The notes point to a missing element in the cosmic recipe

The Hubble Space Telescope image shows the MACS J1206 massive galaxy cluster. Embedded within the cluster are the distorted images of distant background galaxies, which are seen as stained arcs and features. These distortions are caused by the amount of dark matter in the cluster, whose gravity bends and amplifies light from distant galaxies. This effect, called a gravitational lens, allows astronomers to study distant galaxies that otherwise would be too faint to be seen. Many cluster galaxies are massive and dense enough to distort and amplify distant sources. Galaxies in the three clouds are examples of these effects. In the top right and bottom snapshots, there are two distant blue galaxies with a cluster of red galaxies forming rings and multiple images of distant objects. The red dots around the galaxy in the upper left indicate an emission from hydrogen clouds at one distant source. The source, seen four times due to the lens, might be a faint galaxy. These points were detected by the Multi-Unit Spectral Explorer (MUSE) at the Very Large European Southern Observatory (VLT) in Chile. Dots do not appear in Hubble images. MACS J1206 is part of the Cluster Lensing and Supernova survey with Hubble (CLASH) and is one of three galactic groups that researchers studied with Hubble and VLT. Hubble image is a combination of visual and infrared observations captured in 2011 by the Advanced Surveys Camera and Wide Field Camera 3.
Credit: NASA, European Space Agency, B. Natarajan (Yale University), c. Kamenha (University of Groningen), M. Minghetti (INAF Observatory of Astrophysics and Space Sciences in Bologna), CLASH-VLT / Zooming teams; Endorsement: NASA, ESA, M. Postman (STScI), and the CLASH Team

Astronomers have discovered that there may be an element missing in our cosmic description of how dark matter behaves.

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Astronomers have uncovered a discrepancy between theoretical models of how dark matter is distributed in galaxy clusters, and observations of dark matter’s dominance over clusters.

Dark matter does not emit, absorb or reflect light. Its existence is only known by its attractiveness to the visible matter in space. So, dark matter remains as elusive as the Cheshire Cat in Alice in Wonderland – seeing only her smile (in the form of gravity) but not the animal itself.

One way that astronomers can detect dark matter is by measuring how its gravity distorts space, an effect called a gravitational lens.

The researchers found that small concentrations of dark matter in the clusters produced gravitational lens effects ten times stronger than expected. This evidence is based on unprecedented detailed observations of many massive galaxy clusters NASA‘s Hubble Space Telescope And the European Southern Observatory Very large telescope (VLT) in Chile.


Astronomers appear to have uncovered bewildering details of the way dark matter behaves. They found small, dense concentrations of dark matter bend and amplified the light with a much greater force than expected. Credit: NASA’s Goddard Space Flight Center

Galaxy clusters, the largest structures in the universe made up of individual galaxies, are the largest reservoirs of dark matter. Not only are they closely related to each other by the gravitational pull of dark matter, but individual cluster galaxies themselves are filled with dark matter. Therefore, dark matter is distributed in groups of both large and small scales.

“Galactic clusters are ideal laboratories for understanding whether computer simulations of the universe reproduce what we can infer about dark matter and its interaction with luminous matter,” said Massimo Minghetti of the National Institute of Astrophysics – Observatory of Astrophysics and Space Sciences. Bologna in Italy, lead author of the study.

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“We did a lot of rigorous tests comparing the simulations and the data in this study, and we continued to discover the mismatch,” Minghetti continued. “One possible reason for this discrepancy is that we might miss some basic physics in the simulations.”

Priamvada Natarajan Yale University In New Haven, Connecticut, one of the senior theorists on the team added, “There is a feature of the real universe that we simply don’t capture in our current theoretical models. This may indicate a gap in our current understanding of the nature and properties of dark matter, as this impressive data allowed us to probe the detailed distribution of matter. Dark on the smallest scales. “

The team paper will appear in the 9/11 issue of the magazine Science.

The distribution of dark matter in groups is mapped by the bending of light, or the effect of the lens of gravity that produces it. Just like a fun mirror, the dark matter’s gravity amplifies and distorts light from distant background objects, producing distortions and sometimes multiple images of the same distant galaxy. The higher the concentration of dark matter in the cluster, the more dramatic the bending of the light.

The clear Hubble images, along with spectra from the VLT, helped the team produce an accurate, high-resolution map of dark matter. They have identified dozens of background galaxies, both imaged and lensed. By measuring lens distortions, astronomers can track the amount and distribution of dark matter.

The three main galaxy clusters used in the analysis, MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063, were part of two Hubble scans: boundary fields and cluster lens scanning and supernova using Hubble (Clash) software.

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To the surprise of the team, the Hubble images also revealed smaller arcs and distorted images intertwined within the wide-range lens distortions at the core of each group, where the largest of the largest galaxies are.

Researchers believe that the embedded lenses result from the gravitational pull of the dense concentrations of dark matter associated with individual cluster galaxies. The distribution of dark matter in the interior of individual galaxies is known to enhance the cluster’s overall lens effect.

Follow-up of the spectral observations that have been added to the study by measuring the speed of the stars that are orbiting within several clusters. “Based on our spectroscopy, we were able to connect galaxies to each cluster and estimate their distances,” said team member Piero Rosati from the University of Ferrara in Italy.

“The speed of the stars gave us an estimate of the mass of each individual galaxy, including the amount of dark matter,” said team member Pietro Bergamini of the INAF Observatory for Astrophysics and Space Sciences in Bologna, Italy.

The team compared dark matter maps to samples from simulated galaxies of similar masses, located at roughly the same distances as the observed clusters. The clusters in computer simulations did not show the same level of dark matter concentration on the smallest scales – scales associated with individual cluster galaxies as seen in the universe.

The team is looking to continue the stress test of the standard dark matter model to determine its intriguing nature.

NASA’s planned Roman Nancy Grace Space Telescope will discover more distant galaxies through gravitational lenses with massive galaxy clusters. The observations will enlarge sample swarms that astronomers can analyze for further testing of dark matter models.

The reference: “An excess of micro-gravitational lenses observed in galaxy clusters” by Massimo Mingetti, Guido Davoli, Pietro Bergamini, Piero Rosati, Priamvada Natarajan, Carlo Giocoli, and Gabriel B. Kamenya, R. , Francesco Calura, Claudio Grillo, Amata Mercurio and Eros Vanzella, 11 September 2020, Science.
DOI: 10.1126 / science.aax5164

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (European Space Agency). The telescope is operated by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Astronomy Research in Washington, DC

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