#positrons

Antimatter Mystery Likely Due To Pulsars, Not Dark Matter

“Whenever there's an unexplained phenomenon that we've measured or observed, it presents a tantalizing possibility to scientists: that perhaps there's something new at play beyond what's presently known. We know there are mysteries about our Universe that require new physics at some level — mysteries like dark matter, dark energy, or the cosmic matter-antimatter asymmetry — whose ultimate solution has yet to be discovered.

However, we cannot claim evidence for a new discovery until everything that represents what's already known is quantified and accounted for. By factoring in the effect of pulsars, the positron excess observed by the Alpha Magnetic Spectrometer collaboration may turn out to be explicable entirely by conventional high-energy astrophysics, with no need for dark matter. Right now, it appears that pulsars may be responsible for 100% of the observed excess, requiring scientists to go back to the drawing board for a direct signal that reveals our Universe's elusive dark matter.”

Over the past decade, the Alpha Magnetic Spectrometer experiment aboard the International Space Station has taken the best-ever measurements of cosmic rays directly from space. One of the surprises they saw was an excess of positrons, the antimatter counterpart of electrons, in unexpectedly large abundances at high energies. It offered a tantalizing possibility that, just perhaps, dark matter might be the culprit in this cosmic mystery.

Ask Ethan: Is Dark Matter The 'Aliens' Of Astrophysics?

“It's true and remarkable that the positrons implied by the HAWC data explain only 1% of the positrons observed by the other experiments, indicating that something else is responsible. When you see an observation that our conventional ideas can't account for, like an excess of astrophysical positrons, keep in the back of your head that it might be dark matter, exhibiting the long-sought-after interaction properties that have eluded us so far. But it's far more likely that some other astrophysical process is accelerating conventional, known particles to produce these effects. When you have a mystery in science, keep your mind open to a revolution, but place your bets on the mundane. And never, ever believe the hype that claims otherwise.”

When there’s something you can’t explain in space, there’s always an explanation. Before you posit some new, groundbreaking physics, however, the smart move is to examine the mundane explanations, like the matter and the objects we already know must exist. In a recent study, scientists sought to explain the positron/gamma-ray excess seen by astrophysical observatories like PAMELA, AMS, and Fermi by measuring the positrons from a couple of pulsars. While they saw a signal that was consistent with some positron emission, it was too little and too weak to be the answer. So if it isn’t these nearby pulsars, what is it? There could be a great many other possibilities, such as microquasars, magnetars, younger or older pulsars, or supernova remnants, among others. So why go straight to ‘dark matter’ as the primary explanation? There’s no good reason for it at all.