Cosmology

A New, More Accurate Measurement for the Clumpiness of the Universe

Cosmologists are wrestling with an interesting question: how much clumpiness does the Universe have? There are competing but not compatible measurements of cosmic clumpiness and that introduces a “tension” between the differing measurements. It involves the amount and distribution of matter in the Universe. However, dark energy and neutrinos are also in the mix. Now, results from a recent large X-ray survey of galaxy clusters may help “ease the tension”.

The eROSITA X-ray instrument orbiting beyond Earth performed an extensive sky survey of galaxy clusters to measure matter distribution (clumpiness) in the Universe. Scientists at the Max Planck Institute for Extraterrestrial Physics recently shared their analysis of its cosmologically important data.

“eROSITA has now brought cluster evolution measurement as a tool for precision cosmology to the next level,” said Dr. Esra Bulbul (MPE), the lead scientist for eROSITA’s clusters and cosmology team. “The cosmological parameters that we measure from galaxy clusters are consistent with state-of-the-art cosmic microwave background, showing that the same cosmological model holds from soon after the Big Bang to today.”

eROSITA, the Standard Cosmological Model, and Clumpiness

To get a better feel for what this means, let’s look at what the team is trying to confirm. The idea is to figure out just what the Universe has been like through time. That means understanding matter, its distribution (or clumpiness), and what role dark matter and dark energy have played. It all began just after the Big Bang when the Universe was in a hot, dense state. The only things existing were photons and particles. The Universe expanded and began to condense into regions of higher density. Think of these as density variations, or areas of more or less clumpiness in the primordial soup. As things cooled and expanded, the denser clumps in the soup became galaxies and eventually galaxy clusters. The clumpiness was smoother (or “isotropic”) than expected. That raises questions about the role of dark matter and dark energy, among other things.

eROSITA’s observations of galaxy clusters and distribution of matter showed several interesting results. First, both dark matter and visible matter (baryonic matter), make up about 29 percent of the total energy density of the Universe. Presumably, the rest consists of dark energy, which we don’t know much about, yet. Energy density is the amount of energy stored in a region of space as a function of volume. In cosmology, it also includes any mass in that volume of space.

This plot shows the constraints put on the total matter density in the Universe and the S8 “tension”. Constraints from eROSITA galaxy clusters are in orange, from the Cosmic Microwave Background (Planck) in blue, from weak lensing (DES+KiDS) in grey, and from cluster number counts (SPT) in black. Credit: MPE, V. Ghirardini for the eROSITA consortium

The measurement of energy density agrees with measurements of the cosmic microwave background radiation—also known as the CMB. Think of that as a map of the density variations in the early Universe. It’s made up of microwave radiation that permeates the Universe. That radiation is not completely smooth or uniform. That’s the variability in density that eventually became the seeds of the first galaxies.

Measuring Clumpiness

eROSITA’s goal is to measure the assembly of galaxy clusters over cosmic time. By tracing their evolution via the X-rays emitted by hot gas, the instrument traced both the total amount of matter in the Universe and its clumpiness. Those measurements solve the “tension” or discrepancy between past clumpiness measurements that used different techniques. Those included the CMB and observations of weak gravitational lensing.

A computer simulation of what gas and stars in a galaxy cluster look like, and how they look embedded in the cosmic web. The assembly of galaxy clusters has implications for the clumpiness of the Universe throughout time. Credit: Yannick Bahé.

The eROSITA data shows the distribution of matter is actually in good agreement with previous measurements of the CMB. That’s good news because cosmologists were afraid they’d have to invoke “new physics” to explain the tension between measurements. “eROSITA tells us that the Universe behaved as expected throughout cosmic history,” says Dr. Vittorio Ghirardini, the postdoctoral researcher at MPE who led cosmology study. “There’s no tension with the CMB. Maybe the cosmologists can relax a bit now.”

But Wait, There are Neutrinos to Worry About!

Interestingly, the eROSITA measurements of galaxy clusters and other large structures also provide information about neutrinos. They’re the most abundant particles with mass that we know of in the Universe. They come from the Sun and supernovae (for example), but also originated in the Big Bang. eROSITA’s survey offers new information about the mass of neutrinos and their prevalence. “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.

Computer simulations show how neutrinos can form cosmic clumpiness. Credit: Yoshikawa, Kohji, et al

Neutrinos may be small and tough to “see”, but they have mass that contributes to the total density of matter in the Universe. Cosmologists describe them as “hot”, which means they travel at nearly the speed of light. Therefore, they tend to smooth out the distribution of matter—which can be probed by analyzing the evolution of galaxy clusters in the Universe. And, there’s a good chance that eROSITA may help solve the mystery of neutrino mass. “We are even on the brink of a breakthrough to measure the total mass of neutrinos when combined with ground-based neutrino experiments,” added Ghirardini.

How eROSITA Did It

There’s a lot more to explore in the eROSITA data, but it’s also fascinating to look at the extent of the survey data. It comprises one of the most extensive catalogs of clusters of galaxies done so far. The so-called “Western Galactic half” of the all-sky survey contains 12,247 optically identified X-ray galaxy clusters. “Of these, 8,361 represent new discoveries – almost 70%,” said Matthias Kluge, a postdoctoral researcher at MPE who is responsible for the optical identification of the detected clusters. “This shows the huge discovery potential of eROSITA.”

All that data can be charted in three dimensions, and when scientists do that, galaxy clusters show up as intersections of the cosmic web. In addition, there’s a supercluster catalog, which also shows connected clusters and the filaments of matter between them. “We find more than 1,300 supercluster systems, which makes this the largest-ever X-ray supercluster sample,” said Ang Liu, a postdoctoral researcher at MPE.

This new look at clumpiness in the Universe comes from the first release of data from eROSITA. The instrument completed additional surveys in early 2022. Once those data are analyzed, astronomers expect to be probing even deeper into the distribution of matter in the Universe and testing their models against reality. “When the full data are analyzed,” said Esra Bulbul, “eROSITA will again put our cosmological models to the most stringent test ever conducted through a cluster survey.”

For More Information

eROSITA Relaxes Cosmological Tension
The SRG/eROSITA All-Sky Survey: Cosmology Constraints from Cluster Abundances in the Western Galactic Hemisphere

About eROSITA

Carolyn Collins Petersen

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