Hidetoshi Omiya

The nature of dark matter has been puzzling physicists for decades. Among the candidates, axions hold a special place — ultralight particles that could simultaneously solve the strong CP problem. If they exist, they barely interact with light, rotating its polarization plane.

In a close binary system, the hot star, like a searchlight, illuminates its cooler companion. Some of the light scatters off free electrons in its atmosphere — this is Thomson scattering — and acquires linear polarization. As the stars orbit each other, this polarization rotates, like the hand of a peculiar cosmic metronome.

A handful of pebbles drops into a pond: each one sends out ripples. If the pebbles are different, the ripples will be different too: some sharp, some smooth. But when you look at the water from a distance and see only a small part of the ripples, all those differences blur together.