Laura Waterhouse, of Nayland College, asks :-

How do astronomers know the universe is expanding?

Peter Cottrell, an astronomer at the University of Canterbury, responded.

When astronomers take spectra of distant galaxies, these spectra show that the spectral lines of atoms in these objects are not where they normally appear if the same atoms were observed on Earth. The lines from these galaxies are shifted towards longer wavelengths (the red part of the spectrum), hence the term `redshift'. The amount of red-shifting is proportional to the speed that the object is moving away from the Earth and gets progressively larger the further away from Earth we look. This led astronomers to conclude that the universe is expanding. Objects with the largest redshifts show spectral lines in the red part of their spectrum that would appear in the ultraviolet part of the object's spectrum if it were not moving away from us. These correspond to speeds of up to 90 per cent of the speed of light, or about 270,000 kilometres per second.

This shifting is more commonly known as the Doppler shift after the Austrian physicist Christian Doppler, who proposed it in 1842. It applies equally well to sound waves. Consider a flute player who is making a note of a constant pitch. If she was moving away from you at a constant speed, you will hear a note at a constant, but lower, pitch. That is, the wavelength that you hear is longer, equivalent to the `redshift' in the spectra of distant objects. Conversely, if the flute player is moving towards you at constant speed, you would say that she is playing a note of higher pitch. Traffic police use the Doppler shift of radar waves to measure the speed of travelling cars.