Philippa Kaur

Dark matter just got even more puzzling.
This unidentified stuff, which makes up most of the mass in the cosmos, is invisible but detectable by the way it gravitationally tugs on objects like stars. (SN: 11/25/19). Dark matter’s gravity can also bend light traveling from distant galaxies to Earth — but now some of this mysterious substance appears to be bending light more than it’s supposed to. A surprising number of dark matter clumps in distant clusters of galaxies severely warp background light from other objects, researchers report in the Sept. 11 Science.
This finding suggests that these clumps of dark matter, in which individual galaxies are embedded, are denser than expected. And that could mean one of two things: Either the computer simulations that researchers use to predict galaxy cluster behavior are wrong, or cosmologists’ understanding of dark matter is.
Very high concentrations of dark matter can act like a lens to bend light and drastically alter the appearance of background galaxies as seen from Earth — stretching them into arcs or splitting them into multiple images of the same object on the sky. “It’s totally cool. It’s like a fun house mirror,” says astrophysicist Priyamvada Natarajan of Yale University.

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Judging by computer simulations of galaxy clusters, clumps of dark matter around individual galaxies that are dense enough to cause such dramatic gravitational lensing effects should be rare (SN: 10/4/15). Based on cluster simulations run by Natarajan and colleagues, “we would expect to see 1 [strong lensing] event in every 10 clusters or so,” says study coauthor Massimo Meneghetti, an astrophysicist at the Astrophysics and Space Science Observatory of Bologna in Italy.
But telescope images told a different story. The researchers used observations from the Hubble Space Telescope and the Very Large Telescope in Chile to investigate 11 galaxy clusters from about 2.8 billion to 5.6 billion light-years away. In that set, the team identified 13 cases of severe gravitational lensing by dark matter clumps around individual galaxies. These observations indicate there are more high-density dark matter clumps in real galaxy clusters than in simulated ones, Meneghetti says.
The simulations could be missing some physics that leads dark matter in galaxy clusters to glom tightly together, Natarajan says. “Or … there’s something fundamentally off about our assumptions about the nature of dark matter,” she says, like the notion that gravity is the only attractive force that dark matter feels.
Richard Ellis, a cosmologist at the University College London who was not involved in the work, thinks the crux of the problem is more likely in the computer simulations than in the nature of dark matter. “A cluster of galaxies is a very dangerous place. It’s like the Manhattan of the universe,” he says — busy with galaxies whizzing past one another, colliding and getting torn up. “There’s awful physics that goes into predicting how many of these little lensed things they should find,” Ellis says, so the new result “is intriguing, but my suspicion is that there’s something in the simulations … that isn’t quite right.”
Future observations with the upcoming Euclid space telescope (SN: 11/14/17), the Nancy Grace Roman Space Telescope and Vera C. Rubin Observatory (SN: 1/10/20) could help clear matters up, says Bhuvnesh Jain, an astrophysicist at the University of Pennsylvania who was not involved in the work. “These three telescopes are going to produce extremely large samples of galaxy clusters,” he says. That may lead to a new understanding of the physics in these turbulent environments, and help determine whether unrealistic simulations are to blame for this dark matter mystery. More

In one of the most complex cosmic dances astronomers have yet spotted, three rings of gas and dust circle a trio of stars.
The star system GW Orionis, located about 1,300 light-years away in the constellation Orion, includes a pair of young stars locked in a close do-si-do with a third star making loops around both. Around all three stars is a broken-apart disk of dust and gas where planets could one day form. Unlike the flat disk that gave rise to the planets in our solar system, GW Orionis’ disk consists of three loops, with a warped middle ring and an inner ring even more twisted at a jaunty angle to the other two.
The bizarre geometry of this system, the first known of its kind, is reported in two recent studies by two groups of astronomers. But how GW Orionis formed is a mystery, with the two teams providing competing ideas for the triple-star-and-ring system’s birth.
In a Sept. 4 study in Science, astronomer Stefan Kraus of the University of Exeter in England and colleagues suggest that gravitational tugs and torques from the triple-star ballet tore apart and deformed the primordial disk. But in a May 20 study in the Astrophysical Journal Letters, Jiaqing Bi of the University of Victoria in Canada and colleagues think that a newborn planet is to blame.
“The question is how do you actually form such systems,” says theoretical physicist Giuseppe Lodato of the University of Milan, who was not on either team. “There could be different mechanisms that could do that.”

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Astronomers have seen tilted disks of gas and dust around binary star systems, but not systems of more than two stars (SN: 7/30/14). Around half of the stars in the galaxy have at least one stellar companion, and their planets often have tilted orbits with respect to their stars, going around more like a jump rope than a Hula-Hoop (SN: 11/1/13). That misalignment could originate with the disk in which the planets were born: If the disk was askew, the planets would be too.
About a decade ago, astronomers first realized that GW Orionis has three stars and a planet-forming disk, and the scientists scrambled to get a closer look. (At the time, it was impossible to tell if that disk was a single loop or not.) Bi’s team and Kraus’ team aimed the Atacama Large Millimeter/submillimeter Array in Chile at the triple-star system.
Both groups spotted the trio of stars: one about 2.5 times and another about 1.4 times the sun’s mass orbiting each other once every 242 days, and another 1.4 solar mass star orbiting the inner pair about every 11 years.
The observations also revealed three distinct rings of dust and gas encircling the stars. The closest ring to the star trio lies about 46 times the distance from Earth to the sun; the middle one about 185 times the Earth-sun distance; and the outermost ring about 340 times that distance. For perspective, Neptune is about 30 times the distance from Earth to the sun.
That innermost ring is strongly misaligned with respect to the other rings and the stars, the teams found. Kraus’ group added observations from the European Southern Observatory’s Very Large Telescope to show the shadow of the inner ring on the inside of the middle loop. That shadow revealed that the middle ring is warped, swooping up on one side and down on the other.
Astronomers looked at GW Orionis with the ALMA telescope array (left, blue) and the SPHERE instrument on the Very Large Telescope (right, red), both in Chile. The ALMA observations revealed the disk’s tri-ringed structure, while the SPHERE images showed the shadow cast by the innermost ring, allowing scientists to describe the rings’ deformed shapes in detail.Left image: ALMA/ESO, NAOJ, NRAO; Right image: ESO, S. Kraus et al, Univ. of Exeter
Next, both groups ran computer simulations to figure out how the system formed. This is where their conclusions begin to differ, Bi says. His team suggests that a newly formed, not-yet-discovered planet cleared its orbit of gas and dust, splitting the inner ring off from the rest of the disk (SN: 7/16/19). Once the disk was split, the inner ring was free to swing around the stars, settling into its skewed alignment.
Simulations from Kraus’s team, though, found that the chaotic gravity from the triple stars’ orbital dance alone was enough to break up the disk, a phenomenon called disk tearing. Each star tends to keep the disk aligned with itself, and the tug-of-war warped and sheared the disk, and twisted the inner ring even further. Theoretical studies had suggested disk tearing might happen in multiple star systems, but this is the first time it’s been seen in real life, Kraus contends.
“I think it’s plausible that there could be planets somewhere in the system, but they’re not needed to explain the misalignment,” he says. “We don’t need to invoke undiscovered planets to explain what we see.”
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A trio of stars in GW Orionis are surrounded by an enormous, warped disk of gas and dust, new observations reveal. This animation, which is based on computer simulations and observational data, shows the complex geometry of the deformed and broken-apart disk.
The difference may lie in the assumptions that the groups made about the disk’s properties, in particular its viscosity, says astrophysicist Nienke van der Marel, Bi’s colleague at the University of Victoria. A more viscous disk would tear like how Kraus and colleagues propose, but a less viscous disk needs a planet to break apart, she says. She thinks her team’s work is more realistic based on observations of other star systems. But with current technology, there’s no way to tell what the properties of GW Orionis’ disk are really like.
And neither group could explain what made the disk split into three. “We don’t really know what’s causing the outer ring,” Klaus says.
Lodato, who predicted the disk-tearing effect in 2013, thinks GW Orionis is proof that the phenomenon really exists. Back then, Lodato and colleagues were “very worried” that their simulations showed an effect that was introduced by the computations, not real physics, he says. “Now observations tell us that it does happen in reality.”
Future telescopes may also be able to spot the planet if it exists, van der Marel says. More