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Ethereal Wisps

Another possible glimpse of dark matter

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Another possible glimpse of dark matter

One of the great mysteries of the universe is what it is made of. The world, as perceived by those who inhabit it, consists of tangible matter, but this explanation cannot be complete. Most galaxies rotate at a speed that should cause them to fly apart if all that holds their visible matter together is gravity. Physicists think the universe is stuffed with invisible dark matter composed of particles very different from the ones that make up visible matter, and that the gravity of this dark matter holds galaxies together. Now a team of physicists think they may have seen direct evidence of this ghostly material.

Piergiorgio Picozza of the University of Rome Tor Vergata and his colleagues began by investigating a second great mystery: why the universe exists at all. The Big Bang created equal quantities of matter and antimatter. When the two meet, they annihilate each other. So the universe should have been destroyed soon after it was created. That it has not suggests that something must have happened to much of the antimatter, allowing matter to clump together to form stars, planets and, eventually, people.

Dr. Picozza and his colleagues decided to look for clues in cosmic rays. These “rays” are actually streams of particles from outer space that constantly bombard the Earth’s upper atmosphere. Most of them are protons, one of the core constituents of any atomic nucleus, but others are more exotic. Dr. Picozza and his colleagues wanted to examine any antimatter contained in the cosmic rays. To do so, they used a particle detector aboard a satellite.

First they found some relatively mundane low-energy positrons, the antimatter equivalent of electrons. Such positrons are produced by what researchers call secondary sources—mainly collisions between the protons contained in cosmic rays and atoms drifting through space. However, Dr. Picozza and his colleagues then discovered high-energy positrons. These are much more exciting because they must have been produced by primary sources, such as exotic stars or, perhaps, the disintegration of dark matter. Indeed, the team found that, at the highest energies they could measure, the ratio of positrons to electrons was far higher than at lower energies.

Exactly what might be causing the increase is another mystery. It could be a rapidly rotating, extremely dense neutron star called a pulsar. It could be a microquasar, a system in which a sun-like star and a neutron star orbit one another. It could also be explained by the annihilation of dark matter, the physicists report in Nature.

Physicists have long wondered about dark matter. One of the most popular explanations is that it consists of weakly interacting massive particles, also known as WIMPs. As the name suggests, these particles are massive, which is why their presence is revealed by gravity. By calling them “weakly interacting”, physicists mean that the particles experience another of the four fundamental forces, namely, the weak nuclear force that is responsible for radioactive decay.

Dr. Picozza and his colleagues think that the high-energy positrons that they have detected may come from a WIMP either annihilated by a collision with an anti-WIMP or simply decaying, as many particles do after a time. They are now using their satellite-borne particle detector to examine what happens at even higher energies. This should help them to discriminate between the different explanations.

If the alternatives can be ruled out, then this will be the latest glimpse of dark matter. Other possible previous sightings include reports of collisions between clusters of galaxies that would appear to have separated dark matter from its visible counterpart. Indeed, many physicists now suspect that dark matter may provide the scaffold on which visible matter is arranged. When the Large Hadron Collider finally begins work in September at CERN, the European particle-physics laboratory in Geneva, it should also provide more clues. That the universe is filled with matter people cannot see—matter that simply passes through pretty much everything undetected—is hard to accept. But evidence is now accruing to suggest that it is true.

Source: The Economist