In a new era of widespread research of dark matter, the controversial idea of its concentration in thin disks is coming out of scientific oblivion
In 1932, Dutch astronomer Jan Oort counted stars in the Milky Way and found that they were missing. Based on the fact that stars, moving in a circle in the plane of the galaxy, jump up and down like horses on a carousel, Oort calculated that the matter that exerts a gravitational effect on them and sets them in motion should be twice as much as he saw … Oort postulated that the lack is made up for by hidden "dark matter", and suggested that it is concentrated in the disk, which explains the movement of the stars.
However, the discovery of dark matter, as invisible and indeterminate matter is called, which makes up five-sixths of the mass of the universe, is usually attributed to the Swiss-American astronomer Fritz Zwicky, who in 1933 derived its existence from the mutual movements of galaxies. Oort was not well known on the grounds that he was on the wrong track. By 2000, the authors of new studies of the Milky Way, using the Oort method, determined that the "missing" mass was contained in faint stars, gas and dust, and the need for a dark disk was no longer necessary. Hints from 80 years ago indicate that dark matter, whatever it is, forms spherical clouds around galaxies, called "halos".
Well, at least that's what most dark matter hunters say. But although the concept of the dark disc has lost its popularity, it has never been abandoned entirely. More recently, this idea has found an important fanatic in the person of Harvard University physics professor Lisa Randall, who has pulled the disk theory out of scientific oblivion and pushed it into the center of the galactic scene.
Having proposed their model in 2013, Randall and colleagues have since argued that the dark disk could explain gamma rays emanating from the galactic center, the flat distribution of dwarf galaxies in orbit around the Andromeda nebula and the Milky Way, and even periodic comet falls and mass extinctions. species on Earth. She wrote about this in her popular science book Dark Matter and the Dinosaurs, published in 2015.
However, astrophysicists who inventory the Milky Way have protested, arguing that the total mass of the galaxy and the jumps of its stars match too well, leaving no room for a dark disk. "It's much tighter than Lisa Randall thinks," says University of Toronto astrophysicist Jo Bovy.
Now Randall, who has developed a number of big ideas on critical issues in fundamental physics, is striking back. In a paper posted online last week and accepted for publication in The Astrophysical Journal, Randall and her student Eric Kramer found a disk-shaped loophole in their analysis of the Milky Way: “There is an important detail that we still have have not paid attention yet, they write. "The disc can create space for itself."
If a dark disk does pass through the galaxy's midplane, Randall and Kramer argue, then gravity pulls the rest of the matter inward, leading to an increase in the density of stars, gas and dust in the midplane. Scientists usually calculate the total apparent mass of the Milky Way by extrapolating outward based on the density of the midplane. If there is a constricting effect, this extrapolation will lead to an exaggeration of the apparent mass, and then there will be a feeling that the mass corresponds to the motion of the stars. For this reason, the authors of previous studies have not seen evidence of a dark disc, says Kramer. Together with Randall, they believe that a thin dark disc is possible and that in some ways its presence is preferable to its absence.
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“Lisa’s work has revitalized this business,” says Chris Flynn of Swinburne University of Technology in Melbourne, Australia. He, along with Johan Holmberg (Johan Holmberg) in the early 2000s, conducted several "inventories" of the Milky Way, which seemed to completely destroy all possibilities for the existence of a dark disk.
Bowie disagrees. Even if we take into account the constricting effect, according to his estimate, no more than 2% of the total amount of dark matter can be in the dark disk, while the rest of the matter should form a halo. “I think most people want to figure out what 98% of dark matter is, not 2%,” he says.
This debate, like the fate of the dark disc, may soon be resolved. The European Space Agency's Gaia satellite is currently telescoping the positions and velocities of a billion stars, and a definitive register of the Milky Way could be completed by the end of next summer.
Young galaxy cluster Abell 2151 in the constellation Hercules
ESO / INAF-VST / OmegaCAM / A. Fujii / Digitized Sky Survey 2
Opening a dark disc of any size would be extremely revealing. If it exists, dark matter will turn out to be much more complex than scientists have long believed. Matter gathers into a disk shape only if it has the ability to emit energy, and the best way to emit enough energy is to form atoms. The existence of dark atoms would mean that dark protons and dark electrons, charged like visible protons and electrons, interact with each other through the dark force transmitted by dark photons. Even if 98% of dark matter is inert and forms a halo, the existence of the thinnest dark disk would mean the presence of a "dark sector" of unknown particles, as diverse as the visible Universe.
“Ordinary matter is quite complex; there is matter that plays a role in atoms, and there is matter that does not play, says the astrophysicist at the University of California, Irvine, James Bullock. “Therefore, it would not be crazy to imagine that the other five-sixths of the matter in the universe are also quite complex, and that there is a certain part of this dark sector that exists in the form of bound atoms.”
The concept of the complexity of dark matter has recently found more and more supporters, helped by astrophysical anomalies, which do not really fit with the idea of dark matter as passive, slow and "weakly interacting massive particles." These anomalies, as well as the fact that in the course of detailed experiments in different countries of the world such heavy weakly interacting particles (WIMPs) were never detected, weakened this theory and marked the beginning of a new era in which anyone can speculate that it is for such a beast - dark matter.
This era came sometime in 2008, when participants in the PAMELA experiment discovered an excess of positrons coming from space (compared to electrons). This asymmetry has fueled interest in the now popular "asymmetric dark matter" model proposed by Kathryn Zurek and her colleagues. There were few ideas in circulation at the time, other than the concept of wimps. “There were modelers like me who understood that the idea of dark matter was completely underdeveloped in this direction,” says Tsurek, who now works at the National Laboratory. Lawrence Berkeley in California. “So we dived into this work headlong.”
The density of dwarf galaxies was another stimulus. When scientists try to model their formation, dwarf galaxies tend to be too dense at their centers, unless scientists assume that dark matter particles can interact with each other using dark forces. But if we add too strong interaction here, then we destroy the models of structure formation in the early Universe.
“We're trying to figure out what can be tolerated,” says Bullock, who also designs these models. Most modelers allow for weak interactions that do not affect the shape of the dark matter halo. “But what's remarkable is there is a class of dark matter that allows discs to form,” says Bullock. In this case, only a tiny fraction of dark matter particles will interact, but they will interact strongly enough to dissipate energy and then form discs.
Randall and her colleagues JiJi Fan, Andrey Katz and Matthew Reece came up with the idea in 2013 the same way Oort did. They tried to explain the apparent anomaly of the Milky Way. The so-called Fermi line is an excess of gamma rays of a certain frequency coming from the galactic center. "Ordinary dark matter couldn't destroy enough to produce the Fermi line," Randall says, "and so we thought, what if it's much denser?" So the dark disc got a second life. The Fermi line disappeared as more data emerged, but the disk idea was still worth exploring. In 2014, Randall and Rees suggested that it is precisely because of the disk that there are intervals of 30-35 million years between the increasing activity of comets and meteors.which some scientists associate with periodic mass extinctions. Every time the solar system bounces up or down on the Milky Way's merry-go-round, they argue, the gravitational effect of the disk could destabilize asteroids and comets in the Oort cloud, a garbage dump on the outskirts of our solar system named after an astronomer from Holland. These objects fly towards the inner part of the solar system, and some fall to Earth.
But Randall and her team did only a cursory, and as it turned out, incorrect analysis of how much room in the total mass of the Milky Way remains for the dark disk, as judged by the motion of the stars. “They made some outrageous statements,” Bovey said.
Randall, known among her peers for her tenacity (according to Rees), brought Kramer into the case to respond to criticism and “iron out all the wrinkles” in the analysis before Gaia's data became available. A new analysis showed that once a dark disk existed, it couldn't be as dense as her team initially believed. But there was still room for a thin dark disk, due to its constricting effect and the additional uncertainty caused by the pure drift of the stars in the Milky Way being observed.
Chris McKee and colleagues at the University of California at Berkeley have identified a new problem, which they wrote about in The Astrophysical Journal. McKee thinks that a thin, dark disk could still squeeze into the Milky Way's mass storage. But this disc can be so thin that it simply collapses. Citing research from the 1960s and 1970s, McKee and his colleagues write that the disks cannot be much thinner than the disk of visible gas in the Milky Way, as they would collapse. "Perhaps dark matter has properties that are different from the properties of ordinary matter and prevent this, but I do not know what it could be," - said McKee.
Randall has yet to fend off this latest attack, calling it "a tricky question currently under study." She also agreed with the view expressed by Bowie - that the disk of charged dark atoms is insignificant compared to the nature of 98% of dark matter. She is now exploring the possibility that all dark matter can be charged with a single dark force, but because of the excess of dark protons over dark electrons, only a tiny fraction is bound into atoms and carried out into the disk. In this case, the disk and the halo must be composed of the same elements, "which would be more economical," she says. "We thought it could be ruled out, but it didn't work."
So far, the dark disk lives on - as a symbol of everything that is unknown about the dark side of the universe. “I think it's very, very helpful in this area that different people are looking at different ideas,” says Bullock. "Because we have no idea what it is - dark matter, and we must be prepared for a variety of options."