Key takeaways

  • A Harvard scientist discovered a massive hole in a tidal stream within the Milky Way’s halo.
  • The cause of the disruption is unknown, but it is much larger than any known star, possibly a dense dark matter object.
  • Telescopes haven’t found any large, bright object moving away from the hole, ruling out ordinary matter as the cause.
  • The disruption may be due to a dark matter subhalo, which aligns with theories that dark matter is “clumpy.”
  • If confirmed, this discovery could help scientists study dark matter more closely and understand its structure better.

Something looks to have ripped a hole across a portion of the Milky Way’s halo. The “dark substructure” was discovered through Gaia observations—a project aimed at creating the most precise 3D image of our galaxy—after Harvard scientist Ana Bonaca noticed a disruption in a tidal stream.

She presented her findings at the American Physical Society’s meeting.

According to LiveScience, Bonaca was studying tidal streams caused by stars fleeing from globular clusters, which are often located near the galaxy’s outskirts. Tidal streams can be seen across the Milky Way’s star halo.

If there is nothing to disrupt them, the streams are nearly homogeneous in density. However, Bonaca found a hole in one. “The on-sky morphology suggests a recent, close encounter with a massive and dense perturber,” the abstract of her findings stated.

What this “perturber” is, however, remains unknown. “It’s a dense bullet of something,” Bonaca said to Live Science. Telescopes failed to locate the source, so what might it be?

The crater is massive, therefore whatever created it must also have been. “It’s much more massive than a star,” she told the website. “Approximately one million times the mass of the sun. So there aren’t any stars with that mass. We can rule it out. And if it were a black hole, it would be a supermassive black hole like the one at the heart of our galaxy.”

The difficulty with this theory is that there is no evidence of a supermassive black hole nearby.

Observations suggest that no huge bright object (anything formed of ordinary stuff that reflects light) is currently traveling away from the hole. Bonaca suggested that the disruption may have been caused by dark matter. This is the mysterious material that constitutes up about 27% of the cosmos. Scientists know it exists because of the gravitational attraction it exerts on regular matter; yet, because it is “dark,” meaning it does not reflect light, we cannot see what it is.

“Observations permit a low-mass dark-matter subhalo as a plausible candidate,” Bonaca’s abstract states.

If a thick glob of dark matter burst through the tidal stream, it would be an exciting discovery for scientists since it would allow them to investigate the elusive material. The finding of a dark matter “bullet” would also be consistent with existing theories about what dark matter is like—research says it is “clumpy,” meaning it is not smooth and uniformly dispersed across the cosmos.

Identifying a cluster of dark matter “opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures,” the abstract’s conclusion states.

However, she claims that this does not rule out a bright object. “It could be that it’s a luminous object that went away somewhere, and it’s hiding somewhere in the galaxy,” she told me.

Bocana’s study is still in its early phases. She has yet to publish her findings in a peer-reviewed publication; nonetheless, LiveScience says that her presentation was well received by participants.

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