Scientists led by Fumihiro Naokawa have developed a new method to measure cosmic birefringence more precisely, a subtle rotation in the polarization of the cosmic microwave background. Their analysis indicates the birefringence angle may exceed the earlier estimate of 0.3 degrees due to phase ambiguity. The findings, published in Physical Review Letters, could aid in probing new physics related to dark matter and dark energy.
Cosmic birefringence refers to a faint rotation observed in the polarization of the cosmic microwave background (CMB), the afterglow of the Big Bang. Recent studies have detected this effect, potentially linked to particles like axions, through the CMB EB correlation signal. Previous measurements pegged the rotation angle at about 0.3 degrees, but uncertainties lingered due to phase ambiguity akin to a clock's hands, where angles like 0.3, 180.3, or 360.3 degrees appear identical without additional context. Fumihiro Naokawa, a PhD candidate at the University of Tokyo Graduate School of Science, and Toshiya Namikawa, Project Associate Professor at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), quantified this uncertainty for the first time. Their paper, 'nπ Phase Ambiguity of Cosmic Birefringence,' appears in Physical Review Letters (2026; 136(4), DOI: 10.1103/6z1m-r1j5). Naokawa explained: 'Like a clock, the CMB we can observe is only in its current state. Therefore, rotation angles such as 0.3 degrees, 180.3 degrees, and 360.3 degrees should be indistinguishable. This means the birefringence angle has a phase ambiguity of 180 degrees.' The team devised a technique using the detailed shape of the EB correlation signal to resolve this ambiguity, potentially revealing a larger true angle. This advance also affects EE correlation measurements used for the universe's optical depth, prompting a review of past estimates on cosmic reionization. In a companion study, also in Physical Review Letters (DOI: 10.1103/srfg-9fdy), Naokawa proposed verifying birefringence via radio galaxies powered by supermassive black holes to mitigate telescope errors. Future missions like the Simons Observatory and LiteBIRD stand to benefit, enhancing tests of parity-violating physics.
