Asymmetry Detected in the Distribution of Galaxies - Quanta Magazine

Physicists believe they have detected a striking asymmetry in the arrangements of galaxies in the sky.

When they had built every possible tetrahedron from a catalog of 1 million galaxies, they found that tetrahedra oriented one way outnumber their mirror images.

But then, in 1957, the Chinese American physicist Chien-Shiung Wu’s nuclear decay experiments revealed that our universe indeed has a slight handedness to it: Subatomic particles involved in the weak nuclear force, which causes nuclear decay, are always magnetically oriented in the opposite direction from the one they move in, so that they spiral like the threads of a left-handed screw.

The mirror-image particles — the ones like right-handed screws — don’t feel the weak force.

If, for instance, some primordial parity violation was in effect when the universe was in its infancy, it might have imprinted a twist onto the structure of the cosmos.

A roiling, boiling medium where inflaton particles continuously bubbled up and disappeared, the inflaton field was also repulsive; for the brief time it may have existed, it would have caused our universe to rapidly expand to 100 trillion trillion times its original size.

All of those quantum fluctuations of particles in the inflaton field were flung outward and frozen into the cosmos, becoming variations in the density of matter.

The inflaton field could have interacted with another field that could produce right-handed and left-handed particles.

If the inflaton treated right-handed particles differently than the left-handed ones, then it could have preferentially created particles of one handedness over the other.

This so-called Chern-Simons coupling would have imbued the early quantum fluctuations with a preferred handedness, which would have evolved into an imbalance of left-handed and right-handed tetrahedral arrangements of galaxies.

In this scenario, a parity-violating Chern-Simons interaction would occur between inflaton particles and gravitons — the quantum units of gravity — which would have popped up in the gravitational field during inflation.

A statistical analysis of 1 million galaxies in the survey has found evidence of parity violation.

Cahn thought the possibility of solving the matter-antimatter asymmetry puzzle with parity violation in the early universe was “speculative, but also provocative.” In 2019, he decided to look for parity violation in a catalog of galaxies in the Sloan Digital Sky Survey.

To test whether the galaxy distribution respects or violates parity, he and his collaborators knew they needed to study tetrahedral arrangements of four galaxies.

In 2020, Slepian and Cahn came up with a way of defining the “handedness” of a tetrahedral arrangement of galaxies in order to compare the number of left-handed and right-handed ones in the sky.

To determine whether the universe as a whole has a preferred handedness, they had to repeat the analysis for all tetrahedra constructed from their database of 1 million galaxies.

But a factoring trick developed in earlier work on a different problem allowed the researchers to look at the parity of tetrahedra more holistically: Rather than assembling one tetrahedron at a time and determining its parity, they could take each galaxy in turn and group all other galaxies according to their distances from that galaxy, creating layers like the layers of an onion.

The team found a “seven-sigma” level of parity violation in the real data, meaning that the imbalance between left- and right-handed tetrahedra was seven times as large as could be expected from random chance and other conceivable sources of error.

Philcox used similar methods (and had co-authored some earlier papers proposing such an analysis with Hou, Slepian and Cahn), but he made some different choices — for example, grouping the galaxies into fewer layers than Hou and colleagues, and omitting some problematic tetrahedra from the analysis — and therefore found a more modest 2.9-sigma violation of parity.

The ongoing Dark Energy Spectroscopic Instrument survey, for instance, has logged 14 million galaxies so far and will contain more than 30 million when it’s completed.