Tickling the CMB damping tail: scrutinizing the tension between the ACT and SPT experiments

TL;DR

This study investigates tensions between ACT and SPT measurements of the CMB damping tail by allowing both the effective number of relativistic species $N_ ext{eff}$ and the lensing amplitude $A_L$ to vary, while incorporating WMAP9 and external priors. The authors find ACT prefers $N_ ext{eff}\approx3.23\pm0.47$ with $A_L\approx1.65\pm0.33$, whereas SPT prefers $N_ ext{eff}\approx3.76\pm0.34$ with $A_L\approx0.81\pm0.12$, and the two results remain in tension at more than $95\%$ c.l. even after including HST and BAO data. A degeneracy between $A_L$ and the total neutrino mass $\Sigma m_\nu$ means allowing $A_L$ to vary weakens the SPT neutrino-mass signal and can amplify ACT’s inferred lensing amplitude; conversely, fixing $A_L=1$ enhances the SPT mass hint. The authors discuss potential explanations—from dark radiation and modified gravity to unidentified systematics—and highlight Planck data as crucial for resolving this discrepancy.

Abstract

The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have recently provided new, very precise measurements of the cosmic microwave background (CMB) anisotropy damping tail. The values of the cosmological parameters inferred from these measurements, while broadly consistent with the expectations of the standard cosmological model, are providing interesting possible indications for new physics that are definitely worth of investigation. The ACT results, while compatible with the standard expectation of three neutrino families, indicate a level of CMB lensing, parametrized by the lensing amplitude parameter A_L, that is about 70% higher than expected. If not a systematic, this anomalous lensing amplitude could be produced by modifications of general relativity or coupled dark energy. Vice-versa, the SPT experiment, while compatible with a standard level of CMB lensing, prefers an excess of dark radiation, parametrized by the effective number of relativistic degrees of freedom N_eff. Here we perform a new analysis of these experiments allowing simultaneous variations in both these, non-standard, parameters. We also combine these experiments, for the first time in the literature, with the recent WMAP9 data, one at a time. Including the Hubble Space Telescope (HST) prior on the Hubble constant and information from baryon acoustic oscillations (BAO) surveys provides the following constraints from ACT: N_eff=3.23\pm0.47, A_L=1.65\pm0.33 at 68% c.l., while for SPT we have N_eff=3.76\pm0.34, A_L=0.81\pm0.12 at 68% c.l.. In particular, the A_L estimates from the two experiments, even when a variation in N_eff is allowed, are in tension at more than 95% c.l..

Figures (4)

• Figure 1: Constraints in the $A_L$ - $N_\mathrm{eff}$ plane from a CMB only analysis (left panel) and including the HST prior and BAO (right panel). The blue contour includes the ACT data while the red contour refers to the SPT data. The line at $A_L=1$ indicates the standard expectations based on General Relativity. The line at $N_\mathrm{eff}=3.046$ indicates the prediction from the standard model with three neutrino flavours.
• Figure 2: Constraints in the $\Omega_b h^2$ - $n_s$ plane (left panel) and on the $\Omega_ch^2$-$H_0$ plane (right panel) from ACT (blue contours) and SPT (orange contours) including WMAP9, HST and BAO data. The ACT-SPT tension is less pronounced for these parameters.
• Figure 3: Posterior distribution function for the total neutrino mass parameter $\Sigma m_{\nu}$ from a SPT+WMAP+BAO+HST analysis (left panel) and ACT+WMAP+BAO+HST (right panel) in the case of fixing lensing to $A_L=1$ and letting it to vary. As we can see, if we let the $A_L$ parameter to vary the small indication for a neutrino mass from the SPT analysis vanishes. At the same time, letting the $A_L$ parameter to vary weakens the constraints from ACT.
• Figure 4: Constraints in the $A_L$ vs $\Sigma m_{\nu}$ plane for the SPT+WMAP+BAO+HST and ACT+WMAP+BAO+HST datasets. A degeneracy is present between the two parameters: larger values for $A_L$ let larger neutrino masses to be more consistent with the data. The SPT indication for a neutrino mass is driven by the low value of $A_L$ obtained in the neutrino massless case.