Astrophysics

A message from the dark side

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Both astrophysicists and particle physicists are in on the hunt for the elusive dark matter that is thought to pervade the Universe. A high-altitude balloon-borne experiment offers the latest hints as to what it could be.

The ATIC balloon-borne electron detector ready for launch on the Ross Ice Shelf of Antarctica. Credit: T. G. GUZIK

Humiliating as it may sound, you, me and everything we see — the Earth, Moon, Sun and stars — may be little more than cosmic contamination. Most of the 'stuff' in the Universe is thought to be in the form of invisible and elusive particles of dark matter. To date, the existence of this cosmic exotica has been inferred through its gravitational effects. But on page 362 of this issue, Chang and collaborators report1 on a surprising bump detected in the spectrum of celestial electrons that could be a more direct signal of this mysterious substance — or there may be other intriguing explanations.

For 75 years, astronomers have collected data that point to the existence of a type of non-luminous matter that outweighs normal ('baryonic') matter by a factor of about six. Several independent lines of evidence seem to make its reality compelling. For one, the measured rotation speeds of stars and gas within a typical galaxy are such that the galaxy would simply fly apart were it not for the gravitational anchor of copious non-baryonic dark matter. Similarly, the orbital speed of individual galaxies in a galaxy cluster also supports its presence. The slight gravitational warping of space, as evidenced by gravitational lensing (which bends the light from distant objects as the light travels towards Earth), is yet another indicator of its existence2. Dark matter also seems to be a necessary ingredient for making sense of the fluctuations detected in the structure of the cosmic microwave background, the pervasive 'echo' associated with the young, hot Universe3.

However, alternatives to dark matter, such as a modification to the law of gravity (known as MOND4, for 'modified Newtonian dynamics'), have been proposed and can also explain some of the data well. So even though there is not universal consensus regarding the reality of dark matter5,6, on balance the case for its existence seems robust.

But what could these mystery dark-matter particles be? All known fundamental particles — even the ever-elusive neutrinos — are excluded as dark-matter candidates, so we are forced to look beyond the 'standard model' of particle physics. The most promising candidates so far are WIMPs, short for weakly interacting massive particles. And the top suspects within this generic class come in two flavours: the neutralinos, which arise in supersymmetry theories, and Kaluza–Klein (KK) particles, which emerge in theories involving extra dimensions7 and which are named for their first proponents in the 1920s.

Unlike astrophysicists, who look up at the sky and measure the cumulative gravitational effects of dark-matter particles on enormous, galactic scales, particle physicists try to approach the problem from the other end by using precision laboratory experiments situated deep underground. The trouble for these folks lies in the first two letters of the acronym WIMP: these particles are extremely difficult to detect. The putative signal given off when WIMPs interact with normal matter in the laboratory is exceedingly feeble, and it can easily be masked by the background from natural radioactivity and cosmic rays, even in the most exquisitely sensitive of experiments. For this reason, no direct signal of WIMP interactions has yet been confirmed.

Enter Chang and colleagues1 with the latest report. Using a high-altitude balloon-borne detector called ATIC (for advanced thin ionization calorimeter), they have detected a significant bump in the smooth, background spectrum of galactic electrons, confirming earlier hints8 of its existence. This feature is located at an energy of about 620 GeV (1 GeV is 109 electronvolts and, by mass–energy equivalence, corresponds roughly to the mass of a proton). And it is consistent with the type of signal expected when KK WIMP particles interact and annihilate into electron–positron pairs. (The positron is the electron's antiparticle: essentially, an electron but with a positive charge.) The process basically amounts to two KK particles disappearing from 'the dark side' and appearing in our realm in the form of an electron–positron pair. The signature electrons can then be detected and measured.

What makes the ATIC detection especially intriguing is that the 620-GeV energy of the peak is roughly the mass of KK WIMPs expected from particle physics theories7. But in a way, the intensity of the signal is almost too high. To explain its strength requires a large enhancement of local dark matter, such that the Solar System would be whizzing through (or at least near) an especially dense clump of dark matter.

But could there be other explanations for the bump ATIC detected in the electron spectrum? It is certainly possible that known astrophysical objects, such as nearby supernova remnants, spinning pulsars9 or, possibly, microquasars10 — whether catalogued or not — are responsible for the feature around 620 GeV. And let's not forget that a completely new type of astrophysical object could also produce the detected electron excess; after all, pulsars were discovered only in 1967, and until 1992 we were blissfully unaware of microquasars.

However, all is not lost in our attempts to determine the real source of the ATIC electron feature. Luckily, the expected spectral signature of electrons from nearby KK WIMP annihilation is very distinctive: it shows a gradual rise to a sharp peak and a 'cliff-like' drop-off at the high-energy end (see Fig. 4 on page 364). With a longer observation, or a bigger detector, which would markedly increase the number of electrons detected, one ought to be able to say with some confidence whether the ATIC feature is more like a regular bump (for example, from an astrophysical source) or the sharp, discontinuous feature expected from nearby dark-matter KK WIMP annihilation. In fact, existing γ-ray telescopes, both on the ground and in space, should also be able to search for this possible discontinuity in the spectra of integrated diffuse emission they detect.

In the near future, neutrino telescopes on the scale of cubic kilometres, such as IceCube, buried deep beneath the cold, clear Antarctic ice, could also aid our sleuthing for dark matter11. And we may even have our very own dark-matter factory soon: the Large Hadron Collider is due to start regular operations in the next year or so, and its data will be combed through thoroughly in search of any dark-matter signals.

Squeezing the dark-matter problem from both sides is certainly the way to go: astrophysicists staring up and particle physicists working deep underground may finally unveil the secrets of the dark side. And if it turns out that the ATIC detection really is due to an elusive WIMP about 600 times the mass of a proton, it will go to show just how truly insignificant we are — the wimpy, overweight dark side may have us beaten.

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Butt, Y. A message from the dark side. Nature 456, 329–330 (2008) doi:10.1038/456329a

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