Cosmology, Physics, Science, SETI, Space Posted on Sep 20, 2018 in Astronomy

“This idea that there’s something out there that we can’t sense yet is one of those things that sends chills down my spine,” said Harry Nelson, professor of physics at the University of California, Santa Barbara.

“Nature is being coy,” said Enectali Figueroa-Feliciano, an associate professor of physics at the MIT Kavli Institute for Astrophysics and Space Research who works on one of the three experiments searching for dark matter that continue to this day in 2018. “There’s something we just don’t understand about the internal structure of how the universe works. When theorists write down all the ways dark matter might interact with our particles, they find, for the simplest models, that we should have seen it already. So even though we haven’t found it yet, there’s a message there, one that we’re trying to decode now.”

“We’re all looking and somewhere, maybe even now, there’s a little bit of data that will cause someone to have an ‘Ah ha!’ moment,” said Nelson, science lead for the LUX upgrade, called LUX-ZEPLIN. Scientists have long known that dark matter is out there, silently orchestrating the universe’s movement and structure. But what exactly is dark matter made of? And what does a dark matter particle look like? That remains a mystery, with experiment after experiment coming up empty handed in the quest to detect these elusive particles.

With ten times the sensitivity of previous detectors, three dark matter experiments have scientists crossing their fingers that they may finally glimpse these long-sought particles. In past conversations with The Kavli Foundation, scientists working on these new experiments expressed hope that they would catch dark matter, but also agreed that, in the end, their success or failure is up to nature to decide.

While studying over data collected by the European Space Agency’s XMM-Newton spacecraft, a team of researchers observed an odd spike in X-ray emissions coming from two different celestial objects — the Andromeda galaxy (core below) and the Perseus galaxy cluster shown at top of the page that corresponds to no known particle or atom and thus may have been produced by dark matter. The image above shows X-ray emission from the core of the Perseus cluster (in red), as observed by the Chandra X-ray Observatory; the radio emission from the central supermassive black hole is shown in blue.

“The signal’s distribution within the galaxy corresponds exactly to what we were expecting with dark matter — that is, concentrated and intense in the center of objects and weaker and diffuse on the edges,” study co-author Oleg Ruchayskiy, of the École Polytechnique Fédérale de Lausanne (EPFL) said.

The first of the experiments, called the Axion Dark Matter eXperiment, searches for a theoretical type of dark matter particle called the axion. ADMX seeks evidence of this extremely lightweight particle converting into a photon in the experiment’s high magnetic field. By slowly varying the magnetic field, the detector hunts for one axion mass at a time.

“We’ve demonstrated that we have the tools necessary to see axions,” said Gray Rybka, research assistant professor of physics at the University of Washington who co-leads the ADMX Gen 2 experiment. “With Gen2, we’re buying a very, very powerful refrigerator that will arrive very shortly. Once it arrives, we’ll be able to scan very, very quickly and we feel we’ll have a much better chance of finding axions – if they’re out there.”

“We’ve demonstrated that we have the tools necessary to see axions,” said Gray Rybka, research assistant professor of physics at the University of Washington who co-leads the ADMX Gen 2 experiment. “With Gen2, we’re buying a very, very powerful refrigerator that will arrive very shortly. Once it arrives, we’ll be able to scan very, very quickly and we feel we’ll have a much better chance of finding axions – if they’re out there.”

New results this April from the ADMX at the University of Washington suggest that it is now well-tuned enough to detect axions, a theoretical low-mass particle that many physicists believe may account for dark matter.

The ADMX is over 20 years old and first came online at the Lawrence Livermore National Laboratory in 1995. In 2010, it was moved to the University of Washington where it has been in the process of being upgraded ever since. “This experiment heralds a new era of ultrasensitive probes of low mass axionic dark matter,” the researchers wrote in the paper.

The ADMX is technically known as an axion haloscope, which Rybka likened to a large radio receiver.

“If you think of an AM radio, it’s exactly like that,” Rybka said in a statement. “We’ve built a radio that looks for a radio station, but we don’t know its frequency. We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.”

The two other new experiments look for a different type of theoretical dark matter called the WIMP. Short for Weakly Interacting Massive Particle, the WIMP interacts with our world very weakly and very rarely. The Large Underground Xenon, or LUX, experiment, began in 2009, received an upgrade to increase its sensitivity to heavier WIMPs. The experiment is a 370 kg liquid xenon time-projection chamber that aims to directly detect galactic dark matter in an underground laboratory 1 mile under the earth, in the Black Hills of South Dakota.

Meanwhile, the Super Cryogenic Dark Matter Search collaboration, which has looked for the signal of a lightweight WIMP barreling through its detector since 2013,finalized the design for the experiment located in Canada.

The SuperCDMS collaboration has pioneered the use of low-temperature solid-state detectors to search for the rare scattering of dark matter particles with atomic nuclei. This technology provides excellent background rejection, detailed information on each interaction and very low energy thresholds, allowing unparalleled sensitivity especially to dark matter particles with small masses.

The next-generation (G2) SuperCDMS experiment will operate in the deepest underground laboratory in North America, SNOLAB, to provide shielding from high energy cosmic ray particles. It will include a cryogenics system designed to maintain the detectors at temperatures within a fraction of a degree above absolute zero, and special clean shielding materials to exclude radioactive backgrounds from the environment.

the SuperCDMS SNOLAB experiment, which will begin operations in the early 2020s to hunt for hypothetical dark matter particles called weakly interacting massive particles, or WIMPs. The experiment will be at least 50 times more sensitive than its predecessor, exploring WIMP properties that can’t be probed by other experiments and giving researchers a powerful new tool to understand one of the biggest mysteries of modern physics.

“In a way it’s like looking for gold,” said Figueroa-Feliciano, a member of the SuperCDMS experiment. “Harry has his pan and he’s looking for gold in a deep pond, and we’re looking in a slightly shallower pond, and Gray’s a little upstream, looking in his own spot. We don’t know who’s going to find gold because we don’t know where it is.”

Rybka agreed, but added the more optimistic perspective that it’s also possible that all three experiments may find dark matter. “There’s nothing that would require dark matter to be made of just one type of particle except us hoping that it’s that simple,” he said. “Dark matter could be one-third axions, one-third heavy WIMPs and one-third light WIMPs. That would be perfectly allowable from everything we’ve seen.”

Yet the nugget of gold for which all three experiments searched is still undetected three years later as of this writing. And even though the search is difficult, all three scientists agreed that it’s worthwhile because glimpsing dark matter would reveal insight into a large dark portion of the universe. After decades of research, dark matter and dark energy remain elusive. Is it time to admit that cosmology is ensnared by dimly understood forces?

The Daily Galaxy via kavlifoundation.org and Symmetry

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