Scientists have long been fascinated by neutrinos—tiny, nearly invisible particles that pass through matter almost without a trace.
They are among the most abundant particles in the universe and played an important role in shaping how galaxies and cosmic structures formed.
But new research suggests that neutrinos may have an even more mysterious story than we thought.
A study led by physicist Bhupal Dev at Washington University in St. Louis proposes that some neutrinos in the early universe may have transformed into a previously unknown type of radiation, called “dark radiation.”
The findings, published in Physical Review Letters, offer a new way to explain puzzling observations about how the universe evolved.
Neutrinos are often described as “ghostlike” because they interact very weakly with other matter.
According to the standard model of particle physics, neutrinos should rarely interact with each other.
However, recent studies of the universe’s large-scale structure and the cosmic microwave background—the faint afterglow of the Big Bang—hint that neutrinos may have interacted more strongly in the early universe than expected.
This creates a problem. Laboratory experiments on Earth have placed strict limits on how strongly neutrinos can interact. So how can scientists reconcile these two sets of results?
Dev and his team suggest a clever solution. Instead of assuming neutrinos themselves were interacting more strongly, they propose that some neutrinos may have changed into a different kind of fast-moving, lightweight radiation early in cosmic history.
This “dark radiation” would behave similarly to neutrinos in cosmological observations, making it difficult to tell the difference between the two.
Because astronomers can only measure the total amount of fast-moving radiation in the universe, they cannot easily distinguish whether it comes from neutrinos or from something else that behaves in a similar way. As a result, dark radiation could “masquerade” as neutrinos, creating the illusion of stronger interactions.
The researchers believe this transformation would have occurred at a very specific time—after the formation of the first atomic nuclei in the early universe, but before the cosmic microwave background was created. This timing is important because it ensures that the idea remains consistent with both cosmological observations and laboratory experiments.
If this theory is correct, it could help solve several long-standing puzzles in cosmology. One of these is the uncertainty surrounding neutrino masses. Another is the so-called “Hubble tension,” a disagreement between different measurements of how fast the universe is expanding.
The idea also opens up new possibilities for future research. Upcoming experiments that study the cosmic microwave background, map the large-scale structure of the universe, or use 21-centimeter signals from early hydrogen could provide evidence for this hidden radiation. At the same time, laboratory experiments searching for new types of neutrinos or measuring their masses more precisely may offer additional clues.
While neutrinos have always been mysterious, this research suggests they may be hiding an even deeper secret. What appears to be familiar “ghost particles” might actually be part of an unseen form of radiation that shaped the universe in ways we are only beginning to understand.
Source: Washington University in St. Louis.
