Spotlights Gets Such a weakly interacting massive HAWC is not a system of lenses and mirrors a Little Darker particle (WIMP) could account for the but a collection of ultrapure water tanks In the 1970s, astronomers realized that majority of dark matter (with comparatively wired up with ultrasensitive light detectors. are filled with dark matter: little dark matter in the form of chunkier Whenever a very-high-energy particle matter that doesn’t emit, absorb, or block things like black holes and very dim ). from space collides with an air molecule light (like stars, gas, or dust) but still But if a WIMP can pass clean through overhead, a spray of subatomic particles exerts a gravitational force, as measured the earth, then it can also pass clean rains downward and generates a faint flash by the orbital speeds of stars and other through the specialized machines that of blue light in HAWC’s water tanks. It just objects circling around the centers of earthlings build to detect it. Nonetheless, so happens that—in theory, at least— galaxies. For a typical like our own a number of dark matter detectors have WIMPs that collide with one another or , there is about ten times more been built, running for years on end and decay should produce very-high-energy dark matter than all the “regular” forms accumulating enough non-detections to gamma rays capable of triggering of matter combined. rule out various categories of WIMPs and this event. Four decades of intensive scrutiny other dark matter candidates. Key among “HAWC was not designed solely to later, the dark matter, undeniably present, the categories that remain are particles look for dark matter,” Albert says, “but it still hasn’t been identified. The leading somewhat heavier than conventional WIMP is extremely well suited for picking up contender for what might comprise it, or candidates; these could be detectable by gamma-ray signals from higher-mass at least most of it, is a yet-to-be-identified an unconventional detector. dark matter particles.” Unlike conventional elementary particle that swarms invisibly in Astrophysicist Andrea Albert analyzes astronomical observatories, HAWC is tremendous numbers throughout galaxies data from the High-Altitude Water almost always on, day and night. And it and other astronomical structures. These Cherenkov Gamma-ray Observatory (HAWC), doesn’t need to be pointed at any single particles could be passing unhindered a novel Los Alamos-led gamma-ray and astronomical object, but rather takes in through our planet right now, gliding easily cosmic-ray telescope (see “Celestial Mystery many targets at once as it consistently through the empty spaces inside atoms, Machine” in the May 2015 issue of 1663). sweeps across two-thirds of the sky over despite having enough mass to generate Perched on the slope of a volcano in Mexico, the course of the earth’s daily rotation. a powerful collective gravitational pull. the third-highest peak in North America, In addition, it is the only instrument in

2 1663 March 2018 science, a rigorous and gradual chipping- away process, producing ever-tighter limits on the properties that dark matter could possibly have. In the upper-mass range, it can only have x propensity to collide or y rate of decay. “Sometimes in science you have to accept a process of elimination,” Albert says. “If we have to, we’ll find out what dark matter is by crossing off every single thing that it isn’t.”

—Craig Tyler

Capturing Life in Motion Following the inventions of microscopes and x-ray machines, constant technological advances have propelled humans’ ability UGC 12591 is the fastest spinning to understand what is going on inside known, with stars orbiting the center about living organisms. But aside from an invasive twice as fast as in the Milky Way. Stellar orbital endoscopic procedure, most methods of speeds—in this galaxy and pretty much visualizing organs or cells produce only every other—are too fast; without additional one- or two-dimensional still images, gravity from a great deal of unseen dark leaving much to be desired. For instance, matter, galaxies shouldn’ t be able to keep their to look at proteins and other cellular stars from simply flying away. CREDIT: NASA, ESA, Hubble components, one must break open a cell and affix the proteins to a microscope slide or suspend them in a gel. To actually see how a protein moves around and operation that probes for dark matter Dwarf galaxies are what they sound functions with other cellular structures, particles beyond a particular mass like, typically with only a few hundreds scientists would instead need some kind threshold, about 20,000 times the mass of millions of stars, as contrasted with a of molecular video camera. of a proton. full-fledged galaxy like the Milky Way, with Over the last several years, many teams So, has HAWC spotted any dark stars numbering in the hundreds of billions. have been trying to invent one: developing matter? Well, no, it did what every other II appears to contain only new microscopes in an effort to capture the detector has done for decades: it ruled about a thousand. They are gravitationally three-dimensional movements of particles out some types of dark matter based on bound together by an unprecedented inside live cells. In 2008, a Los Alamos what it didn’t see. But strangely, HAWC’s excess of dark matter, with thousands of team led by biophysicist Jim Werner strongest constraints on dark matter times more of it than normal matter. This developed an award-winning 3D-tracking come from gamma rays it didn’t see is a double virtue for Triangulum II and microscope, one of only a few of its kind. from a galaxy that didn’t exist, as far as dwarf galaxies in general: more dark matter Today, the scientists are still improving anyone knew. to collide or decay, producing gamma rays, the technology, adding layers of data “We looked for gamma-ray dark- and fewer normal-matter sources, such capture to help them truly understand matter signals from a number of as supernovae, to produce gamma rays of molecular machinery at work. And with astronomical sources, including our their own. each advancement, they are getting closer neighbor the Andromeda Galaxy,” Albert In all, Albert and colleagues examined to capturing the motion of life at a cellular says. “The most constraining results came 15 nearby dwarf galaxies and found no level. Understanding such interactions from observations of dwarf galaxies, and gamma rays that could be attributed to and movements has a variety of potential our strongest result came from Triangulum dark matter. But as any dark-matter hunter applications, including medical ones, such II, a so faint that it hadn’t knows by now, the search doesn’t stop as testing therapeutics. even been discovered until after we there. Over time, more gamma-ray data The Los Alamos 3D-tracking pooled our dwarf-galaxy data. Luckily, will be collected from these regions, and, microscope is based on the design of a because of HAWC’s wide field of view and with enough patience, a faint signal may typical confocal microscope, which uses nearly 100 percent up time, we already eventually emerge. For now, however, a pinhole combined with an objective had data on it.” there is only the slow march of determined lens on either side of the specimen in an

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