eROSITA relaxes cosmological tension

Results from the first X-ray sky survey resolve the previous inconsistency between competing measurements of the structure of the Universe

The analysis of how galaxy clusters, the largest structures in the Universe, evolve over cosmic time has yielded precise measurements of the total matter content and its clumpiness, report scientists of the German eROSITA consortium, led by the Max Planck Institute for Extraterrestrial Physics. The results affirm the standard cosmological model and alleviate the so-called S8 tension, while at the same time offering insights into the elusive neutrinos' mass and dark energy equation of state. 

Two weeks ago, the German eROSITA consortium released its data from the first all-sky survey. The survey's primary goal is to better understand cosmology via the measurement of the growth over cosmic time of clusters of galaxies, some of the largest structures in our Universe. Tracing the evolution of clusters via the X-rays emitted by hot gas, eROSITA has made precision measurements of both the amount of total matter in the universe and its clumpiness. The eROSITA measurements resolve previous inconsistencies between past clumpiness measurements using different techniques, specifically the cosmic microwave background (CMB) and weak gravitational lensing.

“eROSITA has now established cluster evolution measurement as a tool for precision cosmology,” said Dr. Esra Bulbul (MPE), the lead scientist for eROSITA’s clusters and cosmology team who delivered the ground-breaking results. “The cosmological parameters that we measure from galaxy clusters are consistent with state-of-the-art CMB, showing that the same cosmological model holds from soon after the Big Bang to today.”

Zooming into clusters

This video shows both the full eROSITA cluster catalogue (see figure above) and a zoom to some well-known clusters, including the nearby Virgo cluster, the large overdense region of clusters known as the Shapley supercluster, the Centaurus cluster, the A3391/95 interacting cluster system, and finally, the Fornax cluster.

According to the standard cosmological model, called the Lambda Cold Dark Matter (ΛCDM) model, the infant universe was an extremely hot, dense sea of photons and particles. Over the course of cosmic time, tiny density variations grew into the large galaxies and galaxy clusters we can see today. The eROSITA cluster observations show that matter of all kinds (visible and dark) comprises 29% of the total energy density of the Universe, in excellent agreement with the values obtained from measurements of the cosmic microwave background  radiation, which was emitted when the universe first became transparent.

As well as measuring the total matter density, eROSITA has also measured the clumpiness of the matter distribution, using a parameter called S8. An important development in cosmology in recent years has been the so-called “S8 tension”. This tension arises because cosmic microwave background experiments measure a higher S8 value than, e.g. cosmological weak gravitational lensing surveys. New physics is implied unless this tension can be resolved, and eROSITA has done just that.  “eROSITA tells us that the universe behaved as expected throughout cosmic history,” says Dr. Vittorio Ghirardini, the postdoctoral researcher at MPE who led cosmology study. “There's no tension with the CMB - maybe the cosmologists can relax a bit now.”

The largest structures in the Universe also carry information about the smallest particles: neutrinos. These lightweight particles are nearly impossible to detect. “It may sound paradoxical, but we have obtained tight constraints on the mass of the lightest known particles from the abundance of the largest dark matter haloes in the universe,” said Ghirardini. Even though neutrinos are small, they are “hot”, i.e. they travel with almost the speed of light. Therefore, they tend to smooth out the distribution of matter – which can be probed by analysing the evolution of the largest cosmic structures in the universe. “We are even on the brink of a breakthrough to measure the total mass of neutrinos when combined with ground-based neutrino experiments,” adds Ghirardini. Cluster abundances in eROSITA data alone indicate an upper bound to the total mass of 0.22 eV; combined with CMB data, this reduces even to 0.11eV at a 95% confidence level. This is the tightest combined measurement to date from any observational cosmology probes.

eROSITA’s insights into the nature of the universe may not end there. Theories of gravity predict that large cosmic structures should grow at a certain rate as the universe evolves. The eROSITA data can measure this growth rate. Although it is too early to say for sure, it appears a little slower at late times than Einstein’s Theory of General Relativity predicts. “We might be at the brink of a new discovery,” says Dr. Emmanuel Artis, a postdoctoral researcher at MPE. “If it can be confirmed, eROSITA will pave the way for new exciting theories beyond general relativity.”

All these results are based on one of the largest pure catalogues of clusters of galaxies to date, which is also being released to the public today. In the Western Galactic half of the first eROSITA all-sky survey, the scientists detected 12,247 optically identified clusters of galaxies. “Of these, 8,361 represent new discoveries – more than 80%,” marvels Dr. Matthias Kluge, a postdoctoral researcher at MPE who is responsible for the optical identification of the detected clusters. “This shows the huge discovery potential of eROSITA.”

When charted in three dimensions, the galaxy clusters are situated at the intersections of a so-called cosmic web. The supercluster catalogue, also released today, maps the connected galaxy clusters and the large-scale filaments connecting them. “We find more than 1300 supercluster systems, which makes this the largest-ever X-ray supercluster sample,” said Dr. Ang Liu, a postdoctoral researcher at MPE.

Another key secret of the successful experiment was the ability to fully reproduce the eROSITA observations with large computer simulations. „In this way, we could fully grap the clusters content of the eROSITA data, by understanding those we missed“, says Dr. Nicolas Clerc, researcher at IRAP in Toulouse. „Addressing these so-called ‚selection biases‘ was an extra challenge for our work“.

Finally, in order to measure the mass of each cluster, scientists in the eROSITA team used a weak gravitational signal derived from three optical surveys: the Europe-led KiloDegree Survey, the US-led Dark Energy Survey, and the Japan-led Hyper Suprime-Cam Subaru Strategic Program, a true global endeavour. The so-called weak gravitational lensing effect occurs when the light from background galaxies is distorted through gravitational interactions with the foreground cluster. The cosmologists then decode these distortions to determine the weight of clusters.

„As we reflect on this monumental achievement by the eROSITA team, we eagerly anticipate the exciting future discoveries that will emerge and deepen our understanding of the origins and evolution of the Universe we live in,” emphasized Dr. Esra Bulbul. The eROSITA team is excited to continue analyzing the 4.5 full sky surveys completed in February 2022. “When the full data are analyzed, eROSITA will put our cosmological models to the most stringent test ever conducted through a cluster survey.”

eROSITA clusters in 3D

In this animation, the eROSITA clusters are shown in 3 dimensions in a spherical coordinate system, where Galactic latitude and longitude and redshift are shown. Each dot is a cluster, with the colours indicating the redshift (see first figure).

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