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El Gordo (ACT-CL J0102-4915) is an extremely massive galaxy cluster ($M_{200}\approx 3 \times 10^{15}$ $M_{\odot}$) at redshift $z = 0.87 $ composed of twosubclusters with mass ratio 3.6 merging at speed $V_{infall} \approx 2500$km/s. Such a fast collision between individually rare massive clusters isunexpected in Lambda cold dark matter ($\Lambda$CDM) cosmology at such high$z$. However, this is required for non-cosmological hydrodynamical simulationsof the merger to match its observed properties (Zhang et al. 2015). Here, wedetermine the probability of finding a similar object in a $\Lambda$CDM contextusing the Jubilee simulation box with side length $6 \, h^{-1}$ Gpc. We searchfor galaxy cluster pairs that have turned around from the cosmic expansion withproperties similar to El Gordo in terms of total mass, mass ratio, redshift,and collision velocity relative to virial velocity. We fit the distribution ofpair total mass quite accurately, with the fits used in two methods to inferthe probability of observing El Gordo in the surveyed region. The moreconservative (and detailed) method involves considering the expecteddistribution of pairwise mass and redshift for analogue pairs with similardimensionless parameters to El Gordo in the past lightcone of a $z = 0$observer. Detecting one pair with its mass and redshift rules out $\Lambda$CDMcosmology at $6.16\sigma$. We also use the results of Kraljic & Sarkar (2015)to show that the Bullet Cluster is in $2.78\sigma$ tension once the skycoverage of its discovery survey is accounted for. Using a $\chi^2$ approach,the combined tension can be estimated as $6.43 \sigma$. Both collisions arisenaturally in a Milgromian dynamics (MOND) cosmology with light sterileneutrinos.

monthly notices of the royal astronomical society

A massive blow for $\Lambda$CDM $-$ the high redshift, mass, and  collision velocity of the interacting galaxy cluster El Gordo contradicts  concordance cosmology