A neutrino mismatch can shake cosmology’s foundations
Neutrino Mass Mystery Deepens: Cosmic Observations Clash with Physics Experiments
On the day of September 20 2024. While the universe is congealed under the pull of gravity, matter knotted itself into galaxies, galaxy clusters and filaments, weaving a dazzlingly intricate cosmic web. This web’s structure is thanks, in part, to the handiwork of neutrinos lightweight, subatomic particles that surge through the cosmos in unimaginable numbers. Because they streak about at high speeds and rarely interact with other matter, the particles weren’t easily caught in the gravitational molasses of that latticework. So their presence swept away the cobwebs, hindering the formation of fine details in this cosmic filigree. The masses of neutrinos are less than a millionth that of the next lightest particle, the electron, but no one knows exactly how massive they are. They are the only known type of fundamental subatomic particle for which this basic property is unknown, and some researchers suspect that this missing knowledge could be a gateway to a new understanding of physics.
DESI Data Suggests Unexpectedly Low Neutrino Masses, Creating a Puzzle
“Neutrinos are one of the key particles that we do not understand as well as we do others, but that have nevertheless profound cosmological consequences,” says particle cosmologist Miguel Escudero of the European particle physics lab CERN near Geneva. The little particles’ outsize role in sculpting the universe means they bridge the gap between the subatomic world, usually studied at particle accelerators or physics labs, and the cosmological one discerned by peering out at the heavens. So scientists are using both observations of space and experiments on the ground in an attempt to solve this massive mystery. But if you ask a cosmologist how much neutrinos weigh and ask a particle physicist the same question, you might get two different answers. The two groups’ methods of gauging those masses are showing signs of a disconnect. Recent cosmological data collected by the Dark Energy Spectroscopic Instrument, or DESI, favors masses that are unexpectedly small and creeping close to conflict with those of particle physics experiments. In fact, some interpretations of the DESI data suggest that neutrinos have no mass or even negative mass, normally a forbidden concept in physics
Possible Explanations Range from Evolving Neutrino Mass to the Influence of Dark Energy
The odd result has physicists considering some tantalizing ideas that neutrinos’ masses might change over the history of the universe, or that the apparent negative masses are an illusion caused by dark energy, the mysterious phenomenon causing the universe to expand at an accelerating rate. DESI, located at Kitt Peak National Observatory in Arizona, collects detailed maps of galaxies and other objects. In April, DESI scientists made a splash for suggesting that the density of dark energy might change over the history of the universe. The neutrino weirdness was overshadowed. But in the months since, physicists have realized that DESI might have big implications for neutrinos too. Still, some scientists think the neutrino mass mismatch isn’t universe shattering. Instead, it may result from abstruse details about how the cosmological data are analyzed.
Potential for New Physics as Scientists Grapple with Conflicting Data
But if the effect holds up, it could hint at a massive shift. “I think that our description of the universe is too simple,” says cosmologist Eleonora Di Valentino of the University of Sheffield in England. “Now that we have very strong and very sensitive measurements it’s time to complicate it a bit.” Possible massive confusion of neutrino mass? Neutrinos come in three varieties electron neutrinos, muon neutrinos and tau neutrinos. To make matters more complicated, each type doesn’t have a definite mass but carries a quantum mixture of three different masses. Today, the triumvirate suffuses the cosmos with hundreds of millions of neutrinos per cubic meter, outnumbering protons by a factor of about a billion. In the early universe, the particles were even more densely packed. Although neutrinos are extremely lightweight, there’s strength in numbers. The particles have been throwing their weight around the cosmos for billions of years, indelibly etching the night sky with their presence. They flitted about not only the normal, visible matter that makes up stars and other spacefaring sundries, but also dark matter, a poorly understood source of mass that bulks up galaxies around the cosmos. Neutrinos’ combined numbers were enough not only to alter the cosmic web, but also to influence the expansion rate of the universe. Those two factors allow scientists to gauge neutrino masses by peering into space. Neutrino masses on the bigger side would have resulted in a more rapid expansion of the universe and a less clumpy cosmos than smaller neutrino masses. DESI maps out cosmic structures to determine that expansion rate, through an effect known as baryon acoustic oscillations, sound waves that imprinted circular patterns on the very early universe. By tracing those patterns at different points in the universe’s history, scientists can track its growth, a bit like cosmic tree rings. Meanwhile, the cosmic microwave background, light released 380,000 years after the Big Bang, reveals the clumpiness of the cosmos. As light from the cosmic microwave background traverses space, its trajectory is bent by the pockets of matter on its journey, much like light passing through a lens. The amount of this gravitational lensing tells scientists how clumpy the cosmos is. Combining the measurements of clumpiness from the cosmic microwave background and the expansion rate from DESI two things that neutrinos affect lets scientists zero in on their mass. The DESI data, in combination with cosmic microwave background data from the European Space Agency’s Planck satellite, provide a mass ceiling for neutrinos. Specifically, the sum of the three neutrino masses is less than about 0.07 electron volts at 95 percent confidence level, researchers reported online in April at arXiv.org. (An electron volt is a unit physicists use to quantify mass. An electron’s mass is about 511,000 electron volts.) In addition to a neutrino mass ceiling, there’s also a floor, based on laboratory particle physics experiments. Those experiments measure a phenomenon called neutrino oscillations, which results from the fact that each type of neutrino is a quantum mixture of different masses. The mass mélange means that neutrinos can change from one variety to another as they travel. What starts as a muon neutrino might later be detected as an electron neutrino. Neutrino detectors can spot this shapeshifting. Because oscillations depend on the relationship between the different neutrino masses, these experiments can’t directly measure the masses themselves. But they do indicate that the sum of the three neutrino masses must be greater than about 0.06 electron volts. That means DESI’s rejection of neutrino masses more than about 0.07 electron volts is disconcertingly close to ruling out the entire range of masses allowed by oscillation experiments. The floor and the ceiling are almost touching. There’s still a little leeway a crawl space, perhaps for neutrino masses to live in harmony with both cosmology and oscillation experiments. But the DESI result is surprising for other reasons. For one, the value that DESI pinpoints as most likely for the sum of the neutrino masses is zero no mass at all. What’s more, when additional cosmological data are added to the DESI and Planck data, such as catalogs of exploding stars that also gauge the universe’s expansion rate, the upper limit on the mass shrinks further, to less than 0.05 electron volts, Di Valentino and colleagues reported July 25. The crawl space is essentially eliminated, leaving neutrino masses in a purgatory that’s difficult to explain without proposing new ideas about the cosmos. “If you take everything at face value, which is a huge caveat…, then clearly we need new physics,” says cosmologist Sunny Vagnozzi of the University of Trento in Italy, another author of the paper Even without the addition of the supernova data, the DESI result, if taken seriously, would answer a major question: Which neutrino is heaviest? The three neutrino masses are labeled rather uncreatively with the numbers 1, 2 and 3. In one possible scenario called the normal ordering, mass 3 is heavier than masses 1 and 2. In what’s known as the inverted ordering, masses 1 and 2 are heavier than 3. Another way of stating the problem: Are there two relatively light neutrino masses and one somewhat heavier one or two heavy and one light? If the inverted ordering is correct, oscillation experiments imply the neutrino mass sum would be more than 0.1 electron volts. DESI squeezing the neutrino masses down to less than 0.07 electron volts not only leaves the normal ordering with little leeway, but it also seems to essentially rule out the inverted ordering. “That’s why everybody’s going overboard,” says cosmologist Licia Verde of the University of Barcelona, a member of the DESI collaboration. Nixing the inverted ordering would be a big deal, with repercussions for a slew of theories and experiments. The ordering is so important that scientists designed an enormous experiment the Jiangmen Underground Neutrino Observatory in China, planned to start up this year aimed at measuring it. But particle physicists are not canceling their plans, and no one is popping bottles of champagne to celebrate the demise of the inverted ordering. The reason is that DESI’s mass ceiling exceeded expectations. “It was too good,” says cosmologist Daniel Green of the University of California, San Diego. Given the amount of data DESI collected, scientists would have expected an upper limit that was more than twice as large, pegging the mass to less than about 0.18 electron volts, he says, leaving the possibility of the inverted ordering alive and well. In fact, DESI wasn’t expected to be able to rule out the inverted ordering if the inverted ordering were incorrect until it had taken several more years of data. That has made physicists suspicious that something else is up.