Waves and space · GCSE Physics
Red shift
GCSE Physics only (not Combined Science): teacher-written revision on cosmological red shift, Hubble’s pattern that distant galaxies recede faster, and how that supports an expanding Universe.
Physics only. Light from receding galaxies has increased wavelength — red shift. More distant galaxies recede faster. That is evidence that the Universe is expanding.
The important bits
What you need to know
- 1
This is GCSE Physics (Triple) cosmology, not Combined Science. Combined may meet red shift only if a school adds it; the specification home is Physics.
- 2
Red shift: absorption or emission lines from a galaxy sit at longer wavelength than the same lines in a laboratory. Wavelength has been stretched.
- 3
A source moving away from the observer gives a red shift; a source moving towards gives a blue shift. For distant galaxies, red shift dominates.
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Hubble’s pattern: the further the galaxy, the greater the red shift, so the faster it recedes from us. Nearby galaxies can be gravitationally bound and not follow the pattern.
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If galaxies are receding, space itself is expanding. Reverse that expansion and the Universe was once smaller, hotter and denser — which leads to the Big Bang model.
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Red shift is not “galaxies are red because they are hot”, and it is not light getting tired. It is a stretching of wavelength.
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The Earth is not a special centre: every distant galaxy recedes from every other as space expands, like raisins in a rising bun.
- 8
Use v = fλ thinking: longer λ means lower f for the observed light. The laboratory frequency of the line is a property of the atom; the observed frequency has changed.
Quotations worth analysing
Short evidence. Real method.
“The further away a galaxy is, the greater its red shift and the faster it is moving away.”
Distance, red shift, recession speed — three ideas in one sentence. That correlation is the expansion evidence.
“Red shift is an increase in the observed wavelength of light from a receding source.”
Increase in wavelength, towards the red end of the visible spectrum, though the light may not look red to the eye if it started as ultraviolet.
Go deeper
Red shift is a stretching of wavelength, not a colour filter
Absorption lines in a galaxy’s spectrum sit at longer wavelengths than the same lines in a lab. That is red shift. Hubble found that the further the galaxy, the greater the red shift, so the faster it recedes. Reverse that expansion and the Universe was once hotter and denser — the Big Bang. In an exam, do not say galaxies are “red because they are hot” or that red shift proves the Earth is at the centre. Every distant galaxy recedes from every other as space expands. Nearby galaxies can be gravitationally bound and not follow the Hubble pattern; the question will usually mean distant ones. A sketch of two spectra with labelled lines shifted towards red is worth a paragraph of vague cosmology. This whole argument is Physics-only on standard Combined/Triple splits.
Go deeper
The bun model stops the “we are in the middle” mistake
Raisins in a rising bun all move apart. From any raisin, the others recede, and the more distant ones recede faster because more expanding dough sits between. That is Hubble’s pattern without putting the Milky Way on a throne. Blue shift exists: Andromeda is approaching and will eventually interact with us, because gravity locally wins. Cosmological red shift is about the large-scale expansion of space, not about galaxies burning out fuel as they fly through a fixed void. Students write “the galaxy is going red as it cools”. Cooling stars do change colour, but that is not the red-shift evidence for expansion. Stick to spectral lines moving to longer wavelength, distance, speed, expansion.
See the idea in action
A line that is 656 nm in a laboratory is measured at 662 nm in a distant galaxy. The wavelength has increased, so the galaxy is receding — red shift. A more distant galaxy showing 670 nm for the same line is receding faster, which matches Hubble’s pattern and supports expansion. If a nearby galaxy showed 650 nm, that would be a blue shift (approaching) and would not contradict Hubble, because nearby galaxies can be pulled by local gravity. Percentage increase for the first galaxy: (662 − 656) / 656 × 100 ≈ 0.91%. You are not required to find a speed from that at GCSE, but you must say: larger shift, greater distance, greater recession speed, expanding Universe. Physics only, not Combined.
Exam technique
Turn knowledge into marks
Define red shift as increased wavelength of spectral lines. Link greater distance to greater red shift to greater recession speed to expansion. Say the Earth is not a unique centre. Flag the topic as GCSE Physics only.
Common mistakes
Do not give these marks away
- 01
Explaining red shift as light getting tired, or as galaxies being red-hot.
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Claiming Hubble’s law puts the Earth at the centre of the Universe.
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Treating this as Combined Science content, or ignoring that nearby galaxies can blue-shift.
What does the red shift of distant galaxies provide evidence for?
AThat the Sun will become a black hole
BThat the Universe is expanding, consistent with the Big Bang model
CThat light travels faster in space than in air
DThat planets orbit because there is no gravity
Show the answer
That the Universe is expanding, consistent with the Big Bang model. More distant galaxies show greater red shift and recede faster. Running that expansion backwards supports a hot, dense beginning, alongside evidence from CMBR.
Quick questions
If this is the bit you searched
What is red shift?
An increase in the observed wavelength of light (or other EM waves) from a source that is receding, seen as spectral lines moved towards the red.
What did Hubble observe?
That more distant galaxies have greater red shift, so they recede faster. That pattern supports an expanding Universe.
Does red shift mean we are at the centre?
No. Expansion happens throughout space, so every distant galaxy sees others receding. The bun-and-raisins picture is the standard model.
Is red shift on Combined Science?
It is a GCSE Physics (Triple) cosmology idea on AQA-style routes. Combined students should not assume it is required.