Red-shift revision notes

Edexcel International GCSE Physics (4PH1), chapter 28: Cosmology, specification point 8.15P, 8.17P. Free lesson.

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The Doppler effect changes the wavelength and frequency measured from a moving wave source. Red-shift is the movement of light towards the red end of the spectrum, and it shows that a galaxy is moving away.

How the Doppler effect changes waves

The Doppler effect is the change in the wavelength and frequency an observer measures when a wave source moves. The wave speed stays the same. If the source moves to the right, wavefronts bunch up ahead of it and spread out behind it. Ahead of the source, the wavelength is shorter, so the frequency is higher. Behind the source, the wavelength is longer, so the frequency is lower.

Why light from a moving galaxy is red-shifted

Light is a wave, so the Doppler effect also applies to light from galaxies. When a galaxy moves away from Earth, the light waves reaching us are stretched out. Their wavelength becomes longer and their frequency becomes lower. Longer visible wavelengths are towards the red end of the spectrum, so the light is shifted towards red. This is called red-shift.

How red-shift appears in a spectrum

Red-shift is not identified by looking at the galaxy's overall colour. It is seen by comparing the dark lines in its spectrum with the same pattern of lines from a laboratory spectrum made on Earth. The lines are produced when elements such as hydrogen absorb certain wavelengths. If the lines have moved towards the red end, the wavelength has shifted. The shift of a line can be labelled Δλ. Compare the same pattern of lines in the two spectra. This shows a shift rather than a different set of elements. A galaxy further away can have a larger shift of the same lines.

What galaxy distance tells us about red-shift

Measurements of many galaxies show that the further away a galaxy is, the faster it is moving away. A bigger red-shift indicates a faster speed, so more distant galaxies have bigger red-shifts. Galaxy A is much further from Earth than galaxy B. Light from A has a bigger red-shift than light from B. Both galaxies are moving away from us, and A is moving away faster.

Red-shift means a shift, not a colour

The name can be misleading: red-shift does not mean the galaxy turns red. It means the light's wavelength gets longer, moving the lines in its spectrum towards the red end. For example, a blue line moves a little towards green, rather than becoming red.