Mysterious glow in Milky Way could be evidence of dark matter

Johns Hopkins researchers may have identified what could be a compelling clue in the ongoing hunt to prove the existence of dark matter. 

A mysterious diffuse glow of gamma rays near the center of the Milky Way has stumped researchers for decades, as they’ve tried to discern whether the light comes from colliding particles of dark matter or quickly spinning neutron stars. 

It turns out that both theories are equally likely, according to new research published today in the journal Physical Review Letters. 

If excess gamma light is not from dying stars, it could become the first proof that dark matter exists. 

“Dark matter dominates the universe and holds galaxies together. It’s extremely consequential and we’re desperately thinking all the time of ideas as to how we could detect it,” said co-author Joseph Silk, a professor of physics and astronomy at Johns Hopkins and a researer at the Insitut d’Astrophyque de Paris and Sorbonne University. “Gamma rays, and specifically the excess light we’re observing at the center of our galaxy, could be our first clue.”

Silk and an international team of researchers used supercomputers to create maps of where dark matter should be located in the Milky Way, taking into account for the first time the history of how the galaxy formed. 

Today, the Milky Way is a relatively closed system, without materials coming in or going out of it. But this hasn’t always been the case. During the first billion years, many smaller galaxy-like systems made of dark matter and other materials entered and became the building blocks of the young Milky Way. As dark matter particles gravitated toward the center of the galaxy and clustered, the number of dark matter collisions increased. 

When the researchers factored in more realistic collisions, their simulated maps matched actual gamma ray maps taken by the Fermi Gamma-ray Space Telescope.  

These matching maps round out a triad of evidence that suggests excess gamma rays in the center of the Milky Way could originate with dark matter. Gamma rays coming from dark matter particle collisions would produce the same signal and have the same properties as those observed in the real-world, the researchers said — though it’s not definitive proof. 

Light emitted from reinvigorated, old neutron stars that spin quickly—called millisecond pulsars—could also explain the existing gamma ray map, measurements and signal signature. But, this millisecond pulsar theory is imperfect, the researchers said. To make those calculations work, researchers have to assume there are more millisecond pulsars in existence than what they’ve observed. 

Answers may come with the construction of a huge new gamma ray telescope called the Cherenkov Telescope Array. Researchers believe data from the higher-resolution telescope, which has the capacity to measure high-energy signals, will help astrophysicists break the paradox. 

The research team is planning a new experiment to test whether these gamma rays from the Milky Way have higher energies, meaning they are millisecond pulsars, or are the lower energy product of dark matter collisions. 

“A clean signal would be a smoking gun, in my opinion,” Silk said. 

In the meantime, the researchers will work on predictions about where they should find dark matter in several select dwarf galaxies that circle the Milky Way. Once they’ve mapped their predictions, they can compare them to the hi-res data. 

“It’s possible we will see the new data and confirm one theory over the other,” Silk said. “Or maybe we’ll find nothing, in which case it’ll be an even greater mystery to resolve.”

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