Isotropic cosmic birefringence β ≈ 0.3°: separate a parity-violating rotation from polarization-angle miscalibration
Difficulty 44.5: the signal is at 3–4.8σ depending on calibration priors, and the bottleneck (an absolute angle known to ≲0.05°) has visible routes in drone calibrators, Simons Observatory and LiteBIRD, so it sits below primordial B-modes (55), which need the same calibration plus foreground and delensing control, and above the neutrino-mass tension (37), whose culprits are better isolated. Application value 5.5: none outside physics, just below B-modes (6). Leverage 51: a confirmed parity-violating rotation would be direct evidence for an axion-like field coupled to photons, just below B-modes (54).
The problem
Is the polarization of the Big Bang light twisted slightly, hinting at new physics, or is it a calibration error?
- 1Axion-like field could twist light's polarization
- 2CMB shows hint of 0.3 degree rotation
- 3Tilted detectors give the same signal
- 4Need absolute angle calibration
- 5Confirm 5 sigma or find a systematic
The effect. A parity-violating coupling of an axion-like field φ to photons rotates the plane of linear polarization of CMB photons by an angle β between last scattering and today. This mixes E and B modes: it creates an EB correlation proportional to sin(4β) times the difference between the EE and BB power spectra, in addition to calibration terms.
Built from
earlier problems → this oneThe problems this one is built from: progress or tools it needs (builds on), special cases that come first (generalizes), and weaker results on the way (implies). On the right, what builds on it.
- Start with
- Isotropic cosmic birefringence β ≈ 0.3°: separate a parity-violating rotation from polarization-angle miscalibrationThis problem
- Then
Path to understanding
foundations → problemThe research entries this problem rests on, in the order to read them. Each links to its explainer.
- Start with
- Step 2
- Step 3
- Step 4
- The cosmic microwave backgroundTheory · host
- Then
- Isotropic cosmic birefringence β ≈ 0.3°: separate a parity-violating rotation from polarization-angle miscalibrationThe problem
State of the art
Minami and Komatsu (arXiv:2011.11254, PRL 2020) estimated β and the detector angle simultaneously using the Galactic-foreground EB correlation: β = 0.35 ± 0.14° (68%), 2.4σ. Eskilt and Komatsu (arXiv:2205.13962, PRD 2022) combined WMAP and Planck: β = 0.342° (+0.094/−0.091), excluding zero at 3.6σ, with no evidence of frequency dependence (also Eskilt, arXiv:2201.13347). ACT DR6, separating the instrumental angle with a model of the telescope optics, gives β = 0.215° ± 0.074°, 2.9σ (Diego-Palazuelos and Komatsu, arXiv:2509.13654, PRD). A joint Planck PR4 + ACT DR6 analysis gives β = 0.277° ± 0.057°, 4.8σ, if the instrumental-angle priors are taken at face value, and 3.5σ in a dust-mitigated test (Eskilt, arXiv:2608.06480, August 2026, unrefereed). Lonappan, Keating and Arnold (arXiv:2609.09149, ApJL 2026) apply a birefringence-blind differential-calibration estimator to Planck NPIPE maps and reproduce the Minami–Komatsu calibration pattern (χ² = 8.18 for 7 dof), giving β = 0.37 ± 0.12° conditionally; the authors present it as a consistency check, not an independent measurement. The limiting systematic is the absolute angle: the Crab Nebula (Tau A), the standard celestial calibrator, is known to about 0.27° (Aumont et al. 2020), and drone-borne calibrators (e.g. PROTOCALC for Simons Observatory and CLASS) had first flights in 2024–25.
Realistic applications
A confirmed β ≠ 0 would be evidence of parity-violating physics (axion-like dark energy or dark matter coupled to photons) and set g_{φγ}; absolute polarization-angle calibration is also required for primordial B-mode searches.
What counts as solved
A measurement of β whose instrumental-angle calibration is independent of Galactic foregrounds and of CMB EB itself (an artificial calibrator such as a drone, balloon or satellite source, or a celestial calibrator known to ≲0.05°), giving β ≠ 0 at ≥ 5σ with foreground-model explanations excluded in refereed work; or a result consistent with β = 0 with an uncertainty well below the current ±0.057°.
How it gets verified
ObservationSettled by new measurements or observations.
Needs a new measurement of the birefringence angle with an instrumental-angle calibration independent of Galactic foregrounds and of CMB EB (an artificial calibrator such as a drone, balloon or satellite source, or a celestial calibrator known to about 0.05 degrees), giving a non-zero angle at 5 sigma or more with foreground explanations excluded.
Claims under verification
+ add a claimNo claimed results on the Verification board for this problem yet.
Where the work is
Mostly calibration hardware and new data; the dust modelling needs theory and simulation.
Bottlenecks
ranked by votes- #1EngineeringNo absolute polarization-angle calibrator at the 0.05° level
The Crab Nebula (Tau A), the standard celestial calibrator, is known only to about 0.27°, and artificial calibrators (drone-borne sources such as PROTOCALC) have only just begun flying; CMB data alone cannot distinguish rotation from miscalibration.
Community grades
Sign in to add your own grade.
Connected problems
Progress
- Aug 6, 2026Planck PR4 + ACT DR6: 4.8σ (preprint)source
β = 0.277° ± 0.057° with instrumental priors at face value; 3.5σ in a dust-mitigated test.
- Sep 17, 2025ACT DR6: β = 0.215° ± 0.074°source
2.9σ with the instrumental angle set by an optics model rather than by foregrounds.
- May 27, 2022WMAP+Planck 3.6σsource
β = 0.342° (+0.094/−0.091) with no frequency dependence.
- Nov 23, 2020First 2.4σ hint of birefringencesource
Minami and Komatsu use the foreground EB correlation to separate miscalibration from β.