A neutrino mass mismatch could shake cosmology’s foundations

Because the youthful universe congealed below 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, partially, to the handiwork of neutrinos — lightweight, subatomic particles that surge through the cosmos in unimaginable numbers.

Because they streak about at high speeds and infrequently ever have interaction with other matter, the particles weren’t without problems caught within the gravitational molasses of that latticework. So their presence swept away the cobwebs, hindering the formation of fine important points on this cosmic filigree.

The masses of neutrinos are lower than a millionth that of the next lightest particle, the electron, but no one knows exactly how massive they are. They may possibly even be some of the most effective real known type of fundamental subatomic particle for which this basic property is unknown, and some researchers suspect that this missing knowledge may be a gateway to a new figuring out of physics.

“Neutrinos are among the vital key particles that we wouldn't remember along with to we do others, but that have alternatively 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, in most cases studied at particle accelerators or physics labs, and the cosmological one discerned by peering out on the heavens. So scientists are the usage of both observations of space and experiments on the bottom in an try to solve this huge mystery.

But if you ask a cosmologist how a lot neutrinos weigh and ask a particle physicist the identical question, you are going to probably be in a position to 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 which are small and creeping on the topic of conflict with those of particle physics experiments. In point of fact, some interpretations of the DESI data suggest that neutrinos have no mass or most probably negative mass, in most cases a forbidden concept in physics (SN: eleven/21/14).

The odd result has physicists considering some tantalizing ideas — that neutrinos’ masses may change over the history of the universe, or that the plain negative masses are an illusion attributable to 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 may change over the history of the universe (SN: Four/Four/24). The neutrino weirdness turned into overshadowed. But within the months since, physicists have realized that DESI can have big implications for neutrinos too.

Still, some scientists think the neutrino mass mismatch isn’t universe-shattering. As a replacement, it might possibly result from abstruse important points about how the cosmological data are analyzed.

But if the effect holds up, it might possibly hint at a giant shift. “I feel that our description of the universe is solely too simple,” says cosmologist Eleonora Di Valentino of the University of Sheffield in England. “Now that we now have very strong and really sensitive measurements … it’s time to complicate it a bit bit.”

Massive confusion about neutrino masses

Neutrinos should be found 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.

As of late, the triumvirate suffuses the cosmos with hundreds of millions of neutrinos per cubic meter, outnumbering protons by a component to about a billion. Within the early universe, the particles were even more densely packed.

Though 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 standard, 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 change 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 set up that expansion rate, through an effect is known as baryon acoustic oscillations, sound waves that imprinted circular patterns on the very early universe. By tracing those patterns at different points within the universe’s history, scientists can track its growth, similar to 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, a lot 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 have an effect on — lets scientists zero in on their mass.

The DESI data, at the side of 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 lower than about zero.07 electron volts at 95 p.c.self belief 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 ready 511,000 electron volts.)

As well as to a neutrino mass ceiling, there’s also a floor, according to laboratory particle physics experiments. Those experiments measure a phenomenon is known as neutrino oscillations, which results from the very proven fact that each type of neutrino is a quantum mixture of quite numerous masses. The mass mélange implies that neutrinos can change from one variety to any other as they go backward and forward (SN: 10/6/15). What starts as a muon neutrino may later be detected as an electron neutrino.

Neutrino detectors can spot this shapeshifting. Because oscillations depend upon the relationship between the several neutrino masses, these experiments can’t at once measure the masses themselves. But they do indicate that the sum of the three neutrino masses must be greater than about zero.06 electron volts.

Meaning DESI’s rejection of neutrino masses more than about zero.07 electron volts is disconcertingly on the topic of ruling out the total range of masses allowed by oscillation experiments. The ground and the ceiling are almost touching.

There’s still a bit leeway — a crawl space, most probably — for neutrino masses to live in solidarity with both cosmology and oscillation experiments. Nonetheless the DESI result is surprising for other reasons. For one, the associated fee that DESI pinpoints as possibly for the sum of the neutrino masses is zero — no mass in any respect.

What’s more, when additional cosmological data are added to the DESI and Planck data, comparable to catalogs of exploding stars that also gauge the universe’s expansion rate, the upper limit on the mass shrinks in the same way, to lower than zero.05 electron volts, Di Valentino and colleagues reported July 25 at arXiv.org. The crawl space is in fact eliminated, leaving neutrino masses in a purgatory that’s demanding to supply an explanation for without proposing new ideas in regards to the cosmos.

“Within the event you take your complete lot at face value, which is a gigantic caveat…, then clearly we want new physics,” says cosmologist Sunny Vagnozzi of the University of Trento in Italy, any other author of the paper

Even without the addition of the supernova data, the DESI result, if taken seriously, would answer a prime question: Which neutrino is heaviest? The three neutrino masses are labeled rather uncreatively with the numbers 1, 2 and three. In a single possible scenario is known as the standard ordering, mass Three is heavier than masses 1 and 2. In what’s is known as the inverted ordering, masses 1 and 2 are heavier than Three. Every other way of stating the issue: Are there two relatively light neutrino masses and one a bit bit heavier one or two heavy and one light?

If the inverted ordering is correct, oscillation experiments imply the neutrino mass sum may be more than zero.1 electron volts. DESI squeezing the neutrino masses down to lower than zero.07 electron volts not only leaves the standard ordering with little leeway, however it also seems to in fact rule out the inverted ordering.

“That’s why all and sundry’s going overboard,” says cosmologist Licia Verde of the University of Barcelona, a member of the DESI collaboration.

Nixing the inverted ordering may be a massive deal, with repercussions for a slew of theories and experiments. The ordering is so important that scientists designed a massive experiment — the Jiangmen Underground Neutrino Observatory in China, planned to start off 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 being that DESI’s mass ceiling exceeded expectations. “It turned into too good,” says cosmologist Daniel Green of the University of California, San Diego.

Given the quantity of knowledge DESI collected, scientists would have expected an upper limit that turned into more than twice as large, pegging the mass to lower than about zero.18 electron volts, he says, leaving the chance of the inverted ordering alive and well. In point of fact, DESI wasn’t expected so that you just may possibly rule out the inverted ordering — if the inverted ordering were incorrect — until it had taken several more years of knowledge.

That has made physicists suspicious that something else is up.

May neutrinos have negative mass?

If scientists take seriously DESI’s preference for zero neutrino mass, there are numerous tips on how to supply an explanation for it, in spite of the proven fact that neutrinos within the lab undoubtedly have mass. Neutrinos may possibly decay into other particles or annihilate with each other, Green and colleagues suggest in a paper accepted within the Journal of High Energy Physics. Or most probably neutrinos’ masses vary across time.

But there’s a fair wilder possibility than zero mass: negative mass. Green suspected “all of this funny behavior turned into because the data turned into in fact going the wrong way. [The data] turned into seeing the ‘opposite’ of a neutrino.” Namely, a neutrino with negative mass.

Whereas neutrinos with positive mass make the universe less clumpy, DESI and Planck may possibly even be finding the reverse, a universe this is clumpier than expected, which means it has a larger-than-predicted variation within the density of matter from place to place. Which may be conceptualized by a bizarro neutrino with negative mass.

Within the DESI analysis, scientists didn’t allow the neutrino mass to head negative. Maybe DESI landed on zero only since it turned into forbidden from going lower.

So Green and colleagues tweaked the analysis to permit negative masses. The analysis homed in on –zero.sixteen electron volts, the researchers reported.

Others found similar improve for negative neutrino masses. That’s “kind of a crazy thing to assert,” says cosmologist Willem Elbers of Durham University in England. Negative masses in physics are demanding to define and incorporate in theories, causing all types of conflict in equations. “We don’t in fact think that the neutrino mass is negative,” Elbers says. As a replacement, “it’s a symptom of some problem either within the data or within the assumptions that we make about how the universe evolves.”

The negative mass may be a mirage of dark energy, Elbers and colleagues suggest. The common picture of the universe assumes dark energy has a constant density, what’s is known as a cosmological constant. While the DESI data hint that dark energy is dynamical — that its density changes over time — DESI’s neutrino mass number turned into determined assuming a cosmological constant.

Allowing dynamical dark energy resolves the neutrino mass issue, Elbers and colleagues reported online July 15 at arXiv.org. “It in fact shifts the possibly value from something negative and unphysical to something that’s right on the mark,” Elbers says: zero.06 electron volts.

But not all dynamical dark energy is alike. The perfect models of dynamical dark energy, like that utilized by DESI and by Elbers and colleagues, allow dark energy to head “phantom,” an unexpected situation, theoretically. In scientists’ favorite theories, dark energy’s density either is still constant or gets diluted as space expands. With phantom dark energy, the density instead increases. That type of dark energy is believed of as less plausible — it’s demanding to supply an explanation for within usual physics theories.

The usage of a model wherein dark energy’s variation is illegitimate from going phantom in fact made the neutrino mass mismatch worse, Vagnozzi, Di Valentino and colleagues reported in their paper.

That leaves scientists and not using a winning cosmological reason on the back of why the neutrino masses are smaller than expected.

Problems with Planck’s data

As a replacement of rethinking the universe, some scientists are taking a 2d examine the data.

Subtle issues within the cosmic microwave background data may possibly even be skewing things, some researchers suspect. Specifically, the data from Planck is famous to disclose an unexpected far more than gravitational lensing, that bending of the cosmic microwave background light that helps scientists deduce the neutrino masses.

More gravitational lensing may be what you’d expect from neutrinos with negative masses. In point of fact, earlier attempts to estimate the neutrino masses the usage of Planck data combined with a predecessor of DESI also landed on small estimates. Maybe Planck is the issue.

An updated version of the Planck data, the usage of different methods of mapping out the cosmic microwave background, reduces this excessive gravitational lensing.

An analysis according to that updated Planck data, and removing two outlier DESI data points, eliminated the evidence for negative neutrino masses, Escudero and colleagues reported online July 18 at arXiv.org

On condition that, Escudero says, “it seems premature to conclude there will probably be a tension between the minimum value of neutrino masses we all know from the laboratory and the dearth of detection of neutrino masses in cosmology.”

But, he notes, the analysis still found no evidence of a positive mass for neutrinos.

Taking direct measurements of neutrino mass

The cosmological measurements of neutrino mass depend upon quite numerous observations, and that they hinge on the correctness of scientists’ theory of the cosmos. If there’s a missing link anywhere, that makes the neutrino mass estimates unreliable. So within the very long time, scientists hope to measure the neutrino mass at once, on Earth.

The KATRIN experiment in Karlsruhe, Germany, searches for the influence of neutrinos masses on radioactive decays of tritium, a heavy type of hydrogen (SN: Four/21/21). When tritium’s nucleus decays, it emits an antineutrino (the antimatter twin of a neutrino) and an electron. KATRIN aims to detect the effect of antineutrinos’ masses on the energies of the electrons released within the decay.

But while experiments like this may possibly likely theoretically measure neutrino mass, their results aren’t nearly as precise as those of cosmology. The sum of the neutrino masses must be lower than 1.35 electron volts at ninety p.c.self belief level, KATRIN researchers reported online in June at arXiv.org. That’s a a lot weaker limit than cosmology puts on the mass. So in spite of the proven fact that direct experiments are considered more reliable, they’re not in fact telling scientists a lot that they didn’t already know. Future direct experiments may in the same way zero in on neutrino mass, but when neutrino masses are as tiny as cosmologists think they are, it might possibly take some serious technological advancements.

Still, the chance of higher figuring out numerous of some of the most mysterious particles within the cosmos is tantalizing. “I to search out it particularly interesting that looking up on the sky can tell you something about a particle this is so light and tiny and small and subatomic,” Verde says.

And if scientists can to search out agreement between neutrinos on Earth and in space, they’ll have extra self belief that their theory of the universe is correct, Verde says. “If which you maybe can build a picture where your complete lot hangs together, by combining both experiments that examine at once the infinitely small and experiments that examine the very big, it also offers improve to the picture itself.”