Gravitational lensing gives the CMB a faint circular twist

When light travels through curved spacetime, its trajectory shifts slightly sideways depending on its handedness — the optical Magnus effect. Applied to gravitational lensing of the CMB, this means left- and right-handed photons reaching the same detector originated from slightly different points on the last-scattering surface, mixing temperature fluctuations into circular polarization. The predicted signal is far below any current or near-future instrument's sensitivity, but the calculation identifies a previously unrecognized fundamental mechanism linking spin-Hall physics to CMB observables.