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In the night sky: Orion
In the night sky: Orion

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3.1.3 Evidence of expansion

Astronomers deduced that the Universe was expanding because light from distant galaxies is redshifted. What does this mean?

In order to explain what ‘redshift’ means it is useful to consider an example from everyday life. The phenomenon is known as the Doppler effect and it is probably familiar to you in the context of sound waves, although it applies equally to any wave motion, including electromagnetic radiation such as light. The Doppler effect with sound is perhaps most noticeable when an approaching ambulance sounds its siren or as a speeding car races past.

An image of a demonstration of the Doppler effect with sound.
Figure 4 A demonstration of the Doppler effect with sound

Figure 4 shows how wavelength appears to change with distance between object and receiver. The ambulance sounds its siren as it moves towards observer A. Six successive time intervals are shown in the six sketches, with the curved lines representing successive crests of the sound wave emitted by the siren. You may like to think of the curved lines as being similar to the ripples produced when a stone is dropped into a pond – the wave crests spread out from the centre just as shown here. The wavelength is then just the distance between any two successive crests at any point.

By the time the second wave crest is emitted, the ambulance has caught up slightly with the first wave crest. By the time the third wave crest is emitted, the ambulance has caught up with the second wave crest, and so on. The consequence is that a person at A will perceive a sound wave with a shorter wavelength than that emitted by the siren when at rest, while a person at B will perceive a longer wavelength.

The wavelength of sound waves may be appreciated by the pitch of the sound perceived by the human ear. The wavelength of a light wave is perceived by the human eye as the colour of the light. A shift in the colour to longer wavelengths (towards the red end of the spectrum) is an indication of motion away from the observer, a shift in the colour to shorter wavelengths (towards the blue end of the spectrum) is an indication of motion towards the observer.

The wavelengths of light from distant galaxies, when observed from Earth, are shifted towards the red. This is interpreted as meaning that the galaxies are moving away from us, i.e. the spectrum of light is redshifted.

It is important to realise that although we are using the Doppler effect to explain how a redshift comes about, the Doppler effect results from motion of objects through space (the ambulance is moving towards or away from an observer). Redshift arises from the expansion of space i.e. causing galaxies to be moving further away.

Next, you will view an image of distant galaxies taken by the Hubble Space Telescope.