Red-shift and the Expanding Universe
Red-shift is the observation that light from most distant galaxies has a longer wavelength, shifted towards the red end of the visible spectrum, than it would have if the galaxy were stationary relative to Earth, because the galaxy is moving away. More distant galaxies show greater red-shift, moving away faster than closer ones, providing strong evidence, alongside cosmic microwave background radiation, that the universe is expanding, consistent with the Big Bang model.
Method
- Understand what red-shift means: light, or any electromagnetic wave, from a source that is moving away from an observer is detected with a longer wavelength, shifted towards the red end of the spectrum for visible light, than it would have if the source were stationary; this is an example of the Doppler effect applied to light.
- Learn the key observational pattern: almost all distant galaxies show red-shift, meaning almost all distant galaxies are moving away from Earth and from each other.
- Learn the relationship between distance and red-shift: the further away a galaxy is, the greater its red-shift; since a greater red-shift means a greater speed of recession, more distant galaxies are moving away faster than closer ones.
- Explain why this pattern is evidence for an expanding universe: if every galaxy, in every direction, is moving away from us, and more distant ones are moving away faster, this is consistent with the space between galaxies itself stretching everywhere, rather than Earth being at a special central point that everything happens to be fleeing from.
- Connect this evidence to the Big Bang model: because the universe is currently expanding, running the expansion backwards in time implies it must have been smaller, hotter and denser in the past; the Big Bang model proposes that the universe began from a very small region, extremely hot and extremely dense, and has been expanding and cooling ever since.
- Learn the second, independent piece of evidence for the Big Bang model: cosmic microwave background radiation (CMBR), a faint microwave radiation detected coming from every direction in space, understood as radiation left over from a much hotter, earlier stage of the universe; the Big Bang model predicts this radiation, while a universe that had always existed in its current state would not be expected to produce it.
- For 'explain the evidence' style exam answers, always state both the observation, what is measured, and the inference, what it implies about the universe, since marks are usually awarded separately for describing red-shift or CMBR and for explaining what each observation shows about the universe.
Worked example
A galaxy's light shows a small red-shift. A second, more distant galaxy's light shows a much larger red-shift. Explain what this comparison shows about the motion of the two galaxies, and about the universe as a whole.
- Note the size of each galaxy's red-shift: the nearer galaxy has a small red-shift, and the more distant galaxy has a much larger red-shift.
- Recall what red-shift indicates: red-shift shows that a galaxy is moving away from Earth, and a larger red-shift corresponds to a faster speed of recession.
- Apply this to the nearer galaxy: its small red-shift shows it is moving away from Earth, but relatively slowly.
- Apply this to the more distant galaxy: its much larger red-shift shows it is moving away from Earth considerably faster than the nearer galaxy.
- Draw the general conclusion about distance and speed: the more distant galaxy is both further away and moving away faster, matching the general pattern that the further away a galaxy is, the faster it is moving away.
- Final answer: this pattern, that more distant galaxies recede faster, is evidence that the space between all galaxies is stretching over time, supporting the conclusion that the universe as a whole is expanding.
Practice questions
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Q1State what is meant by red-shift.Show answer
Answer: The light from a source, such as a galaxy, is detected with a longer wavelength, shifted towards the red end of the spectrum, than it would have if the source were stationary, because the source is moving away from the observer.
Q2State what red-shift shows about the motion of most distant galaxies.Show answer
Answer: That they are moving away from Earth (and from each other).
Q3State how the amount of red-shift is related to a galaxy's distance from Earth.Show answer
Answer: The greater the distance to a galaxy, the greater its observed red-shift, and so the faster it is moving away.
Q4Name the model of the origin of the universe that is supported by red-shift observations.Show answer
Answer: The Big Bang model.
Q5State, in one sentence, what the Big Bang model proposes about the early universe.Show answer
Answer: That the universe began from a very small region that was extremely hot and extremely dense, and has been expanding and cooling ever since.
Q6Name the second major piece of observational evidence, besides red-shift, that supports the Big Bang model.Show answer
Answer: Cosmic microwave background radiation (CMBR): faint microwave radiation detected from every direction in space, left over from an earlier, much hotter stage of the universe.
Q7Explain why red-shift observations support the idea that the universe is expanding, rather than Earth simply being at the centre of the universe with everything moving away from it.Show answer
Answer: Red-shift is observed in almost every direction, with more distant galaxies showing greater red-shift; this pattern is exactly what would be seen if the space between all galaxies were stretching uniformly everywhere, so it supports overall expansion of the universe rather than Earth occupying a special central position.
Q8State whether a galaxy moving towards Earth would show a red-shift or a blue-shift in its light.Show answer
Answer: A blue-shift: its light would be shifted towards shorter wavelengths, towards the blue end of the spectrum.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
State two observations astronomers can make about distant galaxies' light that together provide evidence the universe is expanding.
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Two galaxies, X and Y, are studied by astronomers. Galaxy X is closer to Earth than Galaxy Y. (a) State which galaxy, X or Y, would be expected to show the greater red-shift. (b) State which galaxy would be expected to be moving away from Earth at the greater speed. (c) Explain your answers to parts (a) and (b), referring to the relationship between distance and red-shift.
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Describe the observational evidence for the Big Bang model of the universe, and explain how this evidence supports the idea that the universe began from a very small region and has been expanding ever since.
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Free printable worksheet
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This topic is chapter 25 of GCSE Physics Workbook, the whole course as one free printable PDF.
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