By Jialin Yang, Spring 2023.

Imagine looking in a mirror and not seeing yourself. That was the shock astronomers had last year when analyzing the large-scale structure of galaxy distributions. 

Two research teams – one led by Oliver Philcox of Columbia University [1] and the other by Jiamon Hou and Zachary Slepian of University of Florida with Robert Cahn of the Lawrence Berkeley National Laboratory [2] – discovered that the distribution of galaxies is lopsided. While we may expect galaxies to be spread out symmetrically, the researchers find a startling violation of parity, or left-right symmetry. To understand why no known physics explains this phenomenon, we first need to go back to one trillionth of one trillionth of one trillionth of a second after the Big Bang.

In cosmology, the time between 10^-36 seconds to 10^-32 seconds after the Big Bang is called the inflationary epoch. [3] During this time, the Universe rapidly expanded, increasing linearly by about a factor of 10^26. [4] That’s about the equivalent of something the length of a water molecule expanding to the distance to our nearest star. Tiny variations in the distribution of matter caused by quantum fluctuations suddenly became the blueprint for how large pockets of matter like the Milky Way and Andromeda are distributed today. [5]

The current era of our universe is governed by four known fundamental forces: the strong force, weak force, electromagnetism, and gravity. Out of these four, the weak force has been found to violate parity. Certain weak interactions behave differently for left-handed particles compared to right-handed ones (here, handedness refers to the spin of the particle). The weak force, however, only plays a minor role in the formation of the large-scale structure. [6]

The key force responsible for the movement of galaxies is gravity. Over cosmological timescales, gravity shaped the universe as we know it today. But gravity is supposed to respect parity. Everything we know about gravity – from our theories about spacetime to our observations of celestial bodies – is consistent with the idea that gravity does not have spatial preference. [1] [2] As such, we expect the large-scale structure of the universe to be symmetric. If we find an asymmetry or parity violation in the large-scale structure, it must have originated during the inflationary epoch.

So how exactly did Philcox and Hou, Slepian, and Cahn find the parity violation? They looked at the simplest 3-dimensional shape that can be asymmetrical: tetrahedrons. From a dataset of galaxies, researchers built tetrahedrons by choosing groups of four galaxies and assigning one vertex to each galaxy. Each galaxy tetrahedron could be classified as left- or right-handed depending on whether the lengths of the sides of the tetrahedron increased in a clockwise or counterclockwise direction.

Hou, Slepian, and Cahn’s diagram showing how left- and right- tetrahedra are classified.

The teams then reran the same tetrahedron analysis on simulated galaxy data that assumed parity. The simulated data allowed the scientists to form an expectation about how much the difference between left- and right- tetrahedra should vary if parity is preserved. By comparing their empirical data to the simulated data, the teams were able to conclude whether their results came from noise or if they have truly detected a parity violation.

Ultimately, Hou, Slepian, and Cahn found 3.1-sigma and 7.1-sigma deviations using the LOWZ and CMASS datasets [2], and Philcox found a 2.9-sigma deviation using the CMASS dataset. [1] For reference, a 3.1-sigma deviation corresponds to a 0.1% chance that the asymmetry was caused by gravity alone. These two teams may have just found new physics.

As with any unexpected result, rigorous verification is needed. Fortunately, the method of counting tetrahedrons is easily replicable, especially with other data sets. Upcoming galaxy surveys such as the Dark Energy Spectroscopic Instrument Survey (DESI) will produce vast new datasets, meaning the asymmetry result should be even more statistically significant. [2]

Perhaps realizing that galaxies are lopsided will be the key to understanding fundamental questions about how our universe works. In reference to these findings, cosmology expert Marc Kaminowski of John Hopkins University said, “If this result is real, someone’s going to get a Nobel Prize.” [6]

 

[1] Philcox, Oliver. 2022. “Probing Parity-Violation with the Four-Point Correlation Function of BOSS Galaxies.” Physical Review D, Volume 106, Issue 6, article id.063501. https://doi.org/10.48550/arXiv.2206.04227.

[2] Hou, Jiamin, Zachary Slepian, and Robert N. Cahn. 2022. “Measurement of Parity-Odd Modes in the Large-Scale 4-Point Correlation Function of SDSS BOSS DR12 CMASS and LOWZ Galaxies.” eprint arXiv. https://doi.org/10.48550/arXiv.2206.03625.

[3] Saunders, Simon. “Inflation.” Philosophy of Cosmology, University of Oxford. http://philosophy-of-cosmology.ox.ac.uk/inflation.html.

[4] “The Origins of the Universe: Inflation.” The Centre for Theoretical Cosmology, University of Cambridge.  https://www.ctc.cam.ac.uk/outreach/origins/inflation_zero.php.

[5] Wolchover, Natalie. “Cosmic Triangles Open a Window to the Origin of Time.” Quanta Magazine. https://www.quantamagazine.org/the-origin-of-time-bootstrapped-from-fundamental-symmetries-20191029/.

[6] McCormick, Katie. “Asymmetry Detected in the Distribution of Galaxies.” Quanta Magazine. https://www.quantamagazine.org/asymmetry-detected-in-the-distribution-of-galaxies-20221205/.