A new theory suggests dark matter could coalesce into massive structures.
A new theory says physics might allow dark matter to form into compact objects, like planets. But don't try to walk on such a world. (Credit: NASA) Dark stars may not just be for Grateful Dead fans anymore. In a new paper uploaded to arXiv, Rutgers University astrophysics professor Matthew R. Buckley puts forth a truly wild hypothesis: It might be possible to build worlds out of dark matter. But the whole thing came to him from an unusual angle: He wanted to prove that dark matter structures were impossible. In a blog post, Buckley outlined his thinking. He likes sci-fi; he also likes to pick apart bad science. Dark matter planets seemed like an impossibility. But as he delved into the actual math of it, he realized he was wrong. So what was the initial thinking behind why dark matter couldn’t form a planet? It goes something like this: We have indirect evidence of dark matter but don’t know what it’s made out of. When astronomers map our galaxy and other galaxies, they can take out all known objects and visible gasses and discover invisible structures indicating big clouds of dark matter. But we know these aren’t normal (aka baryonic) matter. They clump together in a way that suggests that they don’t coalesce neatly like normal matter. According to Buckley, this may be due to a lack of a cooling mechanism. As he writes, normal matter can be slowed down by photons enough to gather together and accumulate. But under typical conditions, dark matter would just have a series of false starts and stay clumpy, diffuse clouds. “If there is more internal physics for the dark matter, then you can imagine that as the clumps of material start accreting together, you could get some process that releases energy, like fusion does for the baryons,” Buckley says. He and his coauthor, Anthony DiFranzo, didn’t speculate too much on all the internal physics of this. But, they believe “if we want to start making specific predictions for what to look for, we might have to start thinking more about all these different possibilities, since a source of energy in the dark sector will change how dark matter clumps distribute themselves.” So a mechanism for cooling dark matter might not work on a scale to form whole galaxies or other natural megastructures, but it could, under this model, form smaller objects. There have been proposals before for stars and other objects that utilize dark matter alongside baryonic matter to produce a weird chymera. But this model would likely be all or mostly dark matter, rather than mostly baryonic with neutralinos inside producing weird behavior. They suggest that a force of dark electromagnetism could sufficiently cool dark matter to form objects from these halos of dark matter. The largest possible dark matter objects would be one million times the mass of the sun. That’s as big as the largest intermediate mass black holes or the very smallest supermassive black holes. Dark matter might also form something like a dwarf galaxy or cluster of dark matter objects. But according to this paper, such large objects and structures — if they exist — may have broken down over time, leaving behind much, much smaller objects. “The most massive of these objects would end up collapsing to black holes because there would likely not be any internal forces strong enough to arrest that collapse, as there is for the baryons,” Buckley says. “The black holes would be like any other black hole: gravity doesn't distinguish between dark matter and baryons, so a black hole is the same regardless of the material that goes into it.” Finding any of these objects could be difficult. Dark matter doesn’t interact much with baryonic matter … and that includes photons, meaning there’s no light source given off. “There is a force like electromagnetism, but it isn't electromagnetism,” Buckley says. “So you can't actually see the clump of frozen dark matter gas or planetoid or whatever the dark matter shapes itself into, because it isn't interacting with light.” So what would it be like to step on a dark matter planet? “If you tried to land on the surface, you'd sink right through, since there's no electrostatic repulsion between your atoms and the dark matter,” Buckley says. “You'd feel the gravity of the object though, so you'd fall in.”