Fresh X-Rays Reveal a Universe as Clumpy as Cosmology Predicts
Introduction
Clusters of hundreds or thousands of galaxies sit at the intersections of giant, crisscrossing filaments of matter that form the tapestry of the cosmos. As gravity pulls everything in each galaxy cluster toward its center, the gas that fills the space between the galaxies gets compressed, causing it to heat up and glow in X-rays.
The eRosita X-ray telescope, lofted into space in 2019, spent more than two years collecting pings of high-energy light from all over the sky. The data has allowed scientists to map the locations and sizes of thousands of galaxy clusters, two-thirds of them previously unknown. In a slew of papers posted online on February 14 that will appear in the journal Astronomy & Astrophysics, the scientists used their initial catalog of clusters to weigh in on several of cosmology’s big questions.
The results include new estimates of the clumpiness of the cosmos — a much-discussed characteristic of late, as other recent measurements have found it to be unexpectedly smooth — and of the masses of ghostlike particles called neutrinos and of a key property of dark energy, the mysterious repulsive energy that’s speeding up the universe’s expansion.
Cosmologists’ reigning model of the universe identifies dark energy as the energy of space itself and pegs it at 70% of the universe’s contents. A further one-quarter of the universe is invisible dark matter, and 5% is ordinary matter and radiation. All of it is evolving under the force of gravity. But some observations from the past decade defy this “standard model” of cosmology, raising the possibility that the model is missing ingredients or effects that could usher in a deeper understanding.
The eRosita observations, by contrast, bolster the existing picture on all counts. “It’s a remarkable confirmation of the standard model,” said Dragan Huterer, a cosmologist at the University of Michigan who was not involved in the work.
X-Raying the Cosmos
After the Big Bang, subtle density variations in the newborn universe gradually became more pronounced as matter particles glommed onto each other. The denser clumps pulled in more material and grew larger. Today, galaxy clusters are the largest gravitationally bound structures in the cosmos. Determining their sizes and distribution lets cosmologists test their model of how the universe evolved.
To find clusters, the eRosita team trained a computer algorithm to scour for “really fluffy” X-ray sources as opposed to pointlike objects, said Esra Bulbul of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led eRosita’s cluster observations. They whittled a list of candidates down to an “extremely pure sample,” she said, of 5,259 galaxy clusters, out of the nearly 1 million sources of X-rays the telescope detected.
They then had to figure out how heavy these clusters are. Massive objects bend the fabric of space-time, changing the direction of passing light and making the source of the light appear distorted — a phenomenon called gravitational lensing. The eRosita scientists could calculate the masses of some of their 5,259 clusters based on the lensing of more distant galaxies sitting behind them. While only a third of their clusters had known background galaxies lined up in this way, the scientists found that the cluster mass correlated strongly with the brightness of their X-rays. Because of this strong correlation, they could use brightness to estimate the masses of the remaining clusters.
They then fed the mass information into computer simulations of the evolving cosmos to infer the values of cosmic parameters.
Gauging Clumpiness
One number of interest is the “clumpiness factor” of the universe, S8. An S8 value of zero would represent a vast cosmic nothingness, akin to a flat plain with nary a rock in sight. An S8 value closer to 1 corresponds to steep mountains looming over deep valleys. Scientists have estimated S8 based on measurements of the cosmic microwave background (CMB) — ancient light coming from the early universe. Extrapolating from the cosmos’s initial density variations, researchers expect the current S8 value to be 0.83.
But recent studies looking at galaxies today have measured values 8% to 10% lower, implying that the universe is unexpectedly smooth. That discrepancy has intrigued cosmologists, potentially pointing to cracks in the standard cosmological model.
The eRosita catalog of galaxy clusters is plotted here on a map of the half-sky. The colors indicate the distance of the clusters and the sizes of the circles indicate the apparent X-ray brightness of each source.
MPE, J. Sanders for the eROSITA Consortium
The eRosita team, however, found no such discrepancy. “Our result was basically in line with the prediction from the very early time, from the CMB,” said Vittorio Ghirardini, who led the analysis. He and his colleagues calculated an S8 of 0.85.
Some team members were disappointed, Ghirardini said, since hinting at missing ingredients was a more exciting prospect than matching the known theory.
The S8 value sitting a tad higher than the CMB estimate will likely trigger more analysis from other teams, said Gerrit Schellenberger, an astrophysicist who studies galaxy clusters at the Harvard-Smithsonian Center for Astrophysics. “I believe it’s probably not the last paper we have seen on that topic.”
Weighing Neutrinos
Copious neutrinos formed in the early universe — nearly as many as photons (particles of light), said Marilena Loverde, a cosmologist at the University of Washington. But physicists know that neutrinos, unlike photons, must have tiny masses because of how they oscillate between three types. The particles don’t acquire mass through the same mechanism as other elementary particles, so their mass is a much-studied mystery. And the first question is how massive they actually are.
Cosmologists can estimate the mass of neutrinos by studying their effects on the structure of the cosmos. Neutrinos zip around at nearly the speed of light and pass right through other matter rather than glomming onto it. So their presence in the cosmos has attenuated its clumpiness. “The more mass you put on neutrinos, the more of the mass that is smooth on those [large] scales,” Loverde said.
Combining their galaxy cluster measurements with CMB measurements, the eRosita team estimated that the sum of the masses of the three types of neutrinos is no more than 0.11 electron volts (eV), or less than a millionth of the mass of an electron. Other neutrino experiments have established a lower bound, showing that the three neutrino masses must add up to at least 0.06 eV (for one possible ordering of the three mass values) or 0.1 eV (for the inverted order). As the distance shrinks between the upper and lower bounds, scientists are getting closer to pinpointing the value of the neutrino mass. “We are actually at the brink of making a breakthrough,” Bulbul said. In subsequent data releases, the eRosita team could push down the upper bound enough to rule out the inverted-order neutrino mass models.
Caution is warranted. Any other speedy, lightweight particles that might exist — such as axions, hypothetical particles proposed as candidates for dark matter — would have the same effects on structure formation. And they would introduce errors into the neutrino mass measurement.
Tracking Dark Energy
Galaxy cluster measurements can reveal not just how structures grew, but also how their growth was impeded by dark energy — the thin glaze of repulsive energy that permeates space, accelerating space’s expansion and thereby separating matter.
If dark energy is the energy of space itself, as the standard model of cosmology assumes, then it will have a constant density throughout space and time (that’s why it’s sometimes referred to as the cosmological constant). But if its density is instead dropping over time, then it’s something else entirely. “That’s the biggest question that cosmology has,” said Sebastian Grandis, an eRosita team member at the University of Innsbruck in Austria.
From their map of thousands of clusters, the researchers found that dark energy matches the profile of a cosmological constant, although their measurement has a 10% uncertainty, so an ever-so-slightly varying dark energy density remains possible.
Originally, eRosita, which sits aboard a Russian spacecraft, was to conduct eight full-sky surveys, but in February 2022, weeks after the telescope began its fifth survey, Russia invaded Ukraine. In response, the German side of the collaboration, which operates and runs eRosita, put the telescope into safe mode, ceasing all scientific observations.
These initial papers draw from just the first six months of data. The German group expects to find about four times as many galaxy clusters in the additional 1.5 years of observations, which will allow all these cosmological parameters to be pinpointed with more accuracy. “Cluster cosmology could be the most sensitive probe of cosmology other than the CMB,” said Anja von der Linden, an astrophysicist at Stony Brook University.
Their initial results demonstrate the power of a relatively untapped information source. “We’re kind of the new kid on the block,” Grandis said.
