2021MNRAS.500.5249A

El Gordo (ACT-CL J0102-4915) is an extremely massive galaxy cluster (M₂₀₀ ≃ 3 x 10¹⁵ M_☉) at redshift z = 0.87 composed of two subclusters with a mass ratio of 3.6 merging at speed V_infall ≃ 2500 km s^–1. Such a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter (ΛCDM) cosmology at such high z. However, this is required for non-cosmological hydrodynamical simulations of the merger to match its observed properties. Here, we determine the probability of finding a similar object in a ΛCDM context using the Jubilee simulation box with a side length of 6 h^–1 Gpc. We search for galaxy cluster pairs that have turned around from the cosmic expansion with properties similar to El Gordo in terms of total mass, mass ratio, redshift, and collision velocity relative to virial velocity. We fit the distribution of pair total mass quite accurately, with the fits used in two methods to infer the probability of observing El Gordo in the surveyed region. The more conservative (and detailed) method involves considering the expected distribution of pairwise mass and redshift for analogue pairs with similar dimensionless parameters to El Gordo in the past light-cone of a z = 0 observer. Detecting one pair with its mass and redshift rules out ΛCDM cosmology at 6.16σ. We also use the results of Kraljic and Sarkar to show that the Bullet Cluster is in 2.78σ tension once the sky coverage of its discovery survey is accounted for. Using a χ² approach, the combined tension can be estimated as 6.43σ. Both collisions arise naturally in a Milgromian dynamics (MOND) cosmology with light sterile neutrinos.