Scientists at Johns Hopkins University may have uncovered a promising clue in the long-running effort to confirm the existence of dark matter.
For years, astronomers have puzzled over a faint, widespread glow of gamma rays near the Milky Way's center. The source of this mysterious light has remained uncertain, leaving two main possibilities: it could stem from the collision of dark matter particles or from rapidly spinning neutron stars known as millisecond pulsars.
According to a new study published October 16 in Physical Review Letters, both explanations currently appear equally plausible. If the excess gamma radiation turns out not to come from aging stars, it could represent the first concrete evidence that dark matter is real.
"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."
To explore the mystery, Silk and an international team of scientists used advanced supercomputer models to map where dark matter would most likely reside in the Milky Way, incorporating for the first time the galaxy's early history and evolution.
The Milky Way today is a mostly self-contained system, with little material entering or leaving. However, in its first billion years, it absorbed numerous smaller, dark matter-rich galaxies that merged to form its structure. As dark matter particles gathered and concentrated toward the galactic core, the likelihood of their collisions grew.
When the team included these more realistic interactions in their models, the resulting simulations closely matched real gamma-ray observations made by NASA's 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."
Key Takeaways
- A strange glow of gamma-ray light shines from the heart of the Milky Way, and scientists are working to uncover what's behind it.
- One leading theory is that dark matter particles collide and release brief flashes of gamma rays that could explain this glow.
- Using powerful supercomputers, researchers recreated the Milky Way's formation to predict where dark matter and gamma-ray signals from such collisions would appear.
- The simulated maps line up with real telescope data, suggesting the glow may indeed come from dark matter interactions, although more evidence is still needed to confirm it.
