Hidden ancient neutrinos may shape the patterns of galaxies

Shadowy messengers from the Big Bang have seemingly left their mark on ring-shaped patterns imprinted on the sky.

Subatomic particles called neutrinos, released just one second after the universe’s birth 13.8 billion years ago, continually stream through the universe and are exceedingly hard to spot. But circular patterns of galaxies scattered across the sky reveal signs of the shy particles. Those data hint that the neutrinos’ gravity subtly alters the rings, researchers report February 25 in Nature Physics. Since these relic neutrinos were released so early in the universe’s history, scientists hope they can one day use these particles to better understand the cosmos in its first moments.

The study “is certainly new and interesting in that it shows that we can derive the early universe physics” by observing the recent universe, says cosmologist Hee-Jong Seo of Ohio University in Athens, who wasn’t involved in the research.

Spotting signs of the ancient particles is no easy feat. All neutrinos are notoriously difficult to detect. They have no electric charge and can pass straight through other matter. With large, highly sensitive detectors, scientists can spot neutrinos produced by everyday processes such as radioactive decay. But neutrinos released from the Big Bang, known collectively as the “cosmic neutrino background,” are much more elusive. Although these cosmic relics suffuse the universe, the particles have so little energy that they have never been directly spotted.

So rather than trying to observe those relic neutrinos directly, scientists look for their influence on other cosmic signposts. For example: A pattern caused by sound waves in the early universe — known as baryon acoustic oscillations — should be distorted by the neutrinos. Those sound waves spread outward through the universe like circular ripples on a pond, compressing matter into denser pockets. Eventually, that process resulted in galaxies having a tendency to cluster in rings across the sky (SN: 5/5/12, p. 17).

But neutrinos can shift that matter around due to the particles’ gravity, slightly changing the distribution of matter in the rings. “You’re seeing the pull of the neutrinos,” says cosmologist Daniel Green of the University of California, San Diego. Using data from the Baryon Oscillation Spectroscopic Survey, or BOSS, Green and colleagues studied the circular patterns of galaxies and saw evidence that the neutrinos were, in fact, pulling matter around from the inner side of the ring band toward the outer side.

Scientists have previously spotted signs of the ancient neutrinos in a glow leftover from the Big Bang. The cosmic microwave background, light that was released when the universe was just 380,000 years old, is also affected by the cosmic neutrino background. But this is the first time evidence of the particles’ fingerprints on galaxies has been spotted.

“It’s another hallmark of the success of standard cosmology,” says cosmologist Kevork Abazajian, who was not involved with the research. Still, the current result is just scratching the surface of this phenomenon, making the measurement a proof of principle rather than a definitive detection, says Abazajian, of the University of California, Irvine.

In the future, improved surveys of galaxies might be sensitive enough to reveal unexpected tweaks to the ring patterns, which could be caused by the existence of undiscovered phenomena, such as hypothetical new types of neutrinos called sterile neutrinos (SN: 6/23/18, p. 7).

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Editor’s note: This story was updated March 4, 2019, to add in Daniel Green’s affiliation.