Cosmic Handedness Might Show Up in Galaxy Spins
Could galaxies be organized in a preferentially left- or right-handed way? Possible hints of such “parity violation” have been found in the way galaxies group together. In a new study, Junsup Shim of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan and his colleagues explore whether a similar signal might be visible in the way galaxies rotate [1]. The researchers predict that galaxy-rotation maps could exhibit a parity-violating pattern that is strong enough to be detectable in upcoming galaxy surveys.
Most physical processes are blind to handedness, but parity violation does occur in nature. For example, weak interactions distinguish between left- and right-handed particles. Certain cosmological models predict that parity violation arose in the initial distribution of matter, following an early epoch of expansion, or “inflation.” A potential relic of this primordial handedness was recently reported in the statistics of four-galaxy groups—specifically in the way the four galaxies are arranged relative to each other [2, 3].
Another parity-violation signal might be hiding within galaxy data, according to Shim and his colleagues. The researchers ran simulations of a universe seeded with parity violation in its early, pregalaxy distribution of matter. As this simulated matter congealed into galaxies, the team found that the parity violation left a mark on galaxy rotations. The proposed signal would appear in complex clockwise and counterclockwise patterns extracted from galaxy-rotation maps. The researchers show that this signal could potentially be observed by the Dark Energy Spectroscopic Instrument (DESI) experiment, which is expected to provide rotation-relevant data on over 10 million nearby galaxies.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- J. Shim et al., “Probing vector chirality in the early Universe,” Phys. Rev. Lett. 135, 141002 (2025).
- O.H. E. Philcox et al., “Probing parity violation with the four-point correlation function of BOSS galaxies,” Phys. Rev. D 106, 063501 (2022).
- J. Hou et al., “Measurement of parity-odd modes in the large-scale 4-point correlation function of Sloan Digital Sky Survey Baryon Oscillation Spectroscopic Survey twelfth data release CMASS and LOWZ galaxies,” Mon. Not. R. Astron. Soc. 522, 5701 (2023).