Title: Testing Mirror Symmetry in the Universe with LIGO-Virgo Black-Hole Mergers
Authors: Juan Calderón Bustillo, Adrian del Rio, Nicolas Sanchis-Gual, Koustav Chandra, and Samson H. W. Leong
First Author’s Institution: Instituto Galego de Física de Altas Enerxías at the Universidade de Santiago de Compostela in Galicia, Spain and The Department of Physics at The Chinese University of Hong Kong in Hong Kong
Status: Published in Physical Review Letters, Volume 134 [closed access]
Today’s paper uses gravitational waves (GWs) to examine some of the fundamental assumptions that make up one of astronomy’s top 5 ops: the standard cosmological model of the universe. If you aren’t familiar with the long-standing beef between the field of astronomy and the standard cosmological model of the universe, we might be a little surprised because here at Astrobites, we write about it pretty often, but don’t fret! You can read the following articles to catch up on some of the lore: “So how’s it going with the Hubble tension?”, “A New H0pe for the Hubble Constant?”, and “The Hubble Tension is Still Tense”.
While the Hubble Constant, or H0, is one of the most commonly discussed issues with the standard cosmological model of the universe, is not the only troublemaker, and today’s paper takes a closer look at the cosmological principle using gravitational waves. The cosmological principle is the assumption that when viewed at large scales, the universe should be homogenous (e.g., the universe looks the same regardless of where it is viewed from) and isotropic (e.g., the structure of the universe will not look different when viewed from different directions). Prior studies investigate the homogeneity and isotropy of the universe using the distribution of GWs in the sky and orientation of GWs with respect to the observer (us), but today’s paper uses a different approach by studying the “handedness” of GWs.
Handedness describes the spin of an object relative to its motion. Something that is “right-handed” has both its spin and motion in the same direction (mirror symmetric–looks the same when mirrored), while something “left-handed” has its spin and motion in opposite directions (mirror asymmetric–does not look the same when mirrored). Today’s authors measure the handedness of binary black holes (BBHs) that collapse and create GWs and then essentially look at all of the measurements to see if there is an equal amount of right- and left-handed BBHs in the universe–confirming (or disproving…) the cosmological principle.
The symmetry of a BBH
So how do you measure the handedness of a BBH? For isolated astrophysical sources (using general relativity) mirror asymmetry of an object can be quantified using something called the Chern-Pontyagin scalar (a fancy pre-defined integral outlined in the paper). The authors derive the observable parameter VGW, or the gravitational Stokes parameter, from the Chern-Pontryagin scalar, which can be equal to 0 only when there is mirror symmetry and no net flux of circularly polarized gravitational radiation. The authors use the posterior distributions for the masses and spins from 47 BBH mergers with measured GWs to create the posterior distributions for the VGW of each source seen in Figure 1.

With the exception of GW200129 (the green outlier in Figure 1), all of the GW events appear to have a VGW value near 0, indicating mirror symmetry! The authors find a median value of \(\langle V_{GW} \rangle = -0.013 ^{+ 0.142}_{- 0.141}\), which is so close to 0 that there is essentially no evidence that mirror symmetry is violated in this sample of 47 BBHs, so the cosmological principle is safe for now…
GW200129 has a net emission from a circularly polarized GW (a significant non-zero VGW), which is consistent with observations that GW200129 was a precessing BBH (all objects with a non-zero VGW precess). Additionally, since GW200129 is such an outlier, some fraction of the other BBHs must have a non-zero VGW for \(\langle V_{GW} \rangle\) to be so close to zero–even if our GW measurements are not sensitive enough to detect that yet. The authors calculate that of the 47 BBHs, 82% of them must have a non-zero VGW and also have orbital precession!
Finding that such a high percentage of the BBH sample has some amount of precession can help us constrain the formation channels for BBHs, which is highly debated. If each BH in a BBH forms alone and evolves independently, we would expect to see more aligned spins (VGW = 0), but the results from this paper favor dynamical BBH formation and evolution, which are commonly used to explain the current observations of merging BHs.
While the cosmological principle is safe for now, astronomers are always coming up with new ways to challenge the standard cosmological model of the universe–determined to overcome one of our fiercest ops.
Astrobite edited by Sowkhya Shanbhog
Featured image credit: Made with images from LIGO/T. Pyle, NASA/WMAP Science Team, and Imgflip, made with love by Erica Sawczynec