A new analysis of how galaxy clusters have evolved over the 13.8 billion year history of the cosmos could help resolve a long-standing tension around the “lumpiness” of our universe’s matter content. In the future, it could also help scientists solve a host of other cosmic mysteries.
The first data from the eROSITA all-sky survey of cosmic “
These findings could help resolve a discrepancy between theoretical predictions of the Standard Model of cosmology and observations of a cosmic fossil born just after the Big Bang called the cosmic microwave background (CMB). The two currently disagree about the lumpiness of matter in the universe.
This disparity is known as the S8 voltage, where S8 is the parameter scientists use to quantify the amplitude of matter fluctuations on the scale of about 26 million light-years. In other words, that the ‘lumpiness’ of the cosmos is taking place on a large scale.
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While the S8 strain may not be as prominent a problem for cosmology as the “Hubble strain,” which describes a disparity scientists see in calculations of the universe’s expansion rate, it still represents an approaching storm. It has even been suggested that we may have to find entirely new physics to solve the riddle. However, the new eROSITA data offers hope that the S8 strain can be alleviated without such drastic measures.
“eROSITA has now established the measurement of cluster evolution as a tool for precision cosmology,” Esra Bulbul, the lead scientist of eROSITA’s clusters and cosmology team, said in a statement. “The cosmological parameters we measure on galaxy clusters are consistent with state-of-the-art CMB, showing that the same cosmological model holds from shortly after the Big Bang until today.”
Solving an impending cosmic crisis with eROSITA
The Standard Model of cosmology, or the ‘Lambda Cold Dark Matter (ΛCDM) model’ suggests that immediately after the Big Bang, the universe was a hot and dense sea of photons, or light particles, and free electrons and protons.
These electrons are believed to have endlessly scattered photons at this point, meaning the universe would have been essentially opaque. That was until about 400,000 years later, when the universe had expanded and cooled enough for electrons and protons to come close enough to bond and create the first hydrogen atoms.
During this era of reionization, photons were suddenly allowed to travel and the universe became transparent to light. This ‘first light’ now fills the universe almost perfectly uniformly and is known as the CMB, or the ‘surface of final scattering’. And because this light existed before the first stars and galaxies, the CMB is an excellent tool for tracking how the cosmos has evolved.
As cosmic time progressed, the first atoms clumped together to form the first gas clouds, and then the first stars, which gathered into galaxies that nested in the first galactic clusters, eventually leading to some of the largest structures in the universe. known universe.
Observations of these clusters by eROSITA, the main instrument aboard the Russian-German Spectrum-Roentgen-Gamma (SRG) spacecraft, show that visible matter and dark matter make up 29% of the universe’s total energy density, which is consistent with measurements of the universe. CMB.
While observing galactic clusters, eROSITA has also been able to measure the clumpiness of matter using the S8 parameter. Although previous CMB experiments have suggested a higher value for S8 than the Standard Model predicts, the eROSITA observations of this cosmic fossil are in better agreement with those 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 – perhaps the cosmologists can relax a bit now.”
Cosmic ghost hunt
eROSITA’s observations of the galactic clusters have 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. Not only does this make neutrinos notoriously difficult to detect, but it has also earned them the nickname “ghost particles.”
The small mass of these particles also allows them to race through the cosmos at speeds approaching that of light, with astronomers describing them as “hot” for this fact. Temperature is essentially a measure of how fast particles are moving. This means that neutrinos can smooth out the distribution of matter in the universe, and this action can be measured by examining the evolution of the largest cosmic structures known.
Combining eROSITA measurements of galactic clusters with observations from the CMB thus provided the most refined measurements yet of the total neutrino mass value achieved by a cosmological probe.
“It may sound paradoxical, but we have imposed severe constraints on the masses of the lightest known particles by the abundance of the largest dark matter haloes in the universe,” Ghirardini said. “In fact, we are on the verge of a breakthrough in measuring the total mass of neutrinos in combination with ground-based neutrino experiments.”
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eROSITA’s insights into the universe don’t end there; data from the instrument should also be able to reveal the growth rate of the largest structures in the universe, something predicted by Einstein’s 1915 theory of gravity: general relativity.
An early analysis of 12,247 optically identified galaxy clusters observed by eROSITA appears to show that this growth rate is slightly slower at later cosmological times than general relativity predicts.
“We may be on the verge of 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 new exciting theories that go beyond general relativity.”