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Leslie Rosenberg, a physicist with the University of Washington has published a paper in Proceedings of the National Academy of Sciences, describing the current state of research that involves investigating the possibility that axions are what make up dark matter. He also offers some perspective on the work suggesting that at least one project is likely to lead to either proving or disproving that axions are dark matter.
In the late 20th century, cosmology became a precision science. Now, at the beginning of the next century, the parameters describing how our universe evolved from the Big Bang are generally known to a few percent. One key parameter is the total mass density of the universe. Normal matter constitutes only a small fraction of the total mass density. Observations suggest this additional mass, the dark matter, is cold (that is, moving nonrelativistically in the early universe) and interacts feebly if at all with normal matter and radiation. There’s no known such elementary particle, so the strong presumption is the dark matter consists of particle relics of a new kind left over from the Big Bang.
One of the most important questions in science is the nature of this dark matter. One attractive particle dark-matter candidate is the axion. Physicists calculate that dark matter comprises 27 percent of the universe; normal matter 5 percent. WIMPS, weakly interacting massive particles, or axions, are weakly interacting low-mass particles.
Axion searches use a wide range of technologies, and the experiment sensitivities are now reaching likely dark-matter axion couplings and masses.
“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 Harry Nelson, professor of physics at the University of California, Santa Barbara and science lead for the LUX upgrade, called LUX-ZEPLIN. “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.”
With some luck, that may be about to change. With ten times the sensitivity of previous detectors, three recently funded dark matter experiments have scientists crossing their fingers that they may finally glimpse these long-sought particles. In recent 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 last week observed an odd spike in X-ray emissions coming from two different celestial objects — the Andromeda galaxy and the Perseus galaxy cluster that corresponds to no known particle or atom and thus may have been produced by dark matter.
The image at the top of the page shows the central region of the Perseus galaxy cluster, using NASA's Chandra X-ray Observatory and 73 other clusters with ESA's XMM-Newton has revealed a mysterious X-ray signal in the data. The signal is also seen in over 70 other galaxy clusters using XMM-Newton. One intriguing possible explanation of this X-ray emission line is that it is produced by the decay of sterile neutrinos, a type of particle that has been proposed as a candidate for dark matter. While holding exciting potential, these results must be confirmed with additional data to rule out other explanations and determine whether it is plausible that dark matter has been observed.
The Perseus Cluster is one of the most massive objects in the Universe, and contains thousands of galaxies immersed in an enormous cloud of superheated gas. In Chandra's X-ray image, enormous bright loops, ripples, and jet-like streaks throughout the cluster can be seen. The dark blue filaments in the center are likely due to a galaxy that has been torn apart and is falling into NGC 1275 (a.k.a. Perseus A), the giant galaxy that lies at the center of the cluster.
There is uncertainty in these results, in part, because the detection of this emission line is pushing the capabilities of both Chandra and XMM-Newton in terms of sensitivity. Also, there may be explanations other than sterile neutrinos if this X-ray emission line is deemed to be real. For example, there are ways that normal matter in the cluster could have produced the line, although the team's analysis suggested that all of these would involve unlikely changes to our understanding of physical conditions in the galaxy cluster or the details of the atomic physics of extremely hot gases.
"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 new 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.”
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, which began in 2009, is now getting an upgrade to increase its sensitivity to heavier WIMPs. Meanwhile, the Super Cryogenic Dark Matter Search collaboration, which has looked for the signal of a lightweight WIMP barreling through its detector since 2013, is in the process of finalizing the design for a new experiment to be located in Canada.
“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 will 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 search is a very valuable one. 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 portion of the universe.
The physics community has spent three decades searching for and finding no evidence that dark matter is made of tiny exotic particles. Recently, Case Western Reserve University physics professor Glenn Starkman and David Jacobs, who received his PhD in Physics from CWRU in May and is now a fellow at the University of Cape Town, say published observations provide guidance, limiting where to look. The Macros, as Starkman and Jacobs call them, would not only dwarf WIMPS and axions, but differ in an important way. They could potentially be assembled out of particles in the Standard Model of particle physics instead of requiring new physics to explain their existence.
"We've been looking for WIMPs for a long time and haven't seen them," Starkman said. "We expected to make WIMPS in the Large Hadron Collider, and we haven't."
WIMPS and axions remain possible candidates for dark matter, but there's reason to search elsewhere, the theorists argue. "The community had kind of turned away from the idea that dark matter could be made of normal-ish stuff in the late '80s," Starkman added. "We ask, was that completely correct and how do we know dark matter isn't more ordinary stuff— stuff that could be made from quarks and electrons?"
After eliminating most ordinary matter, including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies and neutrinos with a lot of mass, as possible candidates, physicists turned their focus on the exotics. Matter that was somewhere in between ordinary and exotic—relatives of neutron stars or large nuclei—was left on the table, Starkman said. "We say relatives because they probably have a considerable admixture of strange quarks, which are made in accelerators and ordinarily have extremely short lives," he said.
Although strange quarks are highly unstable, Starkman points out that neutrons are also highly unstable. But in helium, bound with stable protons, neutrons remain stable. "That opens the possibility that stable strange nuclear matter was made in the early universe and dark matter is nothing more than chunks of strange nuclear matter or other bound states of quarks, or of baryons, which are themselves made of quarks," he said. Such dark matter would fit the Standard Model.
The Macros would have to be assembled from ordinary and strange quarks or baryons before the strange quarks or baryons decay, and at a temperature above 3.5 trillion degrees Celsius, comparable to the temperature in the center of a massive supernova, Starkman and Jacobs calculated. The quarks would have to be assembled with 90 percent efficiency, leaving just 10 percent to form the protons and neutrons found in the universe today.
The limits of the possible dark matter are as follows:
A minimum of 55 grams. If dark matter were smaller, it would have been seen in detectors in Skylab or in tracks found in sheets of mica. A maximum of 1024 (a million billion billion) grams. Above this, the Macros would be so massive they would bend starlight, which has not been seen. The range of 1017 to 1020 grams should also be eliminated from the search, the theorists say. Dark matter in that range would be massive for gravitational lensing to affect individual photons from gamma ray bursts in ways that have not been seen.
If dark matter is within this allowed range, there are reasons it hasn't been seen.
At the mass of 1018 grams, dark matter Macros would hit the Earth about once every billion years. At lower masses, they would strike the Earth more frequently but might not leave a recognizable record or observable mark. In the range of 109 to 1018, dark matter would collide with the Earth once annually, providing nothing to the underground dark matter detectors in place.
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