The International Space Station carries a suite of instruments conducting scientific experiments and measurements in low-Earth orbit. One of them is the Alpha Magnetic Spectrometer (AMS), which studies antimatter particles in cosmic rays to understand how the universe has evolved since its birth.
Cosmic rays are particles or particle clumps flying through the universe at nearly the speed of light. Since the mid-20th century, scientists have found cosmic-ray particles are emitted during supernovae and in the centres of galaxies that host large black holes. Scientists installed the AMS in May 2011, and by April 2021, it had tracked more than 230 billion cosmic-ray particles.
When scientists from the Massachusetts Institute of Technology (MIT) recently analysed these data — the results of which were published on June 25 — they found something odd. Roughly one in 10,000 of the cosmic ray particles were neutron-proton pairs, a.k.a. deuterons. The universe has a small number of these particles because they were only created in a 10-minute-long period a short time after the universe was born, around 0.002% of all atoms.
Yet cosmic rays streaming past the AMS seemed to have around 5x greater concentration of deuterons. The implication is that something in the universe — some event or some process — is producing high-energy deuterons, according to the MIT team’s paper.
Before coming to this conclusion, the researchers considered and eliminated some alternative explanations. Chief among them is the way scientists know how deuterons become cosmic rays. When primary cosmic rays produced by some process in outer space smash into matter, they produce a shower of energetic particles called secondary cosmic rays. Thus far, scientists have considered deuterons to be secondary cosmic rays, produced when helium-4 ions smash into atoms in the interstellar medium (the space between stars).
This event also produces helium-3 ions. So if the deuteron flux in cosmic rays is high, and if we believe more helium-4 ions are smashing into the interstellar medium than expected, the AMS should have detected more helium-3 cosmic rays than expected as well. It didn’t.
To make sure, the researchers also checked the AMS’s instruments and the shared properties of the cosmic-ray particles. Two in particular are time and rigidity. Time deals with how the flux of deuterons changes with respect to the flux of other cosmic ray particles, especially protons and helium-4 ions. Rigidity measures the likelihood a cosmic-ray particle will reach Earth and not be deflected away by the Sun. (Equally rigid particles behave the same way in a magnetic field.) When denoted in volts, rigidity indicates the extent of deflection the particle will experience.
The researchers analysed deuterons with rigidity from 1.9 billion to 21 billion V and found that “over the entire rigidity range the deuteron flux exhibits nearly identical time variations with the proton, 3-He, and 4-He fluxes.” At rigidity greater than 4.5 billion V, the fluxes of deuterons and helium-4 ions varied together whereas those of helium-3 and helium-4 didn’t. At rigidity beyond 13 billion V, “the rigidity dependence of the D and p fluxes [was] nearly identical”.
Similarly, they found the change in the deuteron flux was greater than the change in the helium-3 flux, both relative to the helium-4 flux. The statistical significance of this conclusion far exceeded the threshold particle physicists use to check whether an anomaly in the data is really real rather than the result of some fluke error. Finally, “independent analyses were performed on the same data sample by four independent study groups,” the paper added. “The results of these analyses are consistent with this Letter.”
The MIT team ultimately couldn’t find a credible alternative explanation, leaving their conclusion: deuterons could be primary cosmic rays, and we don’t (yet) know the process that could be producing them.