer: The universe began from a very small region that was extremely hot and dense.
the universe began from a very small region
that was extremely hot and dense
(b)Answer: Galaxies in every direction are receding, faster the further they are, which is what happens if space itself is expanding.
(almost) all distant galaxies are moving away from us
the more distant galaxies moving faster is what is expected if space itself is expanding (everything is moving apart from everything else)
(c)Answer: It explains the evidence, such as red-shift, which has been confirmed by many independent observations.
the theory explains the observations (e.g. the red-shift of distant galaxies)
the observations have been checked / repeated by many scientists, and predictions made by the theory have been confirmed
Stuck? Get 1-to-1 help. Chhetri Academy tutors GCSE and A level Maths and Science online, with a free 30-minute trial lesson.