Red-shift (physics only) — AQA GCSE Physics
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Red-shift (physics only) explained
Light from a distant galaxy arrives with a longer wavelength than the same light would have if the galaxy were at rest, and the effect is measured using the dark absorption lines in the galaxy's spectrum.
Read the full explanation
Those lines are produced by known elements and appear at known wavelengths in the laboratory, so when the whole pattern is found shifted towards the red end of the visible spectrum, the size of the shift can be measured precisely. A longer wavelength means a lower frequency, and since red is the longest wavelength of visible light the effect is called red-shift. It is observed in almost all galaxies, which tells us they are moving away from us; a few nearby galaxies are blue-shifted instead, because they are approaching.
The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength.
Hubble's key result is a pattern rather than a single number: plot the speed of a galaxy against its distance and the points lie close to a straight line through the origin, so the further away a galaxy is, the faster it is moving away from us. Because a greater speed of recession produces a greater red-shift, distance and red-shift go together too. That is how spectra are read in exam questions. If the dark lines of galaxy A are shifted further towards the red end than those of galaxy B, then A has the greater red-shift, so A is moving away faster, so A is further away. The chain runs shift, then speed, then distance, and each link is usually worth a separate mark.
This effect is called red-shift.
The name is literal. Visible light runs from violet at the short wavelength end to red at the long wavelength end, so when the lines in a galaxy's spectrum shift towards longer wavelengths they shift towards the red end, and the effect is called red-shift. The name says nothing about the colour the galaxy appears to be; it describes the direction the lines have moved. The matching effect for an object approaching us is blue-shift, a shift towards shorter wavelengths, and a few nearby galaxies show it. The idea is similar to the change in pitch you hear as a siren passes, although for the most distant galaxies the wavelength grows because space itself expands while the light is travelling.
The observed red-shift provides evidence that space itself (the universe) is expanding and supports the Big Bang theory.
Every distant galaxy is receding from us, and the further away it is the faster it goes. At first that looks as though we sit at the centre of an explosion, but the same pattern would be seen from any galaxy, and that is the clue: the galaxies are not flying apart through space, but space between them is expanding and carrying them with it. The balloon model helps here. Dots inked onto a balloon all move apart as it is blown up, and the dots that start furthest apart separate fastest. Its weakness is that the dots sit on a surface while real galaxies fill three dimensions. Run the expansion backwards and everything was once together, which is what the Big Bang theory describes.
The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense.
The Big Bang theory states that all of the matter and energy in the universe began in a very small region that was extremely hot and extremely dense, and that it has been expanding and cooling ever since. It is a model of how the universe developed, not of an explosion into space that was already there; space, and time with it, began then too. Observations of supernovae and red-shift provide evidence that space itself is expanding and supports the Big Bang theory. For example, the further away a galaxy is, the faster it is moving away, which extrapolates back to a single hot, dense point.
Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding ever faster.
Supernovae of one particular type always explode with about the same peak brightness, so astronomers can use them as standard candles: compare how bright one looks with how bright it really is and you get its distance, measured independently of its red-shift. From 1998 onwards those measurements gave a surprise. The most distant supernovae were dimmer, and so further away, than an expansion that was slowing under gravity could account for. The conclusion was that the expansion of the universe is not slowing but accelerating, and that distant galaxies are receding ever faster. Nothing in the existing physics explained it, so the term dark energy was introduced for whatever is driving it. New observations had forced a change of theory.
Students should be able to explain: qualitatively the red-shift of light from galaxies that are receding that the change of each galaxy’s speed with distance is evidence of an expanding universe how red-shift provides evidence for the Big Bang model how scientists are able to use observations to arrive at theories such as the Big Bang theory that there is still much about the universe that is not understood, for example dark mass and dark energy.
This is the synoptic question at the end of the topic, and it asks you to run the whole argument. Light from galaxies is red-shifted; the shift is larger for more distant galaxies; so the further away a galaxy is the faster it recedes; so space is expanding everywhere; so running the expansion backwards gives a hot dense beginning, which is the Big Bang. Then the honest ending. Theories are built from observations and are changed when new observations do not fit, which is what happened in 1998. Much of the universe is still unaccounted for: dark mass, also called dark matter, is needed to explain the way galaxies rotate, and dark energy is needed to explain the accelerating expansion. Neither has been detected directly.
Your focus
- Describe red-shift as an observed increase in the wavelength of light from a receding galaxy.
- Explain how the dark lines in a spectrum are used to measure the shift.
- Interpret two galaxy spectra to say which galaxy shows the greater shift.
Show all 21 objectives
- Justify, in the order shift then speed then distance, why a galaxy with the greater red-shift must be the further away.
- Compare two galaxy spectra and deduce which galaxy is further away.
- Interpret a graph of galaxy speed against distance.
- Name the effect in which light from a receding galaxy arrives with an increased wavelength.
- Explain why the name refers to the red end of the visible spectrum.
- Distinguish red-shift from blue-shift by the direction of the change in wavelength.
- Explain how observations of red-shift lead to the conclusion that the universe is expanding.
- Evaluate the balloon model of the expanding universe, giving one strength and one weakness.
- Explain why red-shift observed from the Earth does not mean the Earth is at the centre of the universe.
- Describe the Big Bang theory's model of the early universe.
- Explain what is wrong with describing the Big Bang as an explosion of matter into space that already existed.
- Identify that the universe began from a very small region that was extremely hot and dense.
- Explain how the brightness of a supernova gives the distance of its galaxy independently of that galaxy's red-shift.
- Distinguish between an expanding universe and an accelerating expansion.
- Explain why an unexplained observation leads scientists to propose something such as dark energy.
- Construct the argument that runs from red-shift observations to the Big Bang model.
- Explain why scientists change an accepted theory when new observations appear.
- Distinguish dark mass from dark energy by the observation each was proposed to explain.
Red-shift (physics only) exam tips
Marking Points
- one mark for an observed increase in the wavelength of the light
- one mark for the light coming from galaxies that are moving away from us
- one mark for the shift being measured from the positions of dark lines in the spectrum
- one mark for a longer wavelength corresponding to a lower frequency
- one mark for the greater red-shift meaning the greater increase in wavelength
- one mark for the galaxy with the greater red-shift moving away faster
- one mark for that galaxy therefore being further away
- one mark for describing the relationship as the further away the galaxy, the faster it is receding
- one mark for naming the effect as red-shift
- one mark for linking the name to the red end of the visible spectrum, the longer wavelengths
- one mark for naming blue-shift as the equivalent effect for an approaching source
- one mark for stating that the shift is measured from a spectrum rather than seen as a change of colour
- one mark for the observation that distant galaxies are all moving away from us
- one mark for the more distant galaxies receding faster
- one mark for concluding that space itself, and so the universe, is expanding
- one mark for running the expansion backwards to a single point as support for the Big Bang theory
- one mark for stating the universe began from a very small region
- one mark for stating that this region was extremely hot
- one mark for stating that this region was extremely dense
- one mark for explaining that the universe has been expanding from this point onwards
- one mark for observations of supernovae being used to measure the distances of very distant galaxies
- one mark for the conclusion that the rate of recession is increasing rather than slowing down
- one mark for describing this as an accelerating expansion of the universe
- one mark for naming dark energy as the proposed cause, which is not yet understood
- one mark for red-shift showing that galaxies are receding, with a larger shift for more distant galaxies
- one mark for interpreting this as evidence that the universe is expanding
- one mark for tracing the expansion back to a hot dense start as evidence for the Big Bang model
- one mark for stating that theories are changed when new observations do not fit the current model
- one mark for naming dark mass or dark energy as evidence that much about the universe is not understood
Examiner Tips
- 💡Use the words observed increase in wavelength, because that is the phrasing that earns the mark.
- 💡Mention the dark lines when a spectrum is shown, and say which way the pattern has moved.
- 💡Say that the galaxy is moving away from us, not simply that it is moving.
- 💡Answer in the order shift, speed, distance, and say which galaxy each statement is about.
- 💡If a graph of speed against distance is given, describe it as a straight line through the origin.
- 💡Quote the letters used in the figure rather than saying the top one, so the comparison is unambiguous.
- 💡A one mark naming question wants the term only, so do not spend time explaining it.
- 💡Keep red-shift and blue-shift paired in your notes so that you never swap them over.
- 💡Check the direction of the shift on the figure before you name it.
- 💡Separate the observation from the conclusion, because questions usually award a mark for each.
- 💡When you use the balloon model, say what it represents well and what it fails to represent.
- 💡Use the word evidence rather than proof when writing about scientific theories.
- 💡Learn the one line definition, because it is frequently a straight recall mark.
- 💡Write began from a very small region rather than began from nothing.
- 💡Quote the date, since the specification names 1998 and questions sometimes ask for it.
- 💡Distinguish moving away from moving away ever faster; only the second is the supernova result.
- 💡Use this example when you are asked how scientists change theories in the light of new evidence.
- 💡Write the argument as a chain of so statements, because each link is a mark.
- 💡Name dark mass and dark energy separately and say what each was proposed to explain.
- 💡Use the phrase the current model to show that you understand science is provisional.
Common Mistakes
- writing only that the light waves are stretched, which the mark scheme ignores
- saying the light changes colour to red, when the shift is measured right across the spectrum
- describing a decrease in wavelength, which would be a blue-shift
- saying the galaxies look redder because they are hot, rather than because they are receding
- getting the chain backwards and concluding that the more shifted galaxy is the closer one
- saying the galaxies move faster because they are hotter or lighter
- describing a graph of speed against distance as showing that all galaxies move at the same speed
- identifying the shift but never stating the conclusion about speed or distance
- answering red-shift to a question that asked what causes the effect rather than what it is called
- saying that distant galaxies look red to the eye
- confusing red-shift with the red appearance of a cool star
- calling a shift towards shorter wavelengths a red-shift
- concluding that the Earth is at the centre of the universe because everything is moving away from us
- saying the galaxies are flying outwards from an explosion, rather than that space is expanding
- offering the balloon model with no weakness stated when the question asks for one
- treating red-shift as proof rather than as evidence that supports the theory
- describing the Big Bang as an explosion of matter into empty space that already existed; correct this by stating space itself expands.
- saying that the Big Bang theory explains what caused the universe to begin; correct this by focusing on the development from a small region.
- leaving out either hot or dense from the description of the early universe; correct this by memorising both conditions.
- saying that supernovae push the galaxies apart
- confusing an accelerating expansion with galaxies simply moving away, which was already known
- claiming that dark energy has been detected directly, when it is a name for an unexplained effect
- giving a date earlier than 1998 for these supernova observations
- presenting the Big Bang as a proven fact rather than as the theory that best fits the current observations
- confusing dark matter, which is about missing mass, with dark energy, which is about the accelerating expansion
- saying scientists change theories because they made a mistake, rather than because new evidence appeared
- listing observations without drawing a conclusion from any of them