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

Understand

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).