Bhupal Dev and his colleagues report the results in a recent paper published in Physical Review Letters. Dev is an associate professor of physics in Arts & Sciences and a fellow of the McDonnell Center for the Space Sciences at Washington University in St. Louis.

Neutrinos are among the most abundant particles in the Universe. Often described as ghostlike because they interact so weakly with matter, neutrinos play an important role in shaping how cosmic structures form and evolve.

Recent analyses of cosmological data suggest that neutrinos may interact with one another more strongly than predicted by the Standard Model of particle physics, although laboratory experiments place strict limits on such interactions.

Dev's new study offers a possible explanation for this apparent mismatch. According to the researchers, the cosmological signals interpreted as evidence for strongly interacting neutrinos could instead be produced by an additional component of radiation in the early Universe. 

"Because cosmological observations mainly measure the total amount of fast-moving radiation, they cannot easily distinguish neutrinos from other lightweight particles that behave similarly," Dev said.

He proposes that some fraction of neutrinos converted into a different type of light, fast-moving radiation known as dark radiation during the Universe’s earliest moments.

The transformation must have taken place after Big Bang nucleosynthesis, but before the formation of the cosmic microwave background.

"In this scenario, dark radiation could mimic the cosmological effects attributed to interacting neutrinos while avoiding the experimental constraints that apply to neutrinos themselves," Dev said.

If this dark radiation mechanism occurred, it could also influence several ongoing puzzles in cosmology. These include uncertainties in neutrino masses and the long-standing Hubble tension, which is the discrepancy between different measurements of how quickly the Universe is expanding.

“Our work highlights a broader paradigm in neutrino cosmology,” Dev said. “The degeneracy between neutrinos and neutrino-like dark radiation opens up new avenues for addressing cosmological tensions while respecting terrestrial constraints.”

Future observations may help test the idea. Next-generation measurements of the cosmic microwave background, large-scale structure surveys, and emerging 21-centimeter cosmology experiments could reveal signatures of this hidden radiation. 

Laboratory experiments that measure the absolute mass of neutrinos or search for possible sterile neutrinos may also provide important clues.

In other words, while interactions between neutrinos and dark radiation may be ghostly, they may not remain hidden forever.