The evolving colour of the Universe

Set of three-colour images of intrinsically bright galaxies in the Hubble Deep Field South. The galaxies are arranged horizontally by the age of the Universe when the light left each object. For reference, the Universe is now 13.7 billion years old. The colours of the galaxies have had the effect of redshift removed. That is, the colours indicate the amount of light which is emitted at a given rest-frame wavelength, as observed by someone at the same redshift as each galaxy. These colours provide information about the ages of stars in the galaxies, where redder colours indicate older stars. At the bottom is shown how the mean colour of bright galaxies changes as the Universe gets older. The reddening in colour with time is due to the increasing mean age of the stars.