Published on Mon Dec 07 2020

### A massive blow for ΛCDM – the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology

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 two
subclusters with mass ratio 3.6 merging at speed $V_{infall} \approx 2500$
km/s. Such a fast collision between individually rare massive clusters is
unexpected in Lambda cold dark matter ($\Lambda$CDM) cosmology at such high
$z$. However, this is required for non-cosmological hydrodynamical simulations
of the merger to match its observed properties (Zhang et al. 2015). Here, we
determine the probability of finding a similar object in a $\Lambda$CDM context
using the Jubilee simulation box with side length $6 \, h^{-1}$ 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 lightcone of a $z = 0$
observer. Detecting one pair with its mass and redshift rules out $\Lambda$CDM
cosmology 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 sky
coverage of its discovery survey is accounted for. Using a $\chi^2$ approach,
the combined tension can be estimated as $6.43 \sigma$. Both collisions arise
naturally in a Milgromian dynamics (MOND) cosmology with light sterile
neutrinos.