As researchers learn more about dark matter's complexities, it seems possible that our galaxy lives on top of a shadow galaxy without us even knowing it.
All known particles make up only a small fraction of the energy density in our universe, yet the Standard Model is extremely complicated: three forces, one Higgsed, one conﬁning, plus quarks and leptons organized into three generations. This model — the components of the visible universe — deviates markedly from any apparent principle of minimality. Yet, when considering the 85% of the matter in the universe that is dark, our usual response is to turn to minimal models of a cold, collisionless particle: a WIMP, perhaps, or an axion.
Two recent advances hint at just how much we have been missing about the dark side. In January 2012, Christoph Weniger, a physicist at the University of Amsterdam in the Netherlands, started noticing hints of a strange type of radiation around the center of our galaxy. To his excitement, he realized that the glow could be a signal of dark-matter particles smashing into each other and, in the process, transforming from something invisible to something visible. If so, it might finally be possible to go beyond simply deducing where dark matter gathers, and start learning how it actually behaves.
The other shoe dropped earlier this year, when a group of Harvard University theorists, including Lisa Randall and JiJi Fan, formulated a new theory of dark matter. One of the oddest things about Weniger’s detection, Randall notes, is that it was possible at all. “The signal would be too small for you to see under most reasonable models of dark matter,” she says. But Randall and her collaborators realized they could tidily explain the observation if there were a second type of dark matter out there: one that is not as diffuse as the dominant component of dark matter, but can interact with itself, just like visible matter. Clumps of this interacting kind of dark matter could form a disk, collapsing into a plane that could produce a correspondingly concentrated signal like the one Weniger saw.
Acknowledging that dark matter might have some of the same kind of diversity as visible matter may seem a minor adjustment. But it’s one that has, as Randall narrates in an excited staccato, “super-dramatic consequences.” If one variety of dark matter can clump together, it could form a panoply of previously unimagined dark structures. It could ball up into dark stars surrounded by dark planets made of dark atoms. In the most extravagant leap of possibility, this new kind of dark matter might even allow the existence of dark life.
Getting mainstream scientists to move past their light-matter chauvinism and take that shadow world seriously will require some highly convincing evidence. Weniger frets that the Fermi observations are too ambiguous to do the trick. “What one needs is more data with the same experiment to establish that the signal is there,” he says.
Harvard astronomer Douglas Finkbeiner is making an independent analysis of the Fermi data and likewise is finding that his results hang halfway between verification and falsification. “It’s the most frustrating possible outcome,” he sighs. “One option is that the signal is just not as bright as we thought it was.”
Randall is ready to forge ahead regardless of the fate of this particular observation. “The gamma ray line may not stay, but this just turns out to be independently such an interesting scenario, with so many interesting implications,” she says. And if our galaxy really does live right on top of a shadow galaxy, there are other ways to prove it.
Researchers are working on a new European space observatory called Gaia, scheduled to launch this autumn, which should perform a particularly telling test. Gaia will map the locations and velocities of about 1 billion stars within the Milky Way. Searching for anomalous motions could shade in the outlines of an invisible, dense disk of dark matter pulling on those stars.