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06/11/2019 - Dark Matter is Back

an image of the spiral arms of the milky way galaxy with text that says, dark matter, surrounded by exclamation points.Background image: Artist's concept of the Milky Way Galaxy. Credit: NASA JPL

Everywhere and nowhere; such is the riddle of dark matter. Scientists have quite compelling evidence that normal matter—the kind composing stars, planets, and people—is outnumbered in our universe an astounding five times over by a substance that exudes only gravity. That gravity holds our cosmos together, preventing galaxies from ripping themselves apart. Yet going back decades, this crucial, clandestine matter has evaded all efforts at detection.

Case in point: Just a few years ago, in 2015, one of the most promising trails on dark matter seemingly went cold. Using the Fermi Gamma-ray Space Telescope, researchers had noticed that the center of our galaxy, the Milky Way, is inexplicably bright in gamma rays, the most energetic form of light. A tantalizingly plausible reason why? Dark matter, according to theory, could interact with itself in such a way that it would produce normal matter, which would then decay into gamma rays—presto.

Their hopes up, researchers attempted to rule out other, more mundane sources for the excess energy. Alas, further observations instead ruled in run-of-the-mill pulsars—the dense, radiation-spewing remnants of once-mighty stars—as the likely wellspring of the gamma ray glow.

photos of rebecca leane and tracy slatyerRebecca Leane and Professor Tracy Slatyer

Not so fast, though. Thanks to recent efforts by Rebecca Leane, a theoretical physicist at the Massachusetts Institute of Technology (MIT) and a member of MIT's Kavli Institute for Astrophysics and Space Research (MKI), the cold case has been reopened. "Dark matter is back in the game!" says Leane.

A study she conducted with fellow MKI member and assistant professor of physics, Tracy Slatyer, suggests that the 2015 analyses overly credit pulsars for the core's extra energy, even in the presence of substantial amounts of dark matter. In other words, even if dark matter were there, the previously taken approach would fail to discover it.

"Our finding shows that because we really don't understand well enough how everything else—the visible matter—behaves in the center of our galaxy, you could come to the potentially wrong conclusion that dark matter isn't powering the excess, even if it really were," says Leane. "This means that dark matter annihilation could potentially explain the long-standing mysterious excess of bright energetic light at the center of our galaxy, after all."

To arrive at this conclusion, Leane and Slatyer first built a model Milky Way. Into this computer-simulated version of our galaxy went the known cosmic ingredients of stars and gas, along with the pulsars already on record in the Milky Way's center. The MKI researchers then tweaked the recipe to include additional, unknown pulsars, as well as varying amounts of dark matter.

Subjecting the virtual Milky Way to the same sort of analyses done in 2015 that ixnayed dark matter revealed those analyses' limitations. The excess gamma rays kept being attributed to pulsars, even when the researchers intentionally plugged in a hypothetical dark matter signal. Furthermore, including that signal alongside vast, little-understood structures known as Fermi bubbles—great balloons of gamma ray-spewing plasma extending above and below the Milky Way's disk, which Slatyer helped uncover back in 2010—threw off the results, again chalking up the excess to pulsars without giving the simulated dark matter any potential due.

"If there is no dark matter in the conclusion of the analysis, even though you simulated it, something has gone wrong," says Leane. "This is what we find."

To be clear, the new results do not provide fresh evidence of dark matter actually being present in the Milky Way, Leane explains. "We've just brought into question one of the key pieces of evidence that was against dark matter," she says.

If this really a dark matter signal, it would impact everything about our understanding of the universe as we know it.

To attempt to solve the riddle, new ground-based radio telescopes should be able to spot any additional pulsars, beyond Fermi's capabilities, thus potentially (and finally) linking the excess gamma ray glow to prosaic objects. Theorists, meanwhile, will continue to develop their models to seek accordance with fresh observations and help nail down the happenings in the central Milky Way.

Should the pulsar angle not pan out, it's conceivable that scientists have already captured the long-sought sign of dark matter. "The question of the nature of dark matter is one of the most pressing open questions of modern physics," says Leane, "and this could potentially be our first discovery of interactions of dark matter with visible matter."

In a nice bit of cross-Kavli-institute synergy, the original concept of the gamma ray glow being due to dark matter was cooked up in part by Dan Hooper of the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago. It would prove quite the coup if that idea ultimately proved correct.

"If this really a dark matter signal," Leane continues, "it would impact everything about our understanding of the universe as we know it.

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