Unraveling a Cosmic Antimatter Mystery

Physicists invoke the cosmological collider to demonstrate why matter, not antimatter, dominates the universe.

The universe was filled with equal numbers of matter and “antimatter” – particles that are matter counterparts but have opposite charges — early in its existence, shortly after the Big Bang. The universe then cooled as space expanded. Today’s universe is filled with matter-based galaxies and stars. How did matter come to dominate the universe, and where did antimatter go? Scientists are still perplexed by the cosmic origin of matter.

By invoking the “cosmological collider,” physicists from the University of California, Riverside, and Tsinghua University in China have now opened a new pathway for studying the cosmic origin of matter.

Yanou Cui is an associate professor of physics and astronomy at UC Riverside. Credit: I. Pittalwala/UC Riverside.

Not just any collider

High-energy colliders, such as the Large Hadron Collider, have been designed to generate very heavy subatomic elementary particles that might reveal new physics. However, certain new physics, such as those explaining dark matter and the origin of matter, can involve far heavier particles, necessitating much more energy than a human-made collider can deliver. It turns out that the early universe may have acted as a super-collider.

Yanou Cui, an associate professor of physics and astronomy at UCR, explained that cosmic inflation, a period when the universe expanded at an exponentially increasing pace, is commonly thought to have preceded the Big Bang.

“Cosmic inflation provided a highly energetic environment, enabling the production of heavy new particles as well as their interactions,” Cui said. “The inflationary universe behaved just like a cosmological collider, except that the energy was up to 10 billion times larger than any human-made collider.”

According to Cui, when the universe expanded, tiny structures formed by energetic events during inflation were stretched, resulting in areas of varying density in an otherwise homogeneous universe. These microscopic structures then seeded the large-scale structure of our universe, which is seen today as the distribution of galaxies throughout the sky. Cui noted that analyzing the imprint of the cosmological collider in today’s cosmos’ contents, such as galaxies and the cosmic microwave background, may reveal new subatomic particle physics.

Cui and Zhong-Zhi Xianyu, an assistant professor of physics at Tsinghua University, report in the journal Physical Review Letters that by applying the physics of the cosmological collider and using precision data for measuring the structure of our universe from upcoming experiments such as SPHEREx and 21 cm line tomography, the mystery of the cosmic origin of matter may be unraveled.

“The fact that our current-day universe is dominated by matter remains among the most perplexing, longstanding mysteries in modern physics,” Cui said. “A subtle imbalance or asymmetry between matter and antimatter in the early universe is required to achieve today’s matter dominance but cannot be realized within the known framework of fundamental physics.”

Leptogenesis to the rescue

Cui and Xianyu propose testing leptogenesis, a well-known mechanism that explains the origin of the baryon — visible gas and stars — asymmetry in our universe. Had the universe begun with equal amounts of matter and antimatter, they would have annihilated each other into photon radiation, leaving nothing. Since matter far exceeds antimatter today, asymmetry is required to explain the imbalance.

“Leptogenesis is among the most compelling mechanisms generating the matter-antimatter asymmetry,” Cui said. “It involves a new fundamental particle, the right-handed neutrino. It was long thought, however, that testing leptogenesis is next to impossible because the mass of the right-handed neutrino is typically many orders of magnitudes beyond the reach of the highest energy collider ever built, the Large Hadron Collider.”

The new work proposes to test leptogenesis by decoding the detailed statistical properties of the spatial distribution of objects in the cosmic structure observed today, reminiscent of the microscopic physics during cosmic inflation. The cosmological collider effect, the researchers argue, enables the production of the super-heavy right-handed neutrino during the inflationary epoch.

“Specifically, we demonstrate that essential conditions for the asymmetry generation, including the interactions and masses of the right-handed neutrino, which is the key player here, can leave distinctive fingerprints in the statistics of the spatial distribution of galaxies or cosmic microwave background and can be precisely measured,” Cui said. “The astrophysical observations anticipated in the coming years can potentially detect such signals and unravel the cosmic origin of matter.”

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