Crab nebula. Credit: NASA.

In 2008, physicists found that the number of high-energy positrons — the antiparticle or the antimatter counterpart of the electron — hitting Earth was three times greater than the Standard Model predicted. Some scientists have suggested that nearby pulsars, which expel copious amounts of positrons as they spin erratically, could account for the extra positions. One new study, however, proposes that this antimatter may be the result of decaying particles of dark matter. 

Stranger than fiction

Dark matter is an elusive and hypothetical substance that is supposed to comprise roughly 5/6 of all matter in the universe. Scientists have never been able to directly observe dark matter, which seems to be invisible to current means of observation, but they do have good reasons to believe dark matter exists judging from the gravitational effects it exerts on ordinary matter.

Though not as weird as dark matter, antimatter is also another exotic physical quirk. You’ve likely heard a lot about antimatter in the science fiction universe. Star Trek’s starship Enterprise uses matter-antimatter annihilation propulsion for faster-than-light travel and in the famous book Angels and Demons, (spoiler alert) Professor Langdon tries to save Vatican City from an antimatter bomb. Unlike dark matter, though, we know for sure that antimatter is real.

Every particle of ordinary matter has an antimatter counterpart which is equal in mass but opposite in charge. When the two kinds of particles meet, they annihilate each other while releasing energetic photons called gamma rays.

There are many unsolved riddles surrounding antimatter. For instance, antimatter should have annihilated all ordinary matter during the Big Bang. Scientists think the reason why galaxies, stars, and eventually we exist in the first place is that there was one extra matter particle for every billion matter-antimatter pairs.

Antimatter is closer to us than most people think. Small amounts of antimatter — at a rate ranging from less than one per square meter to more than 100 per square meter — constantly rain down on the Earth in the form of cosmic rays, energetic particles from space. Antimatter can be found closer yet.  The average banana (rich in potassium) produces a positron roughly once every 75 minutes. That’s because potassium-40 will occasionally split out a positron in the process of radioactive decay.

When researchers working with the space-based instrument PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) detected an overabundance of positrons hitting Earth’s atmosphere, things got weird. For a long time, scientists have thought that all of this extra positrons are sourced from pulsars.

A pulsar is a rapidly rotating neutron star which emits electromagnetic signals. These objects are also very dense, concentrating the mass of the sun in a diameter comparable to that of a large city. Pulsars radiate two steady, narrow beams of light in opposite directions from their poles. Although the light from the beam is steady, pulsars appear to flicker because they also spin really fast, and so constantly spin these polar rays around. For this reason, pulsars have earned the nickname of ‘cosmic lighthouses’.

But now, writing in a recently published paper in the journal Science, a team of researchers claims these candidates —  a pair of pulsars less than a thousand light-years away — aren’t the likely culprit.

“When I started this work, I really believed it was pulsars,” lead author Rubén López Coto of the Max Planck Institute for Nuclear Physics told National Geographic. “But these two pulsars actually cannot provide enough positrons in order to account for this positron excess.”

A new suspect: dark matter

HAWC detector consisting of 300 large water tanks, each with four photodetectors. Credit: Jordan A. Goodman

Coto and colleagues refer to observations made at the High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), a science laboratory nestled at the base of a volcano four hours away from Mexico City. When gamma rays from annihilated positrons interact with the atmosphere, they generate showers of particles that hit the planet’s surface. Inside HAWC, scientists have placed 300 corrugated steel tanks filled with water. When the extremely high-energetic particles strike the tanks, they generate flashes of light which can be used to distinguish the signature and origin of the gamma rays that triggered the particle cascade.

By back-tracking from the gamma-ray observations, the HAWC team found that the pulsars’ positrons were not moving fast enough to arrive on Earth. This implies that the interstellar medium between the two pulsars and Earth is likely murky, preventing the positrons from reaching Earth.

“HAWC scans about one third of the sky overhead, giving us the first wide-angle view of high-energy light from the sky,” says study co-author Jordan Goodman, a particle astrophysicist at the University of Maryland, College Park. “Before HAWC there were observatories that were highly sensitive to high-energy gamma rays, but they had relatively limited fields of view. With HAWC, we can see how gamma rays are diffusing from these pulsars across wide regions of sky.”

If the two pulsars, called Geminga and Monogem, aren’t the source of positron overabundance in Earth’s atmosphere, then “other pulsars, other types of cosmic accelerators such as microquasars and supernova remnants, or the annihilation or decay of dark matter particles” must be considered.

Not everyone is convinced that dark matter could be the source, though. Since dark matter is literally everywhere, it follows that the signature generated by dark matter particles annihilating one another should be ubiquitous — but they’re not. It could be that the positrons are sourced from even more unconventional sources such as high-energy particles produced by black holes.