From the July 2019 issue

Do Neutrinos Hold Secrets to the Cosmos?

Could untold types of neutrinos account for dark matter?
By | Published: July 1, 2019 | Last updated on May 18, 2023
BOTTOM LINES. Like strands of high-tech kelp, the Antares detector array rises from the seafloor off Marseille, France. Precisely timing a neutrino’s light flash is the key for tracking a particle’s path through the array.
F. Montanet/Antares

For a quarter of a century, Wolfgang Pauli’s prediction remained an educated guess. In 1930, the Austrian physicist predicted the existence of a ghostly new subatomic particle. 

After observing beta decay in a radioactive nucleus, Pauli noted that an undiscovered particle must exist to explain the resulting spectrum. During beta decay, a proton becomes a neutron by emitting a positron. But Pauli argued the nucleus also emitted an unknown electrically neutral particle. He thought this hypothetical particle had less than 1 percent of a proton’s mass.

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During the 1930s, Italian physicist Enrico Fermi investigated the problem and completed the work Pauli began. Fermi thought the weak nuclear force destabilized atomic nuclei and caused particle transformations. He called Pauli’s ghostly particle the neutrino, Italian for “little neutral one.” 

German physicist Hans Bethe, meanwhile, was attacking the question of how stars shine. While investigating this question, Bethe realized that neutrinos played a key role. Fusion reactions in the Sun’s core create a torrent of neutrinos, a fraction of which passes through Earth eight minutes later. These evanescent particles carry with them a record of what happens inside a star.

COSMIC BASICS. Normal matter comprises electrons and neutrinos, plus particles built from combinations of three quarks, like protons and neutrons. Exchanging force-carrying entities, like photons and gluons, gives rise to electromagnetism and nuclear forces.
Astronomy: Roen Kelly

Neutrinos come in three types — electron, muon, and tau. But these elusive particles don’t interact much with other matter. Neutrinos can pass almost unfettered through us, Earth, the Sun, or the superdense heart of an exploding star. While they exist in tremendous numbers, the challenge of neutrinos is detecting them.   

In the 1950s, physicists Fred Reines and Clyde Cowan began a series of experiments to try. By the mid-1950s, their Project Poltergeist showed that it could be done. Their experiment picked up neutrinos by using a nuclear reactor as a source and a water tank as a detector, both sunk deep in a mine.  

Although Bethe outlined the processes by which stars obtain energy through hydrogen fusion, many neutrino mysteries remain. For a long time, astronomers have known that the universe contains much more matter than the bright stuff we can see. They know this because they track galaxies moving in response to the gravitational pull of large amounts of material that neither emits nor blocks light — dark matter.

SUNKEN SENSORS.  Japan’s Super Kamiokande neutrino detector is a cylinder 130 feet wide and high. Light-sensors lining the water-filled tank hunt for neutrinos.
Kamioka Observatory/University of Tokyo/Tomasz Barszczak
Could untold varieties of neutrinos account for much — or even all — of the dark matter astronomers believe is out there? Unfortunately, scientists now think the answer is no. 

Recent research suggests that while neutrinos do have mass, they do not have nearly enough to account for all the dark matter in the cosmos. Furthermore, neutrinos move at nearly the speed of light, meaning they won’t easily clump together like dark matter is observed doing in galaxies.

Independent of neutrinos’ possible role as dark matter, the hard-to-catch particles may also help astronomers decipher how matter itself came to be. When the Big Bang occurred, matter and antimatter should have been created in equal amounts. And when matter and antimatter meet, they annihilate each other. If the amounts had been equal, then only radiation would have filled the universe. 

Why is there so much matter in the cosmos? Maybe neutrinos played a key role in the universe’s early asymmetry. If so, we owe our existence to them. 

Neutrinos surface in other cosmic mysteries, too. So, expect to hear a lot more about these strange particles as scientists continue to probe matter’s secrets.