A cosmic engine is hurling strange particles at Earth—and new observations, published today in the journal Science, are complicating the hunt for the culprit.
In 2008, the space-based instrument PAMELA detected an overabundance of particles called positrons in Earth’s atmosphere. These particles are a form of antimatter, a substance that is the opposite of normal matter. When an antimatter positron encounters its opposite particle, the two can annihilate one another and vanish in a tiny puff of energy that often includes gamma-rays, which scientists can detect. (Read about the first antimatter found orbiting Earth.)
Figuring out the source of this weird positron excess is exciting, because it helps scientists understand the highest-energy phenomena in the nearby universe, which in turn could help solve some of the big mysteries in physics.
For a long time, scientists thought the particles came from nearby pulsars, or the rapidly spinning corpses of formerly large stars. But according to the team behind the new study, the results point away from the leading candidates, a pair of pulsars less than a thousand light-years away.
Instead, the team argues, something more exotic could be at work, such as interactions among the mysterious substance known as dark matter.
“When I started this work, I really believed it was pulsars,” says study author Rubén López Coto of the Max Planck Institute for Nuclear Physics. “But these two pulsars actually cannot provide enough positrons in order to account for this positron excess.”
However, that suggestion is already sparking controversy among astronomers and physicists, some of whom are not ready to give up on the pulsars. Though the team’s measurements are solid, their interpretation of the data is problematic, says Dan Hooper of the Fermi National Accelerator Laboratory.
“I am as convinced as ever that these pulsars are contributing very significantly to the local positron excess, and very well could dominate it,” Hooper says.
Positrons are the antimatter opposites of electrons, which are a fundamental part of the ordinary matter we interact with on Earth. We don’t see a lot of positrons in nature here on Earth. But in the cosmos, the roiling, violent environments surrounding dead and dying stars can produce electrons and positrons in pairs. Sometimes, those particles are slung through the void and bounced around by magnetic fields until they bump into something.
Pulsars, for instance, basically act as whirling particle accelerators. Sometimes spinning more than 700 times a second, they churn up their surroundings and smash particles into one another; this includes electrons and positrons caught in the pulsar’s whirling grasp.
If those positrons have enough speed and energy, they can escape the pulsar’s environment and take a trip through the cosmos, sometimes ending up at Earth—at least, that’s the leading explanation for the positron excess. Otherwise, they often crash into photons and produce gamma-rays, which scientists on Earth can easily detect.
The new wrinkle in this tale comes courtesy of observations made by the High Altitude Water Cherenkov Gamma-Ray Observatory. Consisting of 300 large water tanks, the observatory is parked on a saddle between two volcanoes in Mexico’s Parque Nacional Pico de Orizaba. When extremely high-energy particles from space strike the water tanks, the minute flashes of light they generate carry both a distinct signature and point the way toward home.
Since 2015, HAWC has been gathering these particles and studying their cosmic sources. And 17 of those months included observing high-energy gamma rays slung toward Earth by two nearby pulsars, called Geminga and PSR B0656+14 (also referred to as Monogem).
By back-tracking from the gamma-ray observations, the HAWC team could calculate how fast particles were moving in the areas around Geminga and Monogem. It turns out that the pulsars’ positrons are not moving fast enough to make it to Earth, López Coto says.
That could mean, he and his colleagues suggest, that the interstellar medium in the direction of those pulsars is particularly murky and prevents particles from rapidly hitting the road.
If so, that’s a problem: Scientists think Geminga and Monogem produce a substantial amount of Earth’s overabundant positrons; they’re near enough and old enough for that to make sense. But if those two are ruled out, the team argues, “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.
But Hooper, whose work has been instrumental in blaming pulsars for those pesky positrons, isn’t ready to concede defeat. He doesn’t think the team’s interpretations about a generally murky interstellar medium are correct. Previous observations using PAMELA and an instrument mounted on the International Space Station agree with the idea that positrons and other particles move efficiently through the space near Earth.
And though the study points to alternate explanations for the excess—including supernova remnants and annihilating dark matter particles—none of them seem to make much sense. Supernova remnants can be ruled out, Hooper says, for the same reason the team disfavors pulsars.
“If the interstellar medium is really as impenetrable to these particles as they’re saying, and Geminga and Monogem are both 250 or so parsecs away, the nearest supernova are at least that far away, and they would suffer from exactly the same problems.”
Tracy Slatyer, a theoretical physicist at MIT, is similarly unconvinced that dark matter particles are a reasonable explanation. When this positron excess first appeared, scientists thought it could be a signature of dark matter particles annihilating one another and producing both matter and antimatter.
But dark matter is thought to make up the vast majority of the universe’s mass, so if that were true, the signatures of such annihilation should be everywhere, and they aren’t, Slatyer says.
“My personal bet is that it’s probably not dark matter annihilation. But if somebody told me, I’ve come back from a hundred years in the future with a time machine, and that’s the case, I’d be surprised, but wouldn’t be like, no, that’s impossible,” she says.
“Assuming I even believed them on that time machine front.”