A thousand years ago, Vikings carried Icelandic spar on their sea voyages. Through this transparent crystal, any light beam splits into two — left and right. This simple effect helped them find the sun even in fog. Today, physicists are searching for something similar in space.

Collisions of black holes and neutron stars in tight binaries give rise to gravitational waves — ripples in spacetime. According to Einstein's theory, the left- and right-handed components of such a wave travel exactly the same way. But cosmic mysteries hint otherwise. As far back as the 1930s, Fritz Zwicky noticed hidden mass in galaxy clusters. Later, Vera Rubin confirmed this for individual galaxies. Then in 1998, Adam Riess and colleagues discovered that the expansion of the universe is accelerating, and this is thought to be driven by dark energy. Data from the cosmic microwave background (the afterglow of the Big Bang) and the large-scale structure of galaxies suggest that left and right might not be on equal footing in the Universe.

To explain this, scientists are building modified gravity theories. One of them adds a special term to Einstein's equations that breaks the symmetry between right and left. Spacetime then becomes akin to Icelandic spar: left- and right-handed gravitational waves experience it differently. The authors of the new study calculated just how large this effect, double refraction, would be in a specific model. It turned out that in one version of the theory, the distortion is enhanced by tens of percent. Sensitive detectors like LIGO already provide an upper limit on this effect, and future gravitational-wave observatories will be able to detect it over vast distances, where the influence of dark energy is stronger and redshift is high. So this ancient “sunstone” helps us peer into the darkest mystery of the cosmos.