The vast majority of galaxies are moving away from us. The more distant galaxies are moving the fastest – in fact the speed of a galaxy is proportional to the distance from us. We do not have a privileged position - the same conclusion is drawn for any observer anywhere in the Universe. We can think, instead of galaxies moving away from us, as space expanding. The expansion of space itself appears to give galaxies a velocity relative to us. In this picture, light waves are stretched between being emitted from some distant galaxy and being received by us be the stretching of space between these two times.
The net effect is that the wavelength of light emitted by some distant galaxy increases in wavelength – it is redshifted – before being received by us. The redshift is given approximately by
where
= change in wavelength and
= wavelength of emitted light respectively
= speed of distant galaxy
= speed of light
Example: An emission line due to hydrogen has a wavelength of
In the spectrum of a distant galaxy this wavelength occurs at
Find the speed of the galaxy.
