Collisions of galaxy clusters challenge the standard model of the universe

Composite color image of the interacting galaxy cluster El Gordo, showing X-ray light from NASA’s Chandra X-Ray Observatory in blue, optical data from the European Southern Observatory’s Very Large Telescope in red, green, and blue, and infrared emission from NASA’s Spitzer Space Telescope in red and orange. Image Credit: X-Ray: NASA/CXC/Rutgers/J. Hughes et al; Optical: ESO/VLT & SOAR/Rutgers/F. Menanteau; IR: NASA/JPL/Rutgers/F. Menanteau

According to recent research by an international team of astronomers, including a physicist from the University of St Andrews, the collision of two giant galaxy clusters when the Universe was half its present age should not have happened on the basis of the usual theory of cosmology.

According to the standard cosmic lambda cold dark matter model (ΛCDM), galaxies form first and only then merge into larger galaxy clusters. Therefore, it takes enough time for galaxy clusters to appear in the universe. New research published in the Astrophysical Journal challenges this claim by showing that two extremely massive galaxy clusters crashed into each other at very high speeds when the Universe was only half its current age. The pair of clusters in question is called El Gordo, which means “The Big One” in Spanish – an appropriate name since its mass is about 2 trillion times that of the Sun (2 followed by 15 zeros). The new study uses updated and very precise estimates of its volume. This eliminates a significant source of doubt in an earlier study by the same author on the El Gordo difficulty for ΛCDM. The mass was estimated using light deflections from background galaxies, whose shapes appear distorted by El Gordo’s gravity – a bit like a magnifying glass.

Quantum Technology eBook A compilation of interviews, articles, and top news from last year. Download a free copy This so-called “weakly lensed” mass was obtained with the Hubble Space Telescope, but is consistent with more recent results from the James Webb Space Telescope and other studies using other methods . The mass currently has a modest uncertainty of 10%. The study was led by Elena Asencio, a Ph.D. student at the University of Bonn, used previously published complex simulations of the interaction to evaluate the speed at which clusters collide. Additionally, the authors performed the study using a less complex ΛCDM cosmological simulation, including a large volume to search for pairs of simulated clusters. The goal is to count how many of them are similar to El Gordo just before impact. This was achieved by the revolutionary “light cone tomography” technique, which considered very distant objects could be seen much further away in times when there was less structure.

The results show that there is significant strain with the ΛCDM model, regardless of the reasonable collision speed considered. Additionally, remaining uncertainties about El Gordo’s mass are no longer considered a major factor contributing to this stress. The results of our previous study were questioned by some scientists following the publication of updated mass estimates for El Gordo and were slightly lower. This effectively reduces the tension with ΛCDM, but it is still very significant for any reasonable impact speed. Hundreds of detailed simulations show that El Gordo cannot resemble photographs given the much slower impact speeds possible in ΛCDM.

Although it is possible to provide an El Gordo-like simulation with very fast collisions, such an event rarely occurs in ΛCDM. Indeed, it would be very strange to identify two such massive star clusters at such a close distance at such an early stage in the history of the universe. Additionally, having to aim them at each other at high speed increases reliability. Newly completed research and more precise mass measurements could lead to more efforts in simulating El Gordo to better understand this mysterious object. El Gordo is not the only example of cluster conflict contrary to ΛCDM. The Bullet Cluster is another example of a high-energy collision between two galaxy clusters, although it occurred later. Combined with El Gordo, the situation becomes even worse for ΛCDM. And some other examples are known and mentioned in our research.