Ghosts haunt our galaxy, passing right through not only walls but entire planets and even heftier objects with ease. Trillions upon trillions are whispering their way through your body even now, as you read this. These subatomic specters, called neutrinos, are harbingers of fiery cosmic processes, such as the nuclear fusion powering our sun and the titanic supernova explosions that herald the deaths of more massive stars.
Neutrinos aren’t mere cosmic messengers, however; they also help shape the universe’s evolution by influencing how structures such as galaxies formed long ago. Neutrinos move so swiftly that they can travel great distances before being captured by the gravitational pull of a dense region. That helped the particles smooth out tiny knots where matter was beginning to clump together, making it harder for bigger structures to form.
Learning more about neutrinos offers a window into the early universe, but they’re extremely difficult to study. They hardly ever interact with normal matter, so it takes state-of-the-art technology to detect them.
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Some fundamentals of the neutrino—such as its mass, as well as the particle’s cosmic abundance—can have surprisingly outsized effects on the number, size and distribution of galaxies and galaxy clusters. By looking at those cosmic megastructures, scientists can sleuth out information about neutrinos indirectly. They can essentially do thought experiments to rewind creation by giving neutrinos different characteristics and playing a virtual universe back to see if its emerging megastructures match those of reality.
Scientists recently used the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) to do this on a particularly grand scale. eROSITA is a German-built x-ray telescope on board the orbiting Russian observatory Spectrum-Roentgen-Gamma (SRG), which launched in July 2019. Using data from the first six months of eROSITA’s operations—which constitutes the most detailed celestial map of x-ray sources yet made—the scientists found more than 12,000 galaxy clusters across one half of the sky, tracing each one by the telltale x-ray glow from surrounding halos of rarefied, 100-million-degree gas. They posted their results last month on the preprint server arXiv.org.
Combined with what we already know about neutrinos, the researchers’ analysis of a subset of galaxy clusters from eROSITA’s data allowed them to fine-tune models that describe the particles and better estimate neutrino masses. They present a new upper limit on how heavy neutrinos could be. The previous mass range was between 0.06 and 0.74 electron volts (a single electron volt is more than a trillion trillion times less massive than a single grain of sugar). The eROSITA results constrain it much further, suggesting an upper bound of 0.22 eV, or 0.11 eV when combined with other data.
“We’ve been wondering about the neutrino’s mass since these particles were first conjectured in the 1930s,” says Joseph Formaggio, a professor of physics at the Massachusetts Institute of Technology, who was not involved with eROSITA. “The cosmology limits on the neutrino mass are the most advanced at the moment. Getting precision measurements across the entire arc of the universe is unique and very compelling.”
Formaggio notes that the neutrino’s mass is particularly interesting because neutrinos are so different from other particles. “We think they get their mass differently from all the other particles,” he says. “That could hint there’s new physics at play.”
A Clumpy Cosmos
And “new physics” is exactly what some researchers are ardently seeking when they poke and prod around the hazy edges of the standard model of cosmology. This framework uses just a handful of parameters to very successfully explain the origin and evolution of the universe. Its simplicity makes it elegant, but many scientists suspect it needs significant tweaks because of escalating tensions between its predictions and actual observations. eROSITA’s unprecedented data offer a wealth of new, independent observations to test against and may ease some of those tensions.
One inconsistency eROSITA explores has to do with how clumpy the universe is. To explain the problem, we’ll have to journey back more than 13.7 billion years, nearly to the birth of the cosmos.
The early universe was filled with a hot, dense and opaque fog of ionized particles. But by about 380,000 years after the big bang, the expanding universe had sufficiently cooled for atoms to form, clearing the fog and allowing light to travel freely. Modern observatories see that ancient light today as an all-sky microwave glow called the cosmic microwave background (CMB), and missions such as the Planck spacecraft from the European Space Agency (ESA) have scrutinized its properties with astounding precision. Many of Planck’s studies focused on tiny temperature fluctuations in the CMB, which scientists can connect with the emergence of larger, later cosmic structures.
Those little fluctuations show that the early universe wasn’t entirely homogenous; there were tiny variations in density (like chocolate chips in cookie dough except far more subtle). Areas that were ever-so-slightly lumpier formed the seeds for eventual galaxies. Studying the CMB allows scientists to better understand the initial conditions of the universe. Then researchers play a grand game of connect-the-dots to figure out how the cosmos produced the grand assemblages of galaxies we see today.
Until recently, there’s been one big problem. In theory, one should be able to pair Planck’s CMB measurements with the standard cosmological model to calculate, from first principles, the clumping of galaxy clusters other telescopes can see and study across the past several billion years of cosmic history. Yet observations from several independent instruments keep coming up with results that are inconsistent with CMB-based extrapolations, which could indicate that the standard model may not be quite right.
But eROSITA’s galaxy-clustering results mark what could be a sea change in this trend: they are in harmony with extrapolations from Planck’s CMB measurements and therefore support the standard model. “We’re not seeing this tension at all,” says Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and lead scientist for eROSITA cluster science and cosmology. “That opens up the question of why nearly all of the other ‘late universe’ probes do see a discrepancy. What is going on?”
Vittorio Ghirardini, a postdoctoral researcher at MPE and the lead author of the recent preprint on eROSITA’s galaxy-clustering results, suggests the telescope offers better measurements than those from other observatories because of its wavelength range. “Optical and infrared telescopes can help identify galaxy clusters,” he says, albeit such telescopes can struggle to distinguish between a cluster’s true members versus objects in the far-distant foreground or background. X-ray measurements are better for such work, he says, because they offer higher contrast between, for instance, the bright x-ray emissions associated with galaxies in a cluster and those far weaker from galaxies in the remote background.
The telescope uses x-rays to trace vast clouds of hot, dilute gas that suffuse and surround a cluster’s galaxies. The extent of this so-called intracluster medium also helps outline halos of dark matter that wreathe and bind together galaxy clusters like invisible gravitational glue. “Scanning the x-ray sky gives us an accurate and efficient way to find galaxy clusters and their dark matter halos,” Bulbul says. “Ground-based optical telescopes can find them, but as an all-sky survey, eROSITA isn’t limited to certain parts of the sky. We can compile huge, pure samples.”
Using those samples, scientists measure the universe’s clumpiness by determining the abundance of normal and dark matter, as well as the prevalence of galaxy clusters and the great filaments, sheets and voids formed by their distribution. “With eROSITA we find the largest dark matter halos in the universe, and by counting them and measuring their mass, what we find is consistent with Planck,” Bulbul says. “So we are a rare late-universe probe confirming Planck.”
But some scientists aren’t convinced. “I think it’s unclear whether this data really removes the tension,” says Yun Wang, a senior research scientist at the Infrared Processing and Analysis Center at the California Institute of Technology, who was not involved with eROSITA. She points out that eROSITA’s joint constraints on matter clumpiness and density only marginally agree with those from Planck. “But to me, that only makes it even more exciting. Here we have the biggest, highest quality set of galaxy cluster data, and the results could be interpreted in multiple ways. Perhaps analyzing additional eROSITA data will offer clues.”
Additional clues could come via fresh results from other missions, such as the forthcoming cosmological measurements from ESA’s Euclid Space Telescope, which launched in July 2023 on a mission to map the size, shape and distribution of galaxies across the past 10 billion years or so of cosmic history.
A Dark Mystery
The eROSITA team says its findings also support the standard model of cosmology in another way.
A wealth of data decisively points to some mysterious force somehow accelerating the universe’s expansion. Although no one yet knows what it is, scientists refer to the culprit as dark energy. One of the ways to study it is by trying to determine its equation of state, which essentially describes how its expansion-accelerating pressure depends on its density.
That’s usually a straightforward relationship; if you compress normal matter, its pressure increases. But dark energy must possess negative pressure to account for its repulsive effect that drives the acceleration of cosmic expansion. “Its equation of state offers us an essential clue to what dark energy might be,” Wang says.
To figure out its equation of state, scientists are exploring whether dark energy’s density weakens as the universe expands—or if, for that matter, its repulsive effect becomes even stronger. The standard model splits the difference by suggesting that its density should remain constant. There is no known physics, however, that can explain such a constant density, which must be vanishingly tiny. “We have to push hard to find out if dark energy’s density is constant or not,” Wang says. “This would have fundamental implications for particle physics one way or another.”
“We can constrain dark energy’s equation of state because seeing how many dark halos there are in the universe as a function of distance tells us the universe’s expansion rate,” Ghirardini says. The standard model says dark energy’s equation of state should be –1. The eROSITA-derived estimate is −1.12 ± 0.12––in keeping with the standard model. “So this is something really spectacular that’s coming from these measurements,” he says.
Science, Interrupted
All these results stem from just a half year’s worth of observations––time in which eROSITA, which slowly scans the heavens as it spins, was able to complete one pass across the entire sky. That this latest result only draws upon half of those data is because of a messy mix of the mission’s origins and geopolitics.
As a joint German-Russian project, exclusive access to eROSITA’s measurements is divided between those two nations, with data from the southern half of the sky allotted to a group based in Germany and a different group in Russia getting the northern half. eROSITA began its observations shortly after its 2019 launch, and it completed four all-sky surveys before its scientific partnership was disrupted by Russia’s invasion of Ukraine. Soon afterward the German government put this collaboration with Russia on ice, and eROSITA was placed in safe mode. It remains functional but is not currently making new observations. The status of the Russian group’s work with its share of the eROSITA data is uncertain; requests to the group for comment were not acknowledged by the time this story went to press.
There’s no way to know if or when eROSITA will begin surveying the sky again, but scientists still have work to do in the meantime because the bulk of the mission’s data remain unpublished. For Bulbul, Ghirardini and their colleagues in the Germany-based group, the plan is to spend the next two years or so compiling their half of the data from the other three surveys to yield even tighter cosmological constraints.
“Understanding the dark energy equation of state and figuring out the neutrino’s mass has the potential to revolutionize physics,” Ghirardini says. “eROSITA has breakthrough capabilities in cosmology research.”
For now we’ll continue to hover on the brink of potentially transcendent cosmology results, with new observations thwarted by dismal world affairs. We humans, it seems, are still deciding whether to deepen our cosmic understanding by mapping enigmatic patterns in the heavens above—or to continue our conflicts and divisions over imaginary lines drawn on maps of Earth.