Published by Australian Academy of Science
Hunting for dark energy with the WiggleZ Box 1 | Doppler shift

Spectroscopy is a vital part of an astronomer’s toolbox. By studying the spectrum or the wavelengths of light from an object in space, astronomers can get a range of information. For example, the change in position of lines in the spectrum from a star can tell astronomers how far away it is, whether it is moving towards or away from us and how fast it is moving.

The Doppler Effect

Listening to a siren or a racing car speeding past it sounds higher in pitch the closer it gets to you and lower as it moves away. Pitch is related to the frequency of sound waves, so the approaching sound seems to have a higher frequency (and shorter wavelength), the receding sound a lower frequency (and longer wavelength). This is called the Doppler Effect, where waves, in this case sound waves, change in frequency and wavelength as the source moves towards or away from you. There is no actual change in sound; the racing car isn’t making a different noise. It just sounds different due to the car’s movement relative to you.

Doppler shift

This apparent change in wavelength can also be observed for electromagnetic radiation, for example, visible light. So if a star is moving towards Earth, it appears to emit light that is shorter in wavelength compared to a source of light that isn’t moving. Because shorter wavelengths correspond to a shift towards the blue end of the spectrum, this is called blueshift. In contrast, the light from a star moving away from us seems to shift towards longer wavelengths. As this is towards the red end of the spectrum, astronomers call it redshift. At the large distances measured by the WiggleZ project, redshift is due to the expansion of the universe rather than a Doppler shift, although they both have the same effect on the spectra.

Lines in the spectrum from an object moving away are redshifted

The degree of shift can also give astronomers information about how fast the object is moving relative to us. So a faster object has a greater shift in wavelength.

Related sites

• The Doppler Effect (CSIRO, Australia)

• Spectra and what scientists can learn from them (NASA, USA)

• Spectroscopy – a vital tool (Nova: Science in the news, Australian Academy of Science)

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