If dark matter isn't made of WIMPs, could neutrinos or axions fit the bill? What if it's not a particle at all but a strange modification of gravity?
ROADS may soon diverge in the dark matter wood, and some physicists want to take the ones less travelled.
The most promising candidate for a dark matter particle could be about to show itself at last, as it is running out of places to hide. But should the hunters fail to bag one of these WIMPs, or weakly interacting massive particles, the search for dark matter could be thrown into crisis.
At a meeting in Cambridge, Massachusetts, last week, researchers debated the best paths forward into the wilder landscape of less-favoured candidates, from alternate particles to changes to our theory of gravity.
"It's really refreshing," says Lisa Randall at Harvard University. "For years I went to conferences where people said, 'We know what dark matter is and we're just cutting out the parameter space'. I thought that was strange, because we really don't know what dark matter is."
So far we have only sensed dark matter's presence through its gravitational effects. But theory says that WIMPs should also brush shoulders with normal atoms occasionally, producing signals we can detect. WIMP champions are pinning their hopes on more sensitive underground detectors that are running or under construction.
"This is a golden decade for dark matter because of detector sensitivity," says Kathryn Zurek at the University of Michigan in Ann Arbor.
The trouble is that background noise can prevent usnoticing the impact of a WIMP. Beyond a certain sensitivity limit, the signal would be swamped by neutrinos, nearly massless particles that are constantly streaming from the sun and from particle collisions in our atmosphere. After just a few more upgrades, WIMP hunters will hit this limit and the desired particles may no longer be detectable.
Indirect methods for spotting WIMPs offer the best chance of a sighting. When WIMPs collide they should annihilate, shattering into other particles. This includes gamma rays, and an excess of these high-energy photons spotted in the centre of our galaxy seems to fit nicely with the simplest models for WIMPs. But one criticism is that the rays could just as easily come from fast-spinning dead stars called pulsars.
So if not WIMPs then what? Some theories modify the classic particle, changing its properties and offering new places to look. Others focus more on runner-up particles, such as axions or sterile neutrinos. And still others say dark matter might not exist at all, and we just need to modify the laws of gravity (see right).
"It's always possible WIMPs are just around the corner," says Avi Loeb at Harvard University. "But when there is no evidence, you have to be careful. We're looking for a black cat in a dark room."
Welcome to WIMP city
There are good reasons to build up a metropolis around WIMPs.
Our best models support the theory that dark matter is the scaffold around which normal matter formed galaxies and clusters. If so, dark matter must have existed since the dawn of the universe.
Early theories hinted that dark matter particles should annihilate themselves, so physicists knew they must have certain properties, in order for enough of the particles to still exist and make up the amount of dark matter we detect today. A particle that interacts via gravity and the weak force but not with photons fits the bill – and that is a WIMP.
"There's a simplistic beauty to the WIMP model. That's why it's so compelling," says James Bullock at the University of California, Irvine.
Signs of exactly this kind of particle are showing up as an excess of gamma rays coming from the galactic centre, says Dan Hooper at the Fermilab in Batavia, Illinois. But alternative explanations have not been ruled out, and other detection techniques have yet to pan out – like waiting for a WIMP to smack into an underground detector such as LUX in South Dakota (pictured above) or creating one at a particle accelerator, for example.
If WIMPs remain elusive even as we whittle down the places to look, the hypothetical particles become less attractive candidates, says Bullock. "Then you start to worry."
With classical WIMPs in a bind, theorists have started expanding their descriptions of the particle, creating a sprawling landscape of WIMP-like alternatives.
One idea is asymmetric dark matter, which would invoke a dark anti-particle. We exist because something in the early universe allowed more matter than antimatter to survive after the big bang. The mechanism for this asymmetry is still unclear, but if something similar happened for dark matter, it should be made of lightweight particles of about 5 to 10 gigaelectronvolts – just below what WIMP detectors can see.
Other models say that dark matter may be a mix of classic WIMPs and WIMP-like cousins that would interact with each other via a hypothetical dark force. Self-interacting dark matter would be harder to find in detectors, but it would build structures.
Some astronomers are already hunting for signs of this shadow cosmos in the motions of stars and colliding galaxies.
In the absence of WIMPs, the runners-up are axions, which behave more like an all-encompassing field than single particles. Theoretically speaking, axions are just as likely as WIMPs but are much harder to find.
Classical WIMP detectors, such as the XENON100 project at Gran Sasso National Laboratory in Italy (pictured below), can also hunt for axions. The best limits so far have been set by the ADMX experiment at the University of Washington in Seattle, but it is only sensitive to a small range of possible particles.
Last April, Peter Graham at Stanford University, California, and his colleagues devised another way to hunt them using the same technology as MRI scanners. "There is still a lot of work to be done, but I think they deserve a similar effort."
Neutrinos seem like natural candidates for dark matter: they have mass, yet they flit through normal matter as if it weren't there. The three known types of neutrinos don't add up to enough mass to explain all the dark matter we see in the universe. But what if there is a fourth flavour of the particle?
This sterile neutrino could fit the bill. Hints of it have popped up and vanished again in several experiments, including the Borexino detector at Gran Sasso (pictured below).
A whiff of X-rays from the centre of the galaxy could be yet another sign of them. In February, two teams saw extra X-rays in data from two telescopes, and a sterile neutrino with a mass of 7 kiloelectronvolts could explain the sighting. If confirmed, the next test would be to see if there is enough of these particles to account for dark matter.
It's still possible that the search for any sort of particle is misguided. Instead, modified Newtonian dynamics, or MOND, suggests rewriting one of our most cherished theories: gravity.
The first evidence for dark matter came from the ability of rotating galaxies to hold themselves together, even though they do not have enough mass in their planets, stars and gas to act as the only gravitational glue.
According to MOND, gravity simply works differently on galactic scales than on the scale of solar systems, and we just need to figure out how.
Some observations of mass in dim galaxies and the motions of dwarf galaxies agree better with MOND than with Newtonian physics, a mystery that convinced Stacy McGaugh at Case Western Reserve University in Cleveland, Ohio, that it could be the way to go. But starting afresh with gravity continues to make many physicists uncomfortable – including some of MOND's grudging supporters.
At the Cambridge conference (see main story), McGaugh made the case for MOND but then left his colleagues with an impassioned plea: "Please detect this stuff! Put me out of the misery of having to give this talk over and over again!"
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