We calculate the expected mass of the most massive object in the universe, finding it to be a cluster of galaxies with total mass M200= 3.8x1015 M☉ at z = 0.22, with the 1σ marginalized regions being 3.3 x1015 M☉< M200< 4.4x1015 M☉ and 0.12 < z < 0.36. We restrict ourselves to self-gravitating bound objects and base our results on halo mass functions derived from N-body simulations. The mass and redshift distribution of the largest objects in the universe are potentially interesting tests of ΛCDM, probing the initial conditions, non-Gaussianity, and the behavior of gravity on large scales. We discuss A2163 and A370 as candidates for the most massive cluster in the universe, although uncertainties in their masses preclude definitive comparisons with theory. We find that the three most massive clusters in the South Pole Telescope (SPT) 178 and 2500 deg2 catalogs match predictions. Since the mass function evolves steeply with redshift, we also investigate the most unlikely clusters in the universe. We find that SPT-CL J2106-5844 is 2σ and XMMU J2235.3-2557 is 3σ inconsistent with ΛCDM, considering their respective redshifts and survey sizes. Our findings motivate further observations of the highest mass end of the mass function, particularly at z > 1, where a number of anomalously massive clusters have been discovered. Future surveys will explore larger volumes, and both the most massive object and the most unlikely object in the universe may be identified within the next decade.
cosmology: theory - galaxies: clusters: general