A new kind of dark matter could help explain some of the universe’s mysteries, a new study suggests.
The study proposes that there are dense clumps of “self-interacting dark matter”, or SIDM, each with about a million times the mass of the Sun. That could solve three very separate puzzles that have troubled astronomers: gravitational lenses, stellar streams, and satellite galaxies.
Dark matter makes up 85 per cent of the matter in the universe, but it cannot be seen directly. That means that it remains mysterious to astronomers who must instead understand it through the effects it has on visible parts of the universe.
Our usual understanding of physics suggests that dark matter is “cold” and passes through other particles without interacting. But there are certain structures in the universe that cannot be explained by that model.
The new work looks instead at SIDM, a model in which dark matter particles can actually collide with each other an exchange energy. That can lead to a “gravothermal collapse” that leaves behind extremely dense cores.
“The difference is like a crowd of people who ignore each other versus one where everyone is constantly bumping into one another,” said Hai-Bo Yu, who led the new research.
“In SIDM, these interactions can dramatically reshape the internal structure of dark matter halos. Dark matter that interacts with itself can become dense enough to explain these observations.”
The new research suggests that this new understanding could help explain a range of phenomena that have puzzled astronomers.
The first is an ultra-dense object that has been through the way it magnifies distant galaxies, the second is a feature in the GD-1 stellar stream that looks as if an unseen, compact object ripped through it and left behind a scar, and a strange star cluster known as Fornax 6 that can be found in a satellite galaxy of the Milky Way.
The latter, for instance, would be explained by a dense clump of dark matter that would work to trap objects in its galaxies, collecting passing stars and pulling them into a tight, compact cluster.
“What’s striking is that the same mechanism works in three completely different settings — across the distant universe, within our galaxy, and in a neighbouring satellite galaxy,” said Professor Yu. “All show densities that are difficult to reconcile with standard model dark matter but arise naturally in SIDM.”
The work is described in a new paper, ‘Core-Collapsed SIDM Halos as the Common Origin of Dense Perturbers in Lenses, Streams, and Satellites’, published in the journal Physical Review Letters.
