ESA Science & Technology03-Aug-2005 11:20:00

Galaxies and the Expanding Universe

A Redshifted Universe

The observed wavelength of light increases if the source of the light is moving away from the observer. This effect is observed in dark absorption lines or bright emission lines in the spectrum of a galaxy. If a galaxy is moving away from us, the wavelengths of all the lines in its spectrum are moved to longer wavelengths, and the lines appear shifted towards the red end of the spectrum. This is effect is known as redshift (z).

Where λ0 is the observed wavelength and λe is the emitted wavelength. 

Astronomers analyse the spectra from distant galaxies and look for key lines such as the K and H lines of ionized calcium and the H-alpha line of hydrogen.  The position of these lines can be found by laboratory experiment and compared to the observed position in the galactic spectrum.

For velocities <0.1c (where c is the speed of light) the redshift equation can be rewritten as:

At relativistic velocities the equation needs to be modified and becomes:


Redshift is an important tool for cosmology, because it indicates that the Universe is expanding. Astronomer Edwin Hubble using the 100-inch Hooker reflector at the Mount Wilson Observatory, was the first to discover that virtually all galaxies show redshifts in their spectra. Initially the cause of the redshifts in the spectra of galaxies was misinterpreted and consequently it is often known as a Doppler shift due its similarities with the Doppler effect for sound waves.

Astronomers, assumed at first, that the galaxies were travelling through space at enormous velocities due to a great explosion. Later it was realised that the Universe appears to expand not because the galaxies are moving, but the space between them galaxies expanding.

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