2026-01-28 東京大学
宇宙複屈折の回転角を測定する際に生じる180度の位相不定性についての概念図（Credit: Naokawa, Namikawa, higgstan.com）
＜関連情報＞
宇宙複屈折の位相の曖昧さ Phase Ambiguity of Cosmic Birefringence
Fumihiro Naokawa, Toshiya Namikawa, Kai Murai, Ippei Obata, and Kohei Kamada
Physical Review Letters Published: 27 January, 2026
DOI: https://doi.org/10.1103/6z1m-r1j5
Abstract
We point out that the rotation angle of cosmic birefringence, which is a recently reported parity-violating signal in the cosmic microwave background (CMB), has a phase ambiguity of ( ∈ℤ). This ambiguity has a significant impact on the interpretation of the origin of cosmic birefringence. Assuming an axionlike particle (ALP) as the origin of cosmic birefringence, this ambiguity can be partly broken by the anisotropic cosmic birefringence and the shape of the CMB angular power spectra. We forecast constraints on from ongoing and upcoming experiments and find that most nonzero values of can be excluded using the cross-power spectrum between and modes. We also find that measuring the reionization bump in the -mode auto-power spectrum can give further constraints, provided that the optical depth is determined independently of the CMB. The change in the low- -mode power spectrum may also provide a potential solution to the tension in suggested by the latest CMB and baryon acoustic oscillation measurements.
