Two of the universe’s most mysterious particles may be colliding invisibly throughout the cosmos — a discovery that could solve one of the biggest lingering problems in our standard model of cosmology.
Those two elusive components — dark matter and neutrinos (or “ghost particles“) — are ubiquitous throughout the cosmos, yet they remain poorly understood. In a study published Jan. 2 in the journal Nature Astronomy, an international team of researchers found evidence that dark matter and neutrinos may collide, transferring momentum between them in the process.
This surprising interaction may help to explain why the universe is less populated by dense regions, like galaxies, than predicted — in other words, the universe is less “clumpy” than cosmologists think it should be, the researchers said in a statement.
Dark matter and neutrinos remain a riddle
Dark matter is the mysterious, invisible substance that constitutes 85% of the matter in the universe. As its name suggests, dark matter does not emit light, so its existence has been only indirectly inferred from its gravitational influence, as observed in cosmological surveys.
Neutrinos are subatomic particles with infinitesimally low masses and no electric charge, so they very rarely interact with other particles. They’re produced by various nuclear processes, including stellar fusion and supernovas, in prodigious quantities: Every second, approximately 100 billion neutrinos pass through each square centimeter of your body, Live Science previously reported.
Yet dark matter and neutrinos should not interact, according to the leading model of cosmology, known as the lambda cold dark matter model (lambda-CDM). This standard model aims to theoretically explain the large-scale structure of the cosmos.
Cosmological conundrum
However, this recent study provides new evidence that dark matter and neutrinos may interact after all, as other researchers have posited over the past two decades.
If dark matter and neutrinos do collide, and transfer momentum to one another in the process, this discovery would inspire a rethink of the lambda-CDM model. Such collisions could also help to explain the “S8 tension,” a mismatch between the expected and actual “clumpiness” of the universe.
“This tension does not mean the standard cosmological model is wrong, but it may suggest that it is incomplete,” Eleonora Di Valentino, study co-author and a senior research fellow at the University of Sheffield in the U.K., explained in the statement. “Our study shows that interactions between dark matter and neutrinos could help explain this difference, offering new insight into how structure formed in the Universe.”
The mismatch stems from researchers’ findings that the current cosmos isn’t as packed together as predicted, based on observations of the cosmic microwave background (CMB) — the first light in the universe, emitted when the cosmos was only 380,000 years old.
“The statement that cosmic structures are ‘less clumped’ is best understood in a statistical sense, rather than as a change in the appearance of individual galaxies or clusters. It refers to a reduced efficiency in the growth of cosmic structures over time,” study co-author William Giarè, a cosmologist at the University of Hawaii, told Live Science via email.
Unraveling multiple threads of evidence
The researchers tried to unite evidence from energy and density fluctuations in the CMB and from baryon acoustic oscillations (BAO) — pressure waves “frozen” in time from the beginning of the cosmos — with more recent observations of the universe’s large-scale structure.
The early-universe data come from the Atacama Cosmology Telescope in Chile and the European Space Agency’s space-based Planck telescope, which was designed to study the CMB. The later-universe data come from the Victor M. Blanco Telescope in Chile and the Sloan Digital Sky Survey, a two-decade effort to create a 3D map of millions of galaxies across more than 11 billion light-years.
The researchers also incorporated cosmic shear data from the Dark Energy Survey. Cosmic shear is the distortion of distant celestial objects due to weak gravitational lensing, which occurs when massive foreground structures bend the fabric of space-time and alter the paths of light traveling from those distant celestial objects to our detectors.
Finally, the researchers combined these data and modeled the evolution of the universe. When accounting for collisions between dark matter and neutrinos and the resulting momentum exchange, the simulations generated a model universe that better agrees with real observations.
There’s reason to remain cautious, however, as the interaction between dark matter and neutrinos has only a 3-sigma level of certainty — meaning there is a 0.3% chance that this result is a fluke. Though short of the scientific gold standard of 5 sigma, it is significant enough to warrant additional research because, if confirmed, the interaction would prove a “fundamental breakthrough in cosmology and particle physics” — and a potential solution to the cosmic clumpiness quandary.
“The final verdict will come from upcoming large sky surveys, such as those from the Vera C. Rubin Observatory, and more precise theoretical work,” research team leader Sebastian Trojanowski, a theoretical physicist at the National Centre for Nuclear Research in Poland, explained in a separate statement. “These will allow us to determine whether we are witnessing a new discovery in the dark sector or whether our cosmological models require further adjustment. However, each of these scenarios brings us closer to solving the mystery of dark matter.”
