Over the past couple of weeks, news sites and science blogs have been buzzing about a(nother) possible sighting of dark matter, based on a paper by Fermilab's Dan Hooper and NYU's Lisa Goodenough. This time, the reports say, cosmologists have found the " most compelling evidence of dark matter particles to date." So how excited should we be?
In the shadow of so much effort, and after many false "discoveries" over the years, one tends to greet new claims that dark matter is discovered with skepticism. However, Hooper and Goodenough's claim looks surprisingly solid (arXiv:1010.2752). But there are also good reasons to be cautious about the slightly odd character of dark matter they seem to have found, which doesn't match with theorists' expectations.
To put this into context, ever since Fritz Zwicky noticed in 1934 that the orbital velocities of clusters of galaxies made them look more massive than a tally of their stars and gas would indicate, the hunt for the missing stuff--later called dark matter--has been on. Many lives and much fortune have been spent digging for evidence for what the identity of dark matter is. Alternate theories of gravity have been proposed to do away with the troublesome matter. Dark matter has lots of potential acronyms to go by, from MACHOs (MAssive Compact Halo Objects) to WIMPs (Weakly Interacting Massive Particles).
Hooper and Goodenough are seasoned dark matter researchers and they have done a very careful, yet entirely conventional and straightforward analysis of the gamma rays that the Fermi Gamma Ray Space Telescope sees coming from the center of our galaxy (image above, courtesy of NASA). Because it's heavy and doesn't really interact with anything else, WIMP-y dark matter (theorists' favored version) tends to sink into the centers of galaxies. This means that if you're going to see any subtle signs that dark matter is around, the center of the galaxy is the place too look. The "W" in WIMP encodes the expectation that dark matter interacts only through the weak nuclear force, the second weakest fundamental force (after gravity)--the one responsible for nuclear decay. This weak interaction can also cause dark matter particles to run into one another and convert into something else, in a process called annihilation. So the idea is that where a lot of dark matter is, so also will there be more of the annihilation products of dark matter--whatever those might be.
Hooper and Goodenough's suspicion was that gamma rays might be something that annihilating dark matter gives off. This is not a new idea, but it's looking like it was a good hunch to follow. Now, the center of the galaxy produces gamma rays through a variety of perfectly ordinary astrophysical processes; otherwise, detection of even a single gamma ray from the center of the galaxy would have constituted discovery of dark matter, and we've been seeing gamma rays from the center of the galaxy for a long time. So what these researchers did was simply make some very reasonable assumptions about how much gamma ray flux there should be at the very center of the galaxy based on the amount seen a bit away from the center of the galaxy. They then compared this prediction to the actual flux seen by the satellite. Lo and behold, the satellite sees an excess--more gamma rays than expected! This is exactly the sign of there being something else going on they were looking for--and that something else, they claim, is dark matter annihilating.
But if they're right, then most theorists are wrong about the character of dark matter. For one thing, it's surprisingly light--7 GeV, or only about 7.5 times as heavy as the proton. Since we've never seen this stuff either directly or indirectly in the laboratory, most physicists expected the dark matter particle to be quite heavy, because it's easier to hide the effects of heavy particles in precision experiments than it is to hide the effects of lighter ones. The other strange character of the Hooper / Goodenough particle is that its annihilation channel--the kind of thing that it makes when it annihilates--is also a bit funny: To explain the observations, it would need primarily to go into tau leptons. The tau is the electron's biggest brother, heavier than the muon, the middle child in the lepton family. It's less natural to come up with a theory involving tau production because their heavy mass makes them less likely to be produced.
Nonetheless, the observational data are compelling enough that the theory mills are even now doubtless cranking out a raft of theories that can explain away these apparent oddities. The only possible flies in the ointment are, first, that the Fermi observatory's experimental team hasn't confirmed the result and, second, that rumors are drifting around that another dark matter experiment, the Cryogenic Dark Matter Search (CDMS) is about to publish a paper that would contradict the Hooper / Goodenough finding by conclusively showing that dark matter cannot have the properties that Hooper and Goodenough claim it needs to have. The Fermi experimental team's official silence is particularly puzzling: Presumably, if they agree, they should be jumping up and down with joy that theirs is the experiment that finally has sighted the elusive dark matter. Their public statements suggest that they suspect the answer is wrong, but can't prove it yet.
In any event, we're left for now in the fuzzy middle, where neither the skeptics nor the hopeful definitively have the upper hand. So for the mean time, stay tuned--it looks like the hounds of physics have the fox of dark matter in sight!