A new analysis of data collected by a space-based telescope has revealed that high-energy particles known as cosmic rays share a common structural feature in their energy spectra, regardless of their atomic composition. The finding, published in Nature, challenges long-standing competing theories about how these particles are accelerated and transported across the galaxy.
Cosmic rays have been observed since their discovery more than a century ago, yet their origins and the mechanisms governing their behavior have remained poorly understood. These particles, the most energetic ever detected, surpassing anything produced by particle accelerators on Earth, are thought to originate in extreme astrophysical environments such as supernova remnants, black hole jets, and pulsars.
A Consistent Signature Across Particle Types
The discovery centers on what physicists call “spectral softening”, a sharper-than-usual drop in the number of particles observed above a certain energy threshold. According to the study by the DAMPE (Dark Matter Particle Explorer) collaboration, this softening occurs at a rigidity of approximately 15 teravolts across all measured cosmic ray species, including protons, helium, carbon, oxygen, and iron nuclei.
Rigidity, in this context, describes how strongly a particle’s trajectory is deflected by magnetic fields and is determined by the particle’s momentum and electric charge, rather than its mass alone. The fact that the softening threshold aligns consistently across nuclei of different masses and atomic numbers is a significant observational result.
It directly supports models in which both the acceleration and propagation of cosmic rays are governed by rigidity, and it places serious pressure on alternative theories that predict spectral features dependent on energy per nucleon, that is, energy divided by the number of particles in the nucleus. The statistical confidence with which a mass-dependent softening is ruled out stands at greater than99.999%, according to the paper.
DAMPE, launched in December 2015, collected the data underpinning this study over nine years of operation in orbit. The mission, which is led by the Chinese Academy of Sciences and includes contributions from several international partners, was designed to study high-energy cosmic rays and search for indirect signatures of dark matter.
The Role of the Geneva Team and What the Findings Mean
Researchers at the University of Geneva’s Department of Nuclear and Particle Physics played a central role in producing the results. The team developed artificial intelligence-based methods to reconstruct individual particle events from detector data and contributed directly to measurements of proton, helium, and carbon fluxes.
They were also responsible for leading the development of the Silicon-Tungsten Tracker, a key instrument aboard DAMPE that enables precise reconstruction of particle trajectories and charge measurements. “Cosmic rays are primarily composed of protons, but also of helium, carbon, oxygen, and iron nuclei,” said Andrii Tykhonov, associate professor at the University of Geneva and a co-author of the study. The team’s measurements extended from roughly 20 gigavolts to around 100 teravolts (or 60 teravolts in the case of iron) representing some of the most precise direct measurements of individual cosmic ray species at these energies.
Spectral energy distributions of five primary cosmic ray species — proton, helium, carbon, oxygen, and iron — with systematic analysis uncertainties shown as quadratic sums of all contributions, excluding hadronic model uncertainties. ©Nature
The implications reach into several areas of astrophysics. By establishing that spectral softening is a rigidity-dependent phenomenon, the results set new observational constraints on theories of particle acceleration in high-energy environments. They also refine existing models of how cosmic rays propagate through interstellar space, where magnetic fields play a central role in shaping particle paths over vast distances.
The paper also discusses possible physical interpretations of the observed softening, including the influence of a nearby cosmic ray source and propagation effects in the interstellar medium, though no single explanation is favored conclusively by the current data.
What the findings do establish more firmly is a structural regularity in cosmic ray spectra that had not been directly confirmed before. Detecting the same spectral feature across five distinct nuclear species (from the lightest to among the heaviest commonly measured) provides a consistent empirical anchor for theoretical work that has, until now, lacked this level of observational precision at high energies.