One of the most direct methods of constraining the epoch at which the first galaxies formed - and thereby to constrain the age of the Universe - is to identify and date the oldest galaxies at high redshift. But most distant galaxies have been identified on the basis of their abnormal brightness in some spectral region1-4; such selection criteria are biased towards objects with pronounced nuclear activity or young star-forming systems, in which the spectral signature of older stellar populations will be concealed. Here we report the discovery of a weak and extremely red radio galaxy (53W091) at z = 1.55, and present spectroscopic evidence that its red colour results from a population of old stars. Comparing our spectral data with models of the evolution of stellar populations, we estimate that we are observing this galaxy at least 3.5 Gyr after star-formation activity ceased. This implies an extremely high formation redshift (z > 4) for 53W091 and, by inference, other elliptical galaxies. Moreover, the age of 53W091 is greater than the predicted age of the Universe at z = 1.55, under the assumption of a standard Einstein-de Sitter cosmology (for any Hubble constant greater than 50km s-1 Mpc-1), indicating that this cosmological model can be formally excluded.
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