Do massive dark matter "nuggets" lurk in our galaxy?
On the numbers, there's nothing to stop dark matter condensing into solids 100 million times bigger than the sun, a US astrophysicist suggests. Richard A Lovett reports.
But does dark matter exist solely in this tenuous form, or can it condense into denser structures, analogous to those formed by normal matter?
Matthew Buckley, a theoretical astrophysicist at Rutgers University in New Brunswick, New Jersey, US, thinks such structures are theoretically possible. Within the next few years, he suggests, it might even be possible to detect them in our own galaxy via their gravitational effects.
Not that he's going so far as to say there might be dark-matter planets, suns, or even people - fascinating as that might be to a science fiction fan. What he's looking for are larger structures, with masses of one million to 100 million times that of the sun. (Since our galaxy contains about a trillion suns worth of dark matter, there could easily be many of these objects around … if they exist at all.)
On first impression, it seems an obvious idea. After all, if normal matter can condense into the gas and dust clouds that eventually form into planets, moons, suns, rocks, poodles, and people, why can't dark matter do the same?
It turns out not to be as simple as that, Buckley says. For dark matter to condense in this manner, he says, there has to be a way for the particles to lose energy, or "cool", as they fall toward each other. Otherwise, they just whizz past too quickly to clump together and head off on their own ways again, like ships in the night.
For ordinary matter, Buckley says, the thing that slows them down is the emission of electromagnetic radiation. This bleeds off energy, gradually slowing the particles' motion by enough to allow them to clump together.
Initially, Buckley says, he thought this was impossible for dark matter, but in a 2009 paper in the journal Physical Review D, he was part of team that calculated the theoretical feasibility of "dark radiation" that would serve the same function for dark matter.
But scientists know that the giant dark matter halos surrounding large galaxies can't collapse in this matter. If they could, they would have done so long ago and would no longer exist.
Buckley's newest realisation, described in a paper currently posted on the pre-print server arXiv, was that this didn't mean smaller blobs of dark matter couldn't condense. "If you fiddle with the parameters," he says, "you can make it so smaller halos cool and big ones don't."
What this means is that there might be "nuggets" of dark matter floating through a haze of dark matter - not just in the dark matter halo, but within the portions of the galaxy where we live.
How large these "nuggets" might be is an open question, based on such key factors as the masses of the dark-matter equivalents of electrons and protons, and the strength of their interaction with dark radiation. But by fiddling with these parameters, Buckley says, it's possible to create a model in which million-stellar-mass blobs of dark matter condense, while the sprawling trillion-solar-mass halo of the entire galaxy does not.
Buckley notes that to date, his dark matter model is very simple - far less complex than our understanding of normal matter's sub-atomic world.
"It's fun to build really complicated models," he says, "but until I have a hint that this is how dark matter works, then spending time writing increasingly baroque models for it is maybe not the best use of my time."
He also chose parameters to produce objects in the size range of 100 million solar masses to one million solar masses. In this case, the reason is simple: that's a size range in which their gravitational effects should soon be detectable with the European Space Agency's Gaia space telescope.
That telescope is currently in the process of a five to nine year mission to monitor the movements of a billion stars. Once the data is in, one of the things it should be able to show are loosely paired binary stars: pairs that orbit each other but are so far apart that the gravitational forces between them are barely strong enough to hold them together.
If dark matter objects of the size Buckley is looking for exist, they would have enough gravity that chance encounters with them should long ago have yanked apart most of these loose binaries. Thus, by looking at how many loose binary pairs exist, he says, it should be possible to put an upper bound on the number of dark matter objects roaming the galaxy.
Even if such objects prove not to exist, he says, it would be a useful find because it would rule out one form that dark matter could take.
"We've known about dark matter for a long time," Buckley says, but "we still don't really know what it is.
"I wrote down this model. I don't know whether it's true or false, but I believe I will be able to answer that question in the near future."
Brad Tucker, an astrophysicist-cosmologist at Australian National University who was not part of Buckley's study team, is impressed.
"I like this paper," he says. "It's particularly interesting that this was done with a 'vanilla/basic' model of dark matter particle physics."
With a more complex model, he says, "you can get even smaller or different effects."
He also agrees that Gaia is a perfect instrument for testing the theory. The best way to detect such dark matter objects he says, "is to see how they gravitationally influence small-scale objects such clusters, stars, and so on".
"The precision of Gaia means now this is possible," he adds.