by A new analysis of how galaxy clusters developed over the 13.8 billion year history of the cosmos could help resolve a long-standing tension about the ‘lumpiness’ of matter in our universe. In this way, it could help scientists solve many other cosmic mysteries.
The first data from the eROSITA all-sky survey of Cosmic X-ray sources, which completed 4.5 full-sky surveys in February 2022, contained precise measurements of the total amount of matter in the universe and the matter’s level of smoothness, or “homogeneity.” “
These results could help resolve the discrepancy between the theoretical predictions of the standard model of cosmology and observations of a cosmic fossil born just after the Big Bang known as the cosmic microwave background (CMB). The two disagree, at the moment, about how lumpy the universe’s matter is.
This difference is called the S8 tension, and S8 is the parameter scientists use to quantify the amplitude of material fluctuations on a scale of about 26 million light years. In other words, that is the lumpiness of the cosmos on a large scale.
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While the S8 tension may not be as prominent a problem for cosmology as the “Hubble Tension,” which describes the difference scientists see when calculating the rate of expansion of the universe, it still represents a brewing storm. It has even been suggested that we may have to invent a whole new physics to solve the problem. However, the new eROSITA data gives hope that the S8 tension can be eased without such drastic measures.
“eROSITA has now established the measurement of cluster evolution as a tool for precision cosmology,” Esra Bulbul, lead scientist for the eROSITA cluster and cosmology team, said in a statement. “The cosmological parameters we measure from galaxy clusters are consistent with a state-of-the-art CMB, indicating that the same cosmological model has existed from shortly after the Big Bang to the present day.”
Solving the brewing cosmic crisis with eROSITA
The standard model of cosmology, or the “Lambda Cold Dark Matter (ΛCDM)) model, suggests that the universe immediately after the Big Bang was a hot and dense sea of photons, or light particles, and free electrons and protons.
Those electrons are now believed to have infinitely scattered photons, meaning the universe would be essentially opaque. That was until about 400,000 years later, when the universe had expanded and cooled enough to allow electrons and protons to get close enough together to bond and form the first hydrogen atoms.
During this period of reionization, photons were suddenly allowed to travel, and the universe became transparent. This “first light” now fills the universe almost uniformly and is called the CMB, or “surface of past scattering.” And because this light has been around since before the first stars and galaxies, the CMB is a great tool to trace how the cosmos developed.
As cosmic time progressed, the first atoms coalesced to form the first gas clouds, then the first stars, which gathered into galaxies which themselves nested in the first Galactic clusters, which resulted in some of the largest structures in the universe being known at last.
Observations of these clusters by eROSITA , the main instrument on board the Russian-German Spectra-Roentgen-Gamma (SRG) spacecraft, show that visible matter and dark matter make up 29% of the total energy density of the universe, consistent with measurements of the universe. CMB.
When observing Galactic clusters, eROSITA was able to measure the lumpiness of the material using the S8 parameter. Although previous CMB experiments suggested a higher value for S8 than predicted by the standard model, eROSITA observations of this cosmic fossil are more in line with these theoretical predictions.
“eROSITA tells us that the universe has behaved as expected throughout cosmic history,” Vittorio Ghirardini, research leader and postdoctoral researcher at the Max Planck Institute for Extraterrestrial Physics, said in the statement. “There is no tension with the CMB – maybe the cosmologists can relax a bit now.”
Cosmic ghost hunt
eROSITA’s observations of the Galactic clusters also helped scientists learn more about tiny particles called neutrinos, which have so little mass and charge that they essentially travel under the radar. In fact, 100 trillion of them pass through our bodies every second, unnoticed. This not only makes neutrinos very difficult to detect, but has earned them the nickname “ghost particles”.
These tiny particles enable them to race through the cosmos at speeds approaching the speed of light, and astronomers describe them as “hot” because of this. Temperature is basically a measure of how fast particles are moving. This means that neutrinos can smooth the distribution of matter in the universe, and this action can be measured by investigating the evolution of the most known cosmic structures.
Thus, combining eROSITA measurements of Galactic clusters with observations of the CMB provided the most refined measurements yet of the total neutrino mass achieved by a cosmological probe.
“It may seem paradoxical, but there are tight constraints on the mass of the lightest particles known from the abundance of the largest dark matter halo in the universe,” said Ghirardini. “We are even on the verge of measuring the total mass of neutrinos when added to ground-based neutrino experiments.”
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eROSITA’s global insights don’t end there; data from the instrument should also be able to reveal the growth rate of the largest structures in the universe, which is predicted by Einstein’s 1915 theory of gravity: General relativity.
An early analysis of 12,247 optically identified galaxy clusters seen by eROSITA appears to show that this growth rate is somewhat slower in recent cosmological times than predicted by general relativity.
“We may be approaching a new discovery,” Emmanuel Artis, a postdoctoral researcher at the Max Planck Institute for Extraterrestrial Physics, said in the statement. “If it can be confirmed, eROSITA will pave the way for exciting new theories beyond general relativity.”
