What does red-shift tell us about the expanding universe and the Big Bang?
Red-shift and the Big Bang: how red-shift shows the universe is expanding, the evidence for the Big Bang theory, and dark matter and dark energy (separate physics).
A focused answer to AQA GCSE Physics 4.8.2, covering red-shift and how it shows distant galaxies are moving away, the evidence that the universe is expanding, the Big Bang theory and the cosmic microwave background, and dark matter and dark energy.
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What this dot point is asking
AQA wants you to explain what red-shift is, describe how it provides evidence that the universe is expanding, outline the Big Bang theory and its supporting evidence, and explain why dark matter and dark energy were proposed. This is part of topic 4.8.2 of the AQA GCSE Physics (8463) specification and is separate physics only.
Red-shift
Red-shift is observed by looking at the dark absorption lines in a galaxy's spectrum. These lines occur at known wavelengths for particular elements, so when the whole pattern of lines is shifted towards longer (redder) wavelengths, astronomers can measure exactly how much the light has been stretched and therefore how fast the galaxy is receding. The same effect for sound, where a source moving away has a lower pitch, is the everyday analogue.
Evidence for an expanding universe
It is important to understand that the galaxies are not flying away through space from a central point; rather, space itself is expanding and carrying the galaxies along with it, so an observer in any galaxy would see all the others receding. This is why a common analogy is dots on the surface of a balloon being inflated: every dot moves away from every other dot as the rubber stretches, and the dots that start further apart separate faster.
The Big Bang theory
Dark matter and dark energy
Try this
Q1. Explain what red-shift tells us about distant galaxies. [2 marks]
- Cue. Their light has a longer (red-shifted) wavelength, showing they are moving away from us, and faster the further away they are.
Q2. State one piece of evidence that supports the Big Bang theory. [1 mark]
- Cue. The cosmic microwave background radiation (or the red-shift of distant galaxies).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksExplain what is meant by red-shift, and describe how observations of the red-shift of distant galaxies provide evidence that the universe is expanding and support the Big Bang theory.Show worked answer →
A top-band level-of-response answer defines red-shift as the increase in the observed wavelength of light from a galaxy, shifting it towards the red end of the spectrum, which happens because the galaxy is moving away from us. It then describes the key observations: light from distant galaxies is red-shifted, and more distant galaxies show greater red-shifts. The reasoning is that a greater red-shift means a faster speed of recession, so the further away a galaxy is, the faster it is moving away from us. This pattern is best explained by the whole of space expanding, carrying the galaxies apart. Tracing this expansion backwards in time leads to the idea that the universe began from a very small, hot, dense point, which is the Big Bang theory. A strong answer links each observation to its conclusion. Markers reward the definition, the distance-speed pattern, the expansion conclusion, and the connection to the Big Bang.
AQA 20214 marksDescribe the cosmic microwave background radiation, and explain why it is considered strong evidence for the Big Bang theory rather than for other models of the universe.Show worked answer →
The cosmic microwave background radiation (CMBR) is a faint microwave radiation that is detected coming from all directions in space, fairly uniformly (1 mark). It is thought to be the leftover radiation from the early, hot universe, now stretched to microwave wavelengths as space has expanded (1 mark). It is strong evidence for the Big Bang because the Big Bang theory is the only model that successfully predicts and explains the existence and properties of the CMBR (1 mark); other proposed models cannot account for radiation arriving uniformly from every direction, so the CMBR has led scientists to accept the Big Bang as the best current theory (1 mark). Markers reward describing the CMBR as uniform microwave radiation from all directions and explaining that only the Big Bang explains it.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)