We live in a universe dominated by normal matter. This wasn’t always true — right after the Big Bang, in fact, nearly equal amounts of matter and antimatter were created, and then soon afterwards destroyed as they annihilated each other. But because the amounts of matter and antimatter weren’t exactly equal, this annihilation was unequal, too, and normal matter won out.
There is, however, still a small amount of antimatter in our universe, and there seems to be an unexplained excess of it. The reason for this antimatter surplus has long been sought, and now it seems astronomers may have finally arrived at a conclusion: It’s not dark matter responsible for the excess, but plain old pulsars.
The antimatter surplus refers to the fact that a greater number of high-energy anti-electrons, called positrons, than are expected have been detected in space. These detections have been confirmed by several observatories over the past decade, including the Alpha Magnetic Spectrometer on the International Space Station.
Based on our current astrophysical models, the ratio of high-energy positrons to electrons should be tipped significantly in the electrons’ favor. But observations show that there’s an unexpected increase in the ratio of positrons to electrons at energies between about 10 and a few hundred giga-electron volts (GeV).
Astronomers have developed two possible explanations for this excess. One explanation says that dark matter particles (such as weakly interacting massive particles, or WIMPs) randomly annihilating each other could produce positron-electron pairs. Because dark matter accounts for up to 85% of the matter in the entire universe, such interactions could lead to the observed positron excess.
The other explanation isn’t as exotic: The excess could be produced by pulsars and their extremely powerful magnetic fields. These magnetic fields accelerate particles around the pulsar to such high energies that they can generate electron-positron pairs, again bumping up the number of positrons counted by observatories.
Now, a team led by Dan Hooper of the Fermi National Accelerator Laboratory has used gamma-ray data from the High-Altitude Water Cherenkov Observatory (HAWC) near Puebla, Mexico, to narrow in on the excess positrons’ source. They set out to determine just how much of the positron excess could be produced by pulsars. In a recent press release, Hooper explained that “Before the HAWC observations, we didn’t know whether pulsars made up 0.1 percent of the excess or 100 percent.”
Using HAWC, the team observed the famous Geminga pulsar and discovered a halo around the stellar remnant that emitted high-energy gamma rays. This halo, they concluded, is produced by high-energy electrons and positrons slamming into and boosting the energy of photons emitted from the pulsar until they appear as gamma rays.
Still, there’s another piece to the puzzle. The electrons and positrons responsible for the halo around Geminga would technically be too energetic (tens of thousands of GeV) to explain the range of energies where the positron excess is actually observed (only up to a few hundred GeV). But based on the amount of high-energy electrons and positrons, Hooper’s team then calculated the amount of lower-energy positrons that should also be created, which would contribute to the excess.
In science, just one observation is never enough. So Hooper’s team repeated the experiment for a second pulsar, B0656+14. They then extrapolated the number of positrons produced to the total number that would be created by the likely thousands of pulsars inhabiting the galaxy.
The result, Hooper says, is that pulsars are the probable cause for “much, if not the entirety” of the observed excess, rather than the more exotic explanation of dark matter.
However, the case may not be closed as neatly as it seems. Hooper did admit that the team’s results are still uncertain, and up to 50 percent of the positron excess may still be due to dark matter, rather than pulsars. More sensitive low-energy gamma-ray observations are needed to further pin down the pulsars’ contribution. Such observations will be possible with the upcoming Cherenkov Telescope Array, due to begin construction next year.