Rapid evolution of the most luminous galaxies during the first 900 million years

The first 900 million years (Myr) to redshift z approximate to 6 ( the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z approximate to 6 (refs 1 - 4) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old ( z approximate to 12 - 15) to z approximate to 6, just 600 Myr later. Here we report the results of a search for galaxies at z approximate to 7 - 8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z approximate to 7 - 8, where ten would be expected if there were no evolution in the galaxy population between z approximate to 7 - 8 and z approximate to 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z approximate to 7 - 8 by the hierarchical merging of small galaxies.