Billions of particles of invisible "dark matter" are probably flying through your body right now, passing through the spaces between your atoms without a trace. According to conventional thinking, these particles should be somewhat less abundant during the winter and should peak around June 1. But a new study suggests this calculation is way off; the real peak is at the beginning of March.
Dark matter is thought to constitute almost 27 percent of the universe's total mass and energy, but its nature is a mystery. One of physicists' best guesses is that theorized particles called WIMPs (weakly interacting massive particles) make up this matter, but WIMPs have so far eluded detection. Whatever dark matter is, it appears to clump into large clouds called haloes that engulf galaxies, including our own Milky Way. As the solar system makes its regular progression around the galaxy, it flies through this halo, causing dark matter to bombard the sun and planets with a steady "wind," just as flies will hit the windshield of a fast-moving car. Earth, however, is also revolving around the sun. Astrophysicists have assumed that when it is moving against the direction of the dark matter wind (which happens to be during summer), we should see an uptick in dark matter particles of a few percent, and a corresponding decrease when Earth is traveling with the tide during winter.
But this accepted wisdom might be wrong. The new study suggests the pattern would be strongly affected by the gravitational effects of the sun, which have so far been dismissed as insubstantial. "As WIMPs stream through the solar system, the gravitational pull from the sun alters their individual trajectories, changing their direction and speed," says Samuel Lee of Princeton University, a co-author of the new paper, which was published January 3 in Physical Review Letters. "To our surprise, we found that the effects could be quite drastic." This gravitational pull could also cause the density of dark matter particles to vary by a few percent, and should shift the seasonal peak earlier by three months or so. "It's important to consider both effects," Lee says.
The sun's pull on dark matter is called gravitational focusing, because the sun acts like a lens to focus the paths of WIMPs toward it. The phenomenon depends on the energy and speed of WIMP particles: Fast-moving WIMPs would be less affected because they would race through the solar system too quickly for the sun's tug to make much difference. The sun could, however, divert to a greater degree lower energy, slower moving WIMPs from their paths. "The overall result is that the date of the maximum of the signal rate is shifted away from the canonical June 1 date, and moves towards March 1," Lee says.
The seasonal effects on dark matter can impact experiments that aim to detect dark matter particles directly. These are designed to catch the largely intangible WIMPs in the rare act of colliding with particles of normal matter. A telltale way of discriminating real WIMPs from mundane particles mimicking them is to find more of them at one time of year than another. Taking gravitational focusing into account could be crucial for identifying a true dark matter signal—only real WIMPs would peak around March every year. "There are many experiments and many relevant situations in which this effect could be very important," says theoretical physicist Peter Graham of Stanford University, who was not involved in the study. "I think this is a great piece of work that significantly enhances our understanding of the signal of dark matter in a direct-detection experiment."
A number of projects are closing in on WIMPs and other dark matter candidate particles and should be able to find them—or effectively rule them out—within a decade or less. One controversial experiment, DAMA/LIBRA (DArk MAtter/Large sodium Iodide Bulk for RAre processes) at Gran Sasso National Laboratory of the National Institute of Nuclear Physics (INFN) in Italy, claimed in 2008 that it had detected WIMPs and saw more of them in May than at other times of the year. Many physicists doubt the find, especially because its results conflict with the null findings of other detectors. Lee and his collaborators point out that the DAMA/LIBRA results do agree with their prediction that the dark matter peak should be offset from June, toward March. "The situation is kind of murky," Lee says. "Using this angle of gravitational focusing as a second check will help us know if these events are coming from dark matter. And future experiments should definitely be able to make use of this effect as a second check."