What is red shift, and how does it support the Big Bang theory?
Red shift of light from distant galaxies, the expanding Universe, the Big Bang theory and the cosmic microwave background radiation.
A focused answer to WJEC GCSE Physics topic 2.6 on the Universe, covering the red shift of light from distant galaxies, the evidence for an expanding Universe, the Big Bang theory, and the cosmic microwave background radiation.
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What this topic is asking
WJEC wants you to explain red shift, describe the expanding Universe, and state the evidence for the Big Bang theory. This is topic 2.6 The Universe in Unit 2 of WJEC GCSE Physics (3420).
Red shift
Red shift is an effect on the light, not a change in the colour of the galaxy itself: the whole pattern of spectral lines is stretched to longer wavelengths. The same idea explains the falling pitch of a siren as an ambulance passes, where the sound waves are stretched, though for light it is the wavelength of the electromagnetic waves that increases. Astronomers detect red shift by looking at the dark lines in a galaxy's spectrum, which are produced by particular elements: the lines appear at the right pattern but shifted towards longer wavelengths, and the size of the shift tells them how fast the galaxy is receding.
Evidence for an expanding Universe
The Big Bang and the cosmic microwave background
Try this
Q1. State what happens to the wavelength of light from a galaxy that is moving away. [1 mark]
- Cue. The wavelength increases (the light is red shifted).
Q2. Name the radiation that fills all of space and supports the Big Bang theory. [1 mark]
- Cue. The cosmic microwave background (CMB) radiation.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20204 marksExplain what red shift is and how it provides evidence that the Universe is expanding.Show worked answer →
A topic 2.6 Explain question. Red shift is the increase in the wavelength (and decrease in frequency) of light from a distant galaxy, shifting it towards the red end of the spectrum (1 mark). It happens because the galaxy is moving away from us (1 mark). Light from almost all galaxies is red shifted, and the more distant galaxies have a greater red shift, so they are moving away faster (1 mark). This shows that space itself is expanding in all directions (1 mark). Markers reward the longer wavelength, motion away, the distance link and the expansion. A common error is to call it a colour change of the galaxy itself.
WJEC 20223 marksDescribe two pieces of evidence that support the Big Bang theory.Show worked answer →
A topic 2.6 Describe question. First, the red shift of light from distant galaxies shows the Universe is expanding, which fits a Universe that began from a single point (1 mark), with more distant galaxies receding faster (1 mark). Second, the cosmic microwave background (CMB) radiation fills all of space and is the leftover radiation from the hot early Universe, exactly as the Big Bang predicts (1 mark). Markers reward red shift/expansion and the CMB. A common error is to give only one piece of evidence.
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)