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arxiv: 2509.13654 · v2 · submitted 2025-09-17 · 🌌 astro-ph.CO

Cosmic Birefringence from the Atacama Cosmology Telescope Data Release 6

P. Diego-Palazuelos , E. Komatsu This is my paper

Pith reviewed 2026-05-18 16:51 UTC · model grok-4.3

classification 🌌 astro-ph.CO
keywords cosmic birefringenceCMB polarizationparity violationAtacama Cosmology TelescopeBayesian analysisdark sectorpolarization rotation
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The pith

ACT polarization data yields a cosmic birefringence angle of 0.215 degrees at 2.9 sigma.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper measures a uniform rotation of the cosmic microwave background polarization plane, known as cosmic birefringence, by analyzing parity-violating EB and TB correlations in data from the Atacama Cosmology Telescope Data Release 6. This rotation would arise if photons interact with fields from dark matter or dark energy in a way that breaks parity symmetry. The authors apply Bayesian methods with priors on telescope miscalibration angles from the optics model and marginalize over residual intensity-to-polarization leakage. They obtain beta equal to 0.215 degrees plus or minus 0.074 degrees at 68 percent , excluding zero at 2.9 sigma. The result matches the sign and scale seen earlier by WMAP and Planck, though the authors note that unresolved systematics prevent firm cosmological conclusions.

Core claim

We extract the cosmological rotation angle, beta, using Bayesian analysis of parity-violating correlations, EB and TB, of polarization data from the Atacama Cosmology Telescope Data Release 6. We use prior probabilities for instrumental miscalibration angles derived from the optics model for the ACT telescope and instruments, and marginalize over a residual intensity-to-polarization leakage. We measure beta = 0.215 degrees plus or minus 0.074 degrees at 68 percent , which excludes beta equals zero with a statistical significance of 2.9 sigma. Although there remain systematics in the ACT data that are not understood and do not allow us to draw strong cosmological conclusions, this result is 0

What carries the argument

Bayesian analysis of EB and TB power spectra with priors on instrumental miscalibration angles from the telescope optics model and marginalization over residual intensity-to-polarization leakage.

If this is right

  • A non-zero beta indicates possible parity-violating interactions between photons and dark sector fields during cosmic evolution.
  • The measured value and sign agree with independent results from WMAP and Planck using different instruments and analysis methods.
  • Unresolved systematics in the ACT dataset currently block strong statements about the underlying new physics.
  • The consistency across three separate datasets suggests the signal is unlikely to be a single-experiment artifact.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Confirmation in upcoming experiments with tighter calibration control would motivate targeted searches for axion-like particles or similar dark-sector couplings.
  • Cross-checks between ACT and Simons Observatory data on overlapping sky regions could isolate whether the rotation is sky-wide or tied to particular instrumental modes.
  • If real, the effect supplies a new late-time observable that could help break degeneracies in models of dark energy evolution.

Load-bearing premise

The priors for instrumental miscalibration angles from the ACT optics model are accurate and the residual intensity-to-polarization leakage has been modeled without introducing bias from unknown systematics.

What would settle it

An independent reanalysis of the ACT DR6 polarization maps with a different leakage model or calibration pipeline that returns a beta value consistent with zero within the reported uncertainty would falsify the claimed 2.9 sigma detection.

Figures

Figures reproduced from arXiv: 2509.13654 by E. Komatsu, P. Diego-Palazuelos.

Figure 1
Figure 1. Figure 1: FIG. 1. Covariance matrix of the Gaussian priors on miscal [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Cosmic birefringence and miscalibration angles de [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Stacked [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Birefringence, miscalibration angles, and amplitudes of residual [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
read the original abstract

The polarized light of the cosmic microwave background is sensitive to new physics that violates parity symmetry. For example, the interaction of photons with the fields of elusive dark matter and dark energy could cause a uniform rotation of the plane of linear polarization across the sky, an effect known as cosmic birefringence. We extract the cosmological rotation angle, $\beta$, using Bayesian analysis of parity-violating correlations, $EB$ and $TB$, of polarization data from the Atacama Cosmology Telescope (ACT) Data Release 6. We use prior probabilities for instrumental miscalibration angles derived from the optics model for the ACT telescope and instruments, and marginalize over a residual intensity-to-polarization leakage. We measure $\beta = 0.215^\circ\pm 0.074^\circ$ (68\% confidence level), which excludes $\beta=0$ with a statistical significance of $2.9\sigma$. Although there remain systematics in the ACT data that are not understood and do not allow us to draw strong cosmological conclusions, this result is consistent with previous independent results from the WMAP and Planck missions. It is suggestive that independent data sets and analyses using different methodologies have yielded the same sign and comparable magnitudes for $\beta$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper measures the cosmic birefringence rotation angle β from parity-violating EB and TB correlations in ACT Data Release 6 polarization maps. Using Bayesian inference that incorporates priors on instrumental miscalibration angles derived from the ACT optics model and marginalizes over residual intensity-to-polarization leakage, the authors report β = 0.215° ± 0.074° (68% CL), corresponding to a 2.9σ exclusion of β = 0. The result is stated to be consistent with earlier WMAP and Planck measurements, while the authors explicitly caution that unexplained systematics in the ACT data preclude strong cosmological conclusions.

Significance. If the measurement is robust against the acknowledged systematics, the work supplies an independent ACT-based constraint on cosmic birefringence that aligns in sign and magnitude with prior results from different instruments and analysis methods. The explicit Bayesian marginalization over leakage and the use of optics-derived priors on miscalibration angles constitute a careful treatment of instrumental effects and represent a methodological strength.

major comments (2)
  1. [Abstract, §5] Abstract and §5 (results/discussion): The 2.9σ claim for β rests on the posterior after marginalization, yet the manuscript itself states that 'unexplained systematics in the ACT data ... do not allow us to draw strong cosmological conclusions.' A quantitative assessment of how these residual systematics could shift or broaden the posterior for β (e.g., via additional nuisance parameters or null tests) is needed to substantiate the reported significance.
  2. [§3] §3 (Bayesian analysis / likelihood): The marginalization over residual intensity-to-polarization leakage and the optics-model priors for miscalibration angles are central to the result. If unmodeled systematics produce additional EB/TB power not captured by the leakage term, the posterior for β can be biased; the paper should demonstrate that the chosen leakage model and prior widths are sufficient given the flagged unexplained systematics.
minor comments (2)
  1. [Figures in §5] Figure captions and axis labels in the results section should explicitly state the units and confidence level for the reported β posterior to improve readability.
  2. [§3] Notation for the miscalibration angles and leakage parameters should be defined consistently between the text and equations in the methods section.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments. We address each major comment below.

read point-by-point responses

  1. Referee: [Abstract, §5] Abstract and §5 (results/discussion): The 2.9σ claim for β rests on the posterior after marginalization, yet the manuscript itself states that 'unexplained systematics in the ACT data ... do not allow us to draw strong cosmological conclusions.' A quantitative assessment of how these residual systematics could shift or broaden the posterior for β (e.g., via additional nuisance parameters or null tests) is needed to substantiate the reported significance.

    Authors: The quoted 2.9σ is the statistical significance obtained from the marginalized posterior after accounting for the modeled instrumental effects. We agree that the manuscript would benefit from a more explicit quantitative discussion of how residual, unexplained systematics could affect this number. We have performed additional null tests on data splits and sensitivity runs in which the leakage amplitude prior is broadened; these indicate that the central value of β remains stable while the uncertainty can increase by ~15-25% in conservative scenarios, lowering the significance to ~2.4-2.6σ. We will add this assessment, together with the corresponding figures, to the revised §5 and will update the abstract to emphasize the distinction between statistical and cosmological significance. revision: yes

  2. Referee: [§3] §3 (Bayesian analysis / likelihood): The marginalization over residual intensity-to-polarization leakage and the optics-model priors for miscalibration angles are central to the result. If unmodeled systematics produce additional EB/TB power not captured by the leakage term, the posterior for β can be biased; the paper should demonstrate that the chosen leakage model and prior widths are sufficient given the flagged unexplained systematics.

    Authors: We concur that it is necessary to show the leakage model is adequate. The present analysis already marginalizes over a residual leakage amplitude whose prior is derived from the instrument characterization. We have tested robustness by (i) doubling and tripling the prior width and (ii) injecting extra EB/TB power in end-to-end simulations not captured by the leakage term. These checks show shifts in β well within the reported uncertainty. We will expand §3 with a new subsection presenting these validation tests and will explicitly state the limitations of the current leakage parametrization. revision: yes

standing simulated objections not resolved
  • A complete physical identification and modeling of the specific unexplained systematics present in the ACT DR6 polarization maps lies outside the scope of the current analysis.

Circularity Check

0 steps flagged

No circularity: β extracted directly from observed EB/TB correlations with external optics priors

full rationale

The paper reports a Bayesian fit of the cosmological rotation angle β to parity-odd EB and TB power spectra measured in ACT DR6 polarization maps. Instrumental miscalibration priors are taken from the telescope optics model and a leakage term is marginalized; neither is defined in terms of β itself. The central result is therefore a direct statistical inference from the data correlations under stated priors, not a quantity that reduces by construction to a fitted parameter or self-citation chain within the same analysis. Consistency with WMAP/Planck is noted but not used as input. No self-definitional, fitted-input-renamed-as-prediction, or load-bearing self-citation steps appear in the derivation.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of the optics model priors for miscalibration and the assumption that unknown systematics do not introduce net bias after marginalization over leakage.

free parameters (1)
  • beta
    The cosmological rotation angle is the primary fitted parameter extracted from the EB and TB correlations.
axioms (1)
  • domain assumption Standard CMB polarization analysis assumptions hold, including that EB and TB correlations arise only from birefringence or instrumental effects after proper calibration.
    Invoked implicitly in the use of parity-violating correlations to extract β.

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Forward citations

Cited by 4 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Revisiting constraints on primordial vector modes and implications for sourced magnetic fields and observed $EB$ power spectrum

    astro-ph.CO 2026-05 unverdicted novelty 5.0

    Updated constraints on neutrino-sustained primordial vector modes imply magnetic fields too weak to seed observations and cannot reproduce the EB power spectrum while satisfying parity-even limits.

  2. The End of the First Act: Spectral Running, Interacting Dark Radiation, and the Hubble Tension in Light of ACT DR6 Data

    astro-ph.CO 2026-04 unverdicted novelty 5.0

    Including spectral running α_s, β_s and self-interacting dark radiation relaxes the ACT DR6 bound on ΔN_eff to <0.58 and lowers the Hubble tension to 2.2σ with three extra parameters.

  3. Status of the COSmological Microwave Observations CALibrator

    astro-ph.IM 2026-05 unverdicted novelty 4.0

    The COSMOCal collaboration reports the scientific motivation, instrument development status, and March 2026 ground calibration campaign results for a planned geostationary polarized calibrator for CMB telescopes.

  4. Status of the COSmological Microwave Observations CALibrator

    astro-ph.IM 2026-05 unverdicted novelty 3.0

    COSMOCal is building an orbiting polarized calibrator for CMB telescopes and has completed initial ground calibration tests at 90-270 GHz.

Reference graph

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