Cosmology: On the trail of great mysteries

The analysis of how galaxy clusters, the largest structures that exist, evolve over time has yielded precise measurements of the total amount of matter in the Universe and its clumpiness. This is what scientists of the German eROSITA consortium, led by the Max Planck Institute for Extraterrestrial Physics, report in a set of studies released now, with important contributions by LMU Munich. The results affirm the cosmological standard model and alleviate the so-called S8 tension, while at the same time offering insights into the elusive mass of neutrinos and pressure provided by dark energy. The analysis is based on one of the largest catalogues of galaxy clusters, also released today, and measurements of weak gravitational lensing from several optical surveys. The new results showcase eROSITA's immense discovery potential and promise to refine our cosmological understanding to unprecedented levels.

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 assembly of clusters of galaxies over cosmic time, 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 total amount of matter in the Universe and its clumpiness. The eROSITA measurements resolve previous inconsistencies between clumpiness measurements using different techniques, specifically the cosmic microwave background (CMB) and weak gravitational lensing.

To scientifically interpret the cluster X-ray emission that eROSITA observes, scientists relate the strength of the X-ray signal to the underlying mass of the galaxy clusters. Following previous analyses by other teams, the eROSITA consortium used measurements of weak gravitational lensing to obtain highly reliable mass estimates. “Weak gravitational lensing imprints tiny distortions on distant background galaxies, and a precise measurement requires large amounts of lensing data. For eROSITA, we combined the information from the latest wide and deep sky images, the Dark Energy Survey, Kilo Degree Survey, and Hyper Suprime-Cam Subaru Strategic Survey, a truly global effort”, explains Prof. Daniel Gruen, Chair of Astrophysics, Cosmology, and Artificial Intelligence at LMU in Munich, who led parts of the calibration of these weak lensing measurements.

Cosmological measurements prove to be extremely reliable

The lensing analysis of Dark Energy Survey data was performed by Dr. Sebastian Grandis, a researcher previously at LMU, now at the University of Innsbruck. “The new results from eROSITA are a big step forward in constraining power” says Dr. Sebastian Bocquet at LMU, who co-led the eROSITA–Dark Energy Survey lensing study. He also led a recent analysis of an independent galaxy cluster sample selected in millimeter-wave data collected by the South Pole Telescope (SPT). “The fact that two independent analyses of the cluster abundance do not result in low S8 measurements compared to Planck is really exciting and gives us additional confidence in the constraints coming from weak-lensing informed cluster cosmology”, Bocquet states.

“eROSITA has now brought cluster evolution measurement as a tool for precision cosmology to the next level,” said Dr. Esra Bulbul, senior staff scientist at MPE and lecturer at LMU, and the lead scientist for eROSITA’s clusters and cosmology team who delivered the groundbreaking 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.”

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 at present, 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 amount of matter, eROSITA has also measured its clumpiness, 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 value of S8 than, e.g. weak gravitational lensing surveys. New physics is implied unless this tension can be resolved. But eROSITA, with the leading measurements of its kind, does not confirm that this puzzle is real. “eROSITA tells us that the Universe behaved as expected throughout cosmic history,” says Dr. Vittorio Ghirardini, the postdoctoral researcher at MPE who led the cosmology study. “There's no tension with the CMB - maybe the cosmologists can relax a bit now.”

Understanding the nature of the universe

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 objects 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. The current analysis has already ruled out a set of extensions of Einstein’s Theory of General Relativity. “But more is to come,” says Dr. Emmanuel Artis, a postdoctoral researcher at MPE. “If we find any hints, eROSITA may pave the way for new exciting theories beyond general relativity.”

The eROSITA dataset is one of many that the scientific community will get access to over the next few years. In Europe, scientists are particularly excited about the Euclid satellite, which delivered first data a few months ago. Prof. Joe Mohr, chair of the structure formation and cosmology group at LMU and eROSITA senior scientist between 2009 and 2020, describes the bigger picture: "This new eROSITA result underscores the power of combining modern weak gravitational lensing data like that from the Dark Energy Survey with X-ray selected galaxy cluster samples. I find it particularly interesting, because the eROSITA constraints are somewhat different from those of most other recent cosmological analyses. We look forward to studying this issue further with the eROSITA cluster sample in combination with the weak gravitational lensing data from the Euclid mission and the Rubin Observatory. An independent analysis using these improved weak lensing data will shed light on whether these differences in cosmological constraints are significant."