# The El Gordo galaxy cluster challenges ΛCDM for any plausible collision velocity

**Authors:** E. Asencio, I. Banik, P. Kroupa  
**arXiv:** [2308.00744](https://arxiv.org/abs/2308.00744)  
**Date:** 2023-08-01  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2308.00744v1/

## Key points

- El Gordo is a massive and luminous galaxy cluster collision
- It challenges the standard cosmological model known as $\Lambda$CDM
- Previous studies show a tension of $6.2\sigma$ with the $\Lambda$CDM model
- Recent weak lensing observations indicate a lower mass for El Gordo
- The revised mass estimate prompts a reevaluation of the tension with $\Lambda$CDM
- In order to reduce the tension below $5\sigma$, subclusters should have an infall velocity lower than $2300~\textrm{km~s}^{-1}$
- Previous simulations suggest an infall velocity of at least $2500~\textrm{km~s}^{-1}$ is required to reproduce observed properties
- El Gordo still presents a significant challenge to the $\Lambda$CDM model
- Further investigations are needed to determine if its properties can be reconciled with a lower infall velocity

## Summary

El Gordo (ACT-CL J0102-4915) is a massive and luminous galaxy cluster collision that poses a significant challenge to the standard cosmological model known as $\Lambda$CDM. Previous studies have shown that El Gordo exhibits a tension of $6.2\sigma$ with the $\Lambda$CDM model when assuming certain values for its mass and infall velocity, derived from hydrodynamical simulations conducted by Zhang et al. Specifically, these simulations suggested a nominal mass of $3.2 \times 10^{15} M_{\odot}$ and an infall velocity of $2500~\textrm{km~s}^{-1}$. However, recent weak lensing observations conducted by Kim et al. have provided new insights into the properties of El Gordo. These observations indicate that the actual mass of El Gordo is significantly lower than previously estimated, measuring at $2.13^{+0.25}_{-0.23} \times 10^{15} M_{\odot}$. This revised mass estimate prompts a reevaluation of the level of tension between El Gordo and the $\Lambda$CDM model, considering various values for the infall velocity. By exploring different scenarios with varying infall velocities, it has been found that in order to reduce the tension below the $5\sigma$ level, the subclusters within El Gordo should have an infall velocity lower than $2300~\textrm{km~s}^{-1}$ ($1800~\textrm{km~s}^{-1}$ when considering combined tension with the Bullet Cluster). However, it is important to note that previous hydrodynamical simulations of El Gordo have indicated that an infall velocity equal to or greater than $2500~\textrm{km~s}^{-1}$ is required to reproduce its observed morphology, as well as its high X-ray luminosity and temperature. In light of these findings, it can be concluded that El Gordo still presents a significant challenge to the $\Lambda$CDM cosmological model. Further investigations are needed in order to determine if its properties can be reconciled with a lower infall velocity; this ongoing research aims to shed more light on this extraordinary galaxy cluster collision and its implications for our understanding of cosmology through new hydrodynamical simulations which explore different configurations of interaction.