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http://www.space.com/28941-colliding-galaxies-rule-out-dark-universe.html
Dark Matter Probably Isn’t a Mirror Universe, Colliding Galaxies Suggest
By Calla Cofield, Space.com Staff Writer | March 26, 2015 02:02pm ETDark matter may not be part of a “dark sector” of particles that mirrors regular matter, as some theories suggest, say scientists studying collisions of galaxy clusters.
When clusters of galaxies collide, the hot gas that fills the space between the stars in those galaxies also collides and splatters in all directions with a motion akin to splashes of water. Dark matter makes up about 90 percent of the matter in galaxy clusters: Does it splatter like water as well?
New research suggests that no, dark matter does not splatter when clusters of galaxies collide, and this finding limits the kinds of particles that can make up dark matter. Specifically, the authors of the new research say it is unlikely that dark matter is part of an entire “dark sector” — a mirror version of the visible universe. [Dark Matter: A Cosmic Mystery Explained (Infographic)]
Colliding galaxy clusters
Our galaxy contains hundreds of billions of stars, and there are hundreds of billions of galaxies in the observable universe. There’s also a lot of gas and dust between the stars and the galaxies. But all of those stars, galaxies, gas and dust make up only about 10 to 15 percent of the matter in the universe.
The other 85 to 90 percent is dark matter. Scientists don’t know what dark matter is made of or where it comes from, only that it doesn’t appear to reflect or radiate light. It does, however, exert a gravitational pull on the regular matter around it.
David Harvey, a postdoctoral researcher at the Swiss Federal Institute of Technology Lausanne, is one of many scientists currently trying to figure out what dark matter is made of. There are lots of ways to go about this, and Harvey decided to see what happens when dark matter collides with itself.
To do this, Harvey and his colleagues at the University of Edinburgh, where Harvey did his PhD work, looked at collisions among entire clusters of galaxies, where as much as 90 percent of the mass involved in the collision is dark matter, according to a statement from the Swiss Federal Institute of Technology Lausanne.
“[Galaxy cluster mergers] are incredibly messy,” Harvey said. “You’ve got [the stars], the highest densities of dark matter and hot gas all swirling together.”
Scientists have tried to use these galaxy cluster crashes to study dark matter for decades, but improved techniques for observing the different components of those mergers has inspired a revival, he said. “We wanted to have a big statistical sample that tries to average over all these different merging scenarios, and try to get a statistical idea of what dark matter is doing during these cosmological crashes.”
During these incredibly large-scale mergers, scientists have observed that individual stars in these galaxies are so far apart that they very rarely run into one another. So, rather than creating a big, messy wreck, the stars sort of neatly fold together.
However, in between the galaxies is a thick gas full of charged particles. When the galaxy clusters collide, the gas splatters in all directions, like water splashed from a puddle, Harvey said.
“If we measure the dark matter [after the collision], and should it lie where the galaxies are, we know the dark matter is completely collisionless, and doesn’t interact with itself at all,” Harvey said. “And if it should lie where the gas is, we’d say that the dark matter is actually interacting with itself a lot, like a liquid.”
The researchers gathered data on a total of 30 galaxy-cluster collisions. In order to see the stars, the gas and the dark matter, they needed observations from NASA’s Hubble Space Telescope and Chandra X-ray Observatory. [Chandra Observatory’s X-ray Universe in Photos]
Dark matter doesn’t radiate or reflect light, but its gravitational pull can help scientists “see” it. Light that is passing near a very massive object will bend around it, in an effect called gravitational lensing. Scientists can see the bending of the light and use that to figure out where dark matter is present.
By looking at 30 galaxy-cluster mergers, the researchers showed that the dark matter behaves more like the stars: It doesn’t splatter during these collisions, but instead remains largely unchanged by the merger.
The dark sector
The implications of the new finding go beyond galaxy mergers: They tell scientists something about what dark matter might be made of.
The gas that is found in between the galaxy clusters tends to splatter during collisions because it interacts with itself, the way a liquid does. Notice how liquids in microgravity tend to join together into bubbles — the material sticks together even though it isn’t bound together like a solid.
Protons — the particles at the heart of every atom — interact with one another in a similar way. Harvey and his colleagues showed that dark matter clearly doesn’t interact with itself the way the gas does; more specifically, it interacts with itself less than protons interact with one another.
Some theories of dark matter posit that it is part of a “dark sector” that is sort of like a mirror of the regular universe — in other words, that it contains dark versions of regular matter particles, like dark photons and dark electrons. In some of those theories, dark matter might be made up of dark protons.
“Chances are that dark matter is not made up of dark protons interacting with dark protons, and chances are, there is not a mirror universe out there with these dark particles,” Harvey said. “The caveat is that theorists could change some of their parameters, so the field is still open to what [dark matter] could be, but we’re narrowing it down.”
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http://www.forbes.com/sites/startswithabang/2016/09/19/dark-matter-proved-real-by-colliding-galaxy-clusters/#4cae45b26becDark Matter Proved Real By Colliding Galaxy Clusters
AgamemnonPublished on Oct 25, 2016
Dark matter permeates the cosmos: The material keeps galaxies from flying apart and has left its imprints in the oldest light in the universe. Despite overwhelming evidence that the exotic form of matter lurks unseen in the cosmos, decades of searches have failed to definitively detect a single particle of dark matter.
Theoretical astrophysicist Dan Hooper of Fermilab in Batavia, Ill, said, “We’ve been looking where our best guess told us to look for all these years, and we’re starting to wonder if we maybe guessed wrong. People are just opening their minds to a wider range of options.”Latest dark matter searches leave scientists empty-handed
Physicists ponder possibilities for bulk of universe’s mass
By
Emily Conover
5:30am, October 25, 2016
galaxy clusterDARK GRAVITY In this cluster of galaxies, the location of dark matter is shown in blue. Scientists indirectly detected this dark matter through its gravitational influence, which bends and distorts the light of galaxies in the background. So far, all efforts to directly detect particles of the invisible matter have fallen flat.
https://www.sciencenews.org/article/latest-dark-matter-searches-leave-scientists-empty-handed
NASA, ESA, D. Harvey/École Polytechnique Fédérale de Lausanne, R. Massey/Durham University, HST Frontier Fields
Magazine issue: Vol. 190, No. 10, November 12, 2016, p. 14EMail logo EMail
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Google+ logo Google+Scientists have lost their latest round of hide-and-seek with dark matter, but they’re not out of the game.
Despite overwhelming evidence that an exotic form of matter lurks unseen in the cosmos, decades of searches have failed to definitively detect a single particle of dark matter. While some scientists continue down the road of increasingly larger detectors designed to catch the particles, others are beginning to consider a broader landscape of possibilities for what dark matter might be.
“We’ve been looking where our best guess told us to look for all these years, and we’re starting to wonder if we maybe guessed wrong,” says theoretical astrophysicist Dan Hooper of Fermilab in Batavia, Ill. “People are just opening their minds to a wider range of options.”
Dark matter permeates the cosmos: The material keeps galaxies from flying apart and has left its imprints in the oldest light in the universe, the cosmic microwave background, which dates back to just 380,000 years after the Big Bang. Indirect evidence from dark matter’s gravitational influences shows that it makes up the bulk of the mass in the universe. But scientists can’t pin down what dark matter is without detecting it directly.
In new results published in August and September, three teams of scientists have come up empty-handed, finding no hints of dark matter. The trio of experiments searched for one particular variety of dark matter — hypothetical particles known as WIMPs, or weakly interacting massive particles, with a range of possible masses that starts at several times that of a proton. WIMPs, despite their name, are dark matter bigwigs — they have long been the favorite explanation for the universe’s missing mass. WIMPs are thought to interact with normal matter only via the weak nuclear force and gravity.
Part of WIMPs’ appeal comes from a prominent but unverified theory, supersymmetry, which independently predicts such particles. Supersymmetry posits that each known elementary particle has a heavier partner; the lightest partner particle could be a dark matter WIMP. But evidence for supersymmetry hasn’t materialized in particle collisions at the Large Hadron Collider in Geneva, so supersymmetry’s favored status is eroding (SN: 10/1/16, p. 12). Supersymmetry arguments for WIMPs are thus becoming shakier — especially since WIMPs aren’t showing up in detectors.
Scientists typically search for WIMPs by looking for interactions with normal matter inside a detector. Several current experiments use tanks of liquefied xenon, an element found in trace amounts in Earth’s atmosphere, in hopes of detecting the tiny amounts of light and electric charge that would be released when a WIMP strikes a xenon nucleus and causes it to recoil.
The three xenon experiments are the Large Underground Xenon, or LUX, experiment, located in the Sanford Underground Research Facility in Lead, S.D.; the PandaX-II experiment, located in China’s JinPing underground laboratory in Sichuan; and the XENON100 experiment, located in the Gran Sasso National Laboratory in Italy. Teams of scientists at the three locations each reported no signs of dark matter particles. The experiments are most sensitive to particles with masses around 40 or 50 times that of a proton. Scientists can’t completely rule out WIMPs of these masses, but the interactions would have to be exceedingly rare.
Story continues after image
BETTER LUX NEXT TIME The LUX detector, shown above, failed to detect dark matter through interactions with liquid xenon. A souped-up version of the experiment, known as LZ, will continue the search.
C.H. FahamIn initial searches, proponents of WIMPs expected that the particles would be easy to find. “It was thought to be like, ‘OK, we’ll run the detector for five minutes, discover dark matter, and we’re all done,’” says physicist Matthew Szydagis of the University at Albany in New York, a member of LUX. That has turned into decades of hard work. As WIMPs keep failing to turn up, some scientists are beginning to become less enamored with the particles and are considering other possibilities more closely.
One alternative dark matter contender now attracting more attention is the axion. This particle was originally proposed decades ago as part of the solution to a particle physics quandary known as the strong CP problem — the question of why the strong nuclear force, which holds particles together inside the nucleus, treats matter and antimatter equally. If dark matter consists of axions, the particle could therefore solve two problems at once.
Axions are small fry as dark matter goes — they can be as tiny as a millionth of a billionth the mass of a WIMP. The particles interact so feebly that they are extremely difficult to detect. If axions are dark matter, “you’re sitting in an enormous, dense sea of axions and you don’t even notice them,” says physicist Leslie Rosenberg of the University of Washington in Seattle, the leader of the Axion Dark Matter eXperiment. After a recent upgrade to the experiment, ADMX scientists are searching for dark matter axions using a magnetic field and special equipment to coax the particles to convert into photons, which can then be detected.
Although WIMPs and axions remain the front-runners, scientists are beginning to move beyond these two possibilities. In between the featherweight axions and hulking WIMPs lies a broad range of masses that hasn’t been well explored. Scientists’ favorite theories don’t predict dark matter particles with such intermediate masses, says theoretical physicist Kathryn Zurek of Lawrence Berkeley National Laboratory in California, but that doesn’t mean that dark matter couldn’t be found there. Zurek advocates a diverse search over a broad range of masses, instead of focusing on one particular theory. “Dark matter direct detection is not one-size-fits-all,” she says.
Nuclear recoilXenon dark matter experiments work by watching for dark matter interactions that cause xenon nuclei to recoil. Such interactions would theoretically release photons (orange lines) and electrons (red lines), which create two consecutive bursts of light that can be observed by light-detecting photomultiplier tubes (circles) at the top and bottom of the detector, as seen in this schematic of the LZ experiment.
SLAC National Accelerator Laboratory
In two papers published in Physical Review Letters on January 7 and September 14, Zurek and colleagues proposed using superconductors — materials that allow electricity to flow without resistance — and superfluids, which allow fluids to flow without friction, to detect light dark matter particles. “We are trying to broaden as much as possible the tools to search for dark matter,” says Zurek. Likewise, scientists with the upcoming Super Cryogenic Dark Matter Search SNOLAB experiment, to be located in an underground lab in Sudbury, Canada, will use detectors made of germanium and silicon to search for dark matter with smaller masses than the xenon experiments can.
Scientists have not given up on xenon WIMP experiments. Soon some of those experiments will be scaling up — going from hundreds of kilograms of liquid xenon to tons — to improve their chances of catching a dark matter particle on the fly. The next version of XENON100, the XENON1T experiment (pronounced “XENON one ton”) is nearly ready to begin taking data. LUX’s next generation experiment, known as LUX-ZEPLIN or LZ, is scheduled to begin in 2020. PandaX-II scientists are also planning a sequel. Physicists are still optimistic that these detectors will finally find the elusive particles. “Maybe we will have some opportunity to see something nobody has seen,” says Xiangdong Ji of Shanghai Jiao Tong University, the leader of PandaX-II. “That’s what’s so exciting.”
In the sea of nondetections of dark matter, there is one glaring exception. For years, scientists with the DAMA/LIBRA experiment at Gran Sasso have claimed to see signs of dark matter, using crystals of sodium iodide. But other experiments have found no signs of DAMA’s dark matter. Many scientists believe that DAMA has been debunked. “I don’t know what generates the weird signal that DAMA sees,” says Hooper. “That being said, I don’t think it’s likely that it’s dark matter.”
But other experiments have not used the same technology as DAMA, says theoretical astrophysicist Katherine Freese of the University of Michigan in Ann Arbor. “There is no alternative explanation that anybody can think of, so that is why it is actually still very interesting.” Three upcoming experiments should soon close the door on the mystery, by searching for dark matter using sodium iodide, as DAMA does: the ANAIS experiment in the Canfranc Underground Laboratory in Spain, the COSINE-100 experiment at YangYang Underground Laboratory in South Korea, and the SABRE experiment, planned for the Stawell Underground Physics Laboratory in Australia.
Scientists’ efforts could still end up being for naught; dark matter may not be directly detectable at all. “It’s possible that gravity is the only lens with which we can view dark matter,” says Szydagis. Dark matter could interact only via gravity, not via the weak force or any other force. Or it could live in its own “hidden sector” of particles that interact among themselves, but mostly shun normal matter.
Even if no particles are detected anytime soon, most scientists remain convinced that an unseen form of matter exists. No alternative theory can explain all of scientists’ cosmological observations. “The human being is not going to give up for a long, long time to try to search for dark matter, because it’s such a big problem for us,” says Ji.
More than you ever wanted to know about dark matter.
Agamemnon
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