THE DOPPLER EFFECT
2 marks · EasyExplain how observing red-shifted galaxies supports the idea that the universe is expanding.
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Seeing red-shifted galaxies means they are moving away from us in all directions, showing that space between galaxies is increasing, which indicates the universe is expanding.
Also accept: Red-shift means galaxies are receding, and if most galaxies are moving away, the universe must be expanding.
Common mistakes: Describing red-shift but not linking to expansion.
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Marking points
M1 · 1 mark · Explains that red-shift shows galaxies are moving away.
M1 · 1 mark · Links red-shift of galaxies in all directions to expansion of the universe (space between galaxies increasing).
Worked solution
Red-shift shows that galaxies are moving away from us.
If most galaxies are moving away, it means the space between them is increasing, which is evidence that the universe is expanding.