A massive protocluster of galaxies at a redshift of z ≈ 5.3

Massive clusters of galaxies have been found that date from as early as 3.9 billion years(1) (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs(2,3). Cosmological simulations using the current cold dark matter model predict that these systems should descend from 'protoclusters'-early overdensities of massive galaxies that merge hierarchically to form a cluster(4,5). These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts(4-6). Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe(6,7). Here we report a protocluster region that dates from 1 Gyr (z = 55.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas(8). These massive galaxies place a lower limit of more than 4 x 10(11) solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters(4,5,7).