A flashlight beam glides over uneven glass. The glass is massive galaxy clusters, and the flashlight is ancient light born shortly after the big bang. Passing by such clusters, light doesn't just bend around them like water around a rock. Its path shifts slightly sideways, and the amount of shift depends on how the light is 'twisted'—right or left. This is like a soccer ball that curves in flight due to its spin. In astrophysics, this behavior is called the optical Magnus effect.
The authors applied this idea to gravitational lensing — the phenomenon where massive bodies curve the path of light. They considered the cosmic microwave background — the faint glow left over from the hot early universe. Usually, we measure its temperature and linear polarization. But the researchers showed that if you account for the transverse shift of the beam, circular polarization should also arise — light begins to spiral. The reason is that left- and right-handed waves reach us from slightly different points on the last scattering surface: that distant 'wall' where light first could travel freely.
This mechanism is not just a mathematical curiosity. It links two stages of the universe's history: the moment it became transparent and the epoch of large-scale structure formation — galaxies and their clusters. Someday, ultra-sensitive detectors might catch this faint signal, giving us a new tool to probe the invisible web of dark matter and dark energy.
as written by the authors arXiv:2605.16945
Polarization of the cosmic microwave background (CMB) brings out information not only on the early universe but also on the late-time large-scale structure via weak gravitational lensing. Here, we show that circular polarization is induced in principle from CMB temperature fluctuations when the optical Magnus effect is incorporated into gravitational lensing. This is a consequence of the transverse shift of a trajectory of light depending on its helicity that requires right-handed and left-handed components at the same observation point to be sourced from different points of the surface of last scattering. Whereas the resulting circular polarization is found far beyond the scope of current detection, our work establishes the optical Magnus effect on gravitational lensing as a new fundamental mechanism to produce circular polarization of CMB.
The optical Magnus effect is a close cousin of what makes a spinning ball curve through the air. Only here, instead of air, it's curved spacetime, and instead of a ball, it's a light wave.
In Stanislaw Lem's novel 'Solaris', the planet's ocean interacted with radiation, altering its properties. Something similar, but on a cosmic scale, happens to the relic light as it passes through gravitational lenses.