Constraints on Cosmic Birefringence from SPIDER, Planck, and ACT observations

TL;DR

The paper investigates cosmic birefringence arising from a Chern-Simons coupling in an Early Dark Energy (EDE) context, linking parity-violating photon propagation to a dynamical scalar field. By reformulating the problem in terms of a total rotation angle $α+β$, the authors fit $gM_{Pl}$ and $α+β$ to EB/TB power spectra from SPIDER, Planck, and ACT using the CLASS_EDE framework. They find Planck data strongly favor $α+β>0$ with high significance, while ACT data show tensions in $gM_{Pl}$ but compatible $α+β$ values; SPIDER constrains are weaker and contribute modestly when combined with Planck/ACT. Across datasets, a Planck+ACT combination yields the most robust constraints, whereas all-three-dataset fits exhibit notable tension, highlighting the need for improved low-$ℓ$ measurements and cross-calibrations in future missions like LiteBIRD and AliCPT to decisively test cosmic birefringence and its EDE origin.

Abstract

The Early Dark Energy (EDE) model has been proposed as a candidate mechanism to generate cosmic birefringence through a Chern-Simons coupling between a dynamical scalar field and the cosmic microwave background (CMB) photon. Such birefringence induces a nonzero cross-correlation between the CMB $E$- and $B$-modes, providing a direct observational signature of parity violation. Recent measurements of the $EB$ and $TB$ power spectra, however, cannot yet unambiguously separate instrumental miscalibration ($α$) from a true cosmic-rotation angle ($β$). For this reason, we perform a model-independent analysis in terms of the total effective rotation angle $α+β$. We analyze the latest $EB$ and $TB$ measurements from the SPIDER, Planck, and ACT experiments and derive constraints on the Chern-Simons coupling constant $gM_{Pl}$ and on the polarization rotation angle $α+β$. We find that the coupling $gM_{Pl}$ is not compatible with the SPIDER data, while it provides reasonable fits to the Planck and ACT measurements. The fits for $α+β$ prefer a value larger than zero: when combined, Planck+ACT yield a detection significance of approximately 7$σ$. We also find that ACT data alone do not provide sufficiently tight constraints on either $gM_{Pl}$ or $α+β$, whereas the combination Planck+ACT improves the statistical consistency of ACT's high-$\ell$ results and leads to a better PTE for those measurements.

Figures (8)

• Figure 1: Best fitting results of $gM_{Pl}$ and a constant rotation angle $\alpha+\beta$ using the CMB $EB$ power spectrum from SPIDER. Similar to Table \ref{['tab:chi2-single']}, Base, Base+SH0ES, and BSL, refer to fixing the remaining nine EDE parameters to the best-fit results from Eskilt:2023nxmKochappan:2024jyf. The constraint result of $\alpha+\beta$ from SPIDER-$EB$ is $0.35^\circ\pm{0.69^\circ}$
• Figure 2: The CMB power spectrum of $EB$ mode, that $EB$ data from Planck 2018. The other EDE parameters were fixed by the Base, Base+SH0ES, and BSL datasets. The Chern-Simons constant $gM_{Pl}$ and the minimum of $\chi^2$ are obtained by Planck-$EB$. The constraint result of $\alpha+\beta$ from Planck-$EB$ is $0.29^\circ\pm{0.03^\circ}$.
• Figure 3: The CMB power spectra of $EB$ mode with the ACT-$EB$ data (a), and the CMB $TB$ mode with ACT-$TB$ data (b). The green, red, and blue lines from the different values of $gM_{Pl}$ constraints by ACT and with other parameters fixed in Base, Base+SH0ES, and BSL datasets, respectively.
• Figure 4: The probability density functions for a constant rotation angle, $\alpha+\beta$, using the independent CMB $EB$ measurements from SPIDER, Planck, and $EB$ ($TB$) measurements from ACT (a), and the corresponding results using different combinations of the datasets (b).
• Figure 5: The summary result of CMB-$EB$ power spectra for the individual datasets of SPIDER, Planck, and ACT. The left-hand side (a) shows that during the other EDE parameters in Base, Base+SH0ES, and BSL results, the green, red, pink, blue, and gray lines come from the constraints of Chern-Simons coupling constant $gM_{Pl}$ in SPIDER-$EB$, Planck-$EB$, ACT-$EB$, ACT-$TB$, and SPIDRER+Planck+ACT datasets, respectively. The right-hand side (b) shows the model-independent $\alpha+\beta$ results in corresponding $EB$ datasets.