For years, scientists have had a tough nut to crack when it comes to antimatter and its quantity – why we see more matter than antimatter around us if there should have been equal amounts created in the Big Bang. It’s also a problem that more antimatter is reaching Earth than it should, and the latest pulsar observations not only don’t solve this puzzle, they make it even worse.
For years, scientists have had a tough nut to crack when it comes to antimatter and how much of it there is – why we see more matter than antimatter around us when equal amounts should have been created in the Big Bang. The problem is also that there is more antimatter reaching the Earth than there should be, and recent observations of pulsars not only do not solve this puzzle, but make it even worse.
9 years ago scientists operating the detector with the graceful name PAMELA (extension of the acronym is less graceful – Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) noticed that in cosmic radiation reaching the Earth there are too many positrons – the equivalent of electrons in antimatter. There were as many as ten times more of them than theory predicts, and two hypotheses arose that could explain the excess – one pointed to pulsars, very fast-rotating neutron stars, or objects that are burned-out cores left over from supernovae, and the other to an even more mysterious source – namely, dark matter.
The positrons themselves do not pass through the Earth’s magnetic field (usually) so they are hard to detect, but they interact with the Big Bang’s residual microwave background radiation and give the photons that make up the radiation high energy, creating a source of particles that can be detected at the High-Altitude Water Cherenkov (HAWC) Observatory, a water-filled tank near Mexico City that is packed with optical sensors.
When particles (photons) are accelerated to speeds higher than the speed of light for a given medium – making the water glow blue, a phenomenon called Cherenkov radiation, it is often observed in water that is the coolant of nuclear reactors. And by studying this radiation it is possible to determine the source of the cosmic radiation that is its source, the positrons we are interested in.
And the data collected at HAWC indicate that it can’t come from pulsars – to reach us it would have to have left before the pulsars formed.
So this is a blow to the hypothesis that antimatter can come from pulsars, so the dark matter theory remains the leading one at the moment. Although this is not good news for us, because we know almost nothing about dark matter.