The expansion of the universe means that the light from distant galaxies is red-shifted when observed on Earth. The larger the red shift, the greater the distance to the galaxy. Moreover, when we view a galaxy with a high redshift, we are seeing it as it was billions of years ago. By measuring the red shifts of galaxies, therefore, astronomers can tell how far away the galaxy is and how old it was when it emitted the light that they are observing.

The Durham team, working with the 4.2 metre William Herschel Telescope in the Canary islands, the Calar Alto Telescope in Spain and the Hubble Space Telescope, counted the number of galaxies at different redshifts in a small region of the sky. They found that the number density of galaxies with very high redshifts - about 5 or 6 - was similar to the number of galaxies with small redshifts. A redshift of 6 corresponds to light emitted about 10 billion years ago, so the results imply that most galaxies were formed before this time - which is early in the history of the universe.

Most theories predict that galaxies formed fairly recently, at redshifts of about 1. Observing galaxies so much older than this means that a radical rethink is clearly required. Furthermore, the results open up the question of whether galaxies exist at even higher redshifts. The Durham team plan to search for these galaxies with the VISTA telescope, which is due to open in Chile in 2004, and the Infrared Telescope on Hawaii, which will be upgraded soon.