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Reliance on Indirect Evidence Fuels Dark Matter Doubts

Pinning down the universe's missing mass remains one of cosmology's biggest challenges
Infrared picture of M31 from WISE



NASA/JPL-Caltech/UCLA

Most of the matter in the universe remains missing in action—at least, that's long been the standard cosmological paradigm.

Now, however, a small but vocal group of cosmologists is challenging the dark matter tenets of the widely accepted cosmological model, which holds that the universe is composed of roughly 70 percent dark energy, 25 percent dark matter, and only 5 percent normal (or baryonic) matter. Dark matter, whatever it is, exerts a gravitational pull but only interacts with ordinary matter very weakly, if at all, beyond that. Light seems to have no effect on dark matter—hence its name.

Evidence of dark matter's influence on the cosmos stretches back to the 1930s and has only gotten stronger in recent years. NASA's groundbreaking cosmology satellite, the Wilkinson Microwave Anisotropy Probe, has in the decade since its launch delivered a robust indirect detection of dark matter's footprint on the ancient echo of light known as the cosmic microwave background. And dark matter's effects are also inferred in gravitational interactions around clusters of galaxies as well as around individual galaxies themselves.

But the dark stuff itself has yet to be detected, either directly, in particle physics laboratories as a new subatomic particle, via neutrino telescopes also operating in the subatomic realm, or with concrete evidence of such hidden matter using telescopes operating in the electromagnetic spectrum. Some astrophysicists are hopeful that the Fermi Gamma-Ray Space Telescope will deliver corroborating, if still somewhat indirect, evidence for the mutual annihilation of dark matter particles in the galaxy.

"Dark matter comes about because people unquestionably find mass discrepancies in galaxies and clusters of galaxies," says Mordehai Milgrom, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel.

Stars at the very edges of spiral galaxies, for instance, rotate much faster than can be explained by Newtonian gravity alone; the picture makes sense only if astrophysicists either modify gravity itself or invoke additional gravitational acceleration due to an unknown source of mass such as dark matter.

"The mass of visible matter falls very short of what is needed to account for the gravity shown by these systems," Milgrom says. "The mainstream assumes it is due to the presence of dark matter, while others, like me, think that the theory of gravity has to be modified."

Milgrom's doubts about dark matter have long kept him on the fringe of professional astronomical circles. But as Rutgers University astronomer Jerry Sellwood notes, "people are beginning to think that we should have found some independent evidence for dark matter, and that hasn't happened."

That is arguably largely a result of the fact that dark matter is theorized to interact minimally with normal matter. But some observational campaigns have not seen the effects of dark matter where it is expected to exist. Theory predicts that spiral galaxies, including our own Milky Way, are enveloped by massive dark matter halos that provide the galaxy's missing mass. But the Milky Way's own dark matter halo has also yet to be detected, even indirectly. Its putative existence is primarily inferred from the anomalous rotations of satellite galaxies such as the Magellanic Clouds, which orbit the Milky Way too quickly to be explained by ordinary gravity alone.

More recently, there have also been predictions of a disk of dark matter that would reside in the galactic plane, co-rotating with the Milky Way itself. But in an analysis of the movements of some 300 stars located at least 6,000 light-years beyond the galactic plane, Christian Moni Bidin, an astronomer at the University of Concepción in Chile, and his colleagues conclude that there is "no compelling evidence" for such a dark disk. Given uncertainties in their own analysis, however, they acknowledge that such a disk's existence cannot be completely ruled out.

Moni Bidin, the lead author on a paper detailing the finding in the November 20 issue of The Astrophysical Journal Letters, says that one can always conclude that dark matter escapes detection because it has an exotic nature or unexpected properties. "But failing to detect it in indirect kinematical measurements such as ours," he says, "means finding a way out is harder."

Another dynamical complication comes from the so-called Tully-Fisher relationship, which describes the relation between a galaxy's luminosity and its rotation velocity: the higher the luminosity, the faster a galaxy rotates.

The measured rotation speeds on the outskirts of a spiral galaxy, Milgrom says, depend in "a very strict manner only on the total visible mass of the galaxy." But if the theory of dark matter is correct, then the speed of stars rotating on the galaxy's outskirts should also depend on the shape of the galaxy's dark matter halo.

"Dark matter halos should be lumpy, underinflated football shapes; not spherical," says Stacy McGaugh, an astronomer at the University of Maryland, College Park. "Statistically, that means we should see many [different galactic rotation] velocities for the same luminosity. We don't."

Instead, McGaugh says, the "baryonic tail wags the dark matter dog." In other words, astronomers can predict just what the galactic rotation curves will be from a given galaxy's stellar distribution. McGaugh makes the claim that if dark matter is dominant, observers shouldn't be able to predict the galactic rotation curves by what they see in normal luminous matter.

"Because each dark matter halo should be unique, you should see lots of variation in rotation curves for the same galaxy," he says. "You don't expect the kind of uniformity that we observe in hundreds of galactic rotation curves."

Even if dark matter raises questions on such large galactic scales, particle physicists are hopeful that it will be detected in the lab. If dark matter particles in the sun, for instance, undergo self-annihilation, then such annihilation events could create high-energy neutrinos that would potentially be detectable with ground-based neutrino telescopes.

Then there are detectors, such as the Xenon100 experiment at Italy's National Laboratory in Gran Sasso, built to register direct hits from particulate dark matter. Xenon100 is designed to search for the most favored dark matter particle candidate—the weakly interacting massive particle (WIMP)—by watching for signs that a WIMP has recoiled off an atom in a tank of liquid xenon. A recent analysis of an 11-day observing run in 2009, however, failed to identify any such dark particles, casting doubt on two competing groups' prior claims of possible dark matter signals.

One problem in making such detections is the uncertainty over dark matter's density in the local universe, says Chris Mihos, an astrophysicist at Case Western Reserve University. "Does the dark matter particle not exist," he wonders, "or are we just unlucky in terms of the local dark matter density?"

Current direct detection scenarios include potential dark matter particles with masses between one and 1,000 times the mass of a proton and with interaction "cross-sections" roughly one trillionth the size of a neutron.

After each non-detection, McGaugh says, theorists continually redefine the interaction cross-section of WIMPs to safely undetectable levels. This kind of behavior, he adds, can spark a never-ending game of leapfrog between experimental physicists and theoreticians, allowing them to continue business as usual without ever revising their cosmology.

"There is a lot of misplaced certainty in the dark matter model—a feeling that it's not 'if' we directly detect dark matter, but 'when,'" Mihos says.

Or, as McGaugh puts it, "Once you convince yourself that the universe is full of an invisible substance that only interacts with ordinary matter through gravity, then it is virtually impossible to disabuse yourself of that notion. There is always a way to wiggle out of any observation."

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