Revisiting the $A_L$ Lensing Anomaly in Planck 2018 Temperature Data

TL;DR

This paper investigates the lensing anomaly in Planck 2018 temperature data, revealing its dependence on frequency, sky masking, and foreground treatment, and discusses implications for cosmological models.

Contribution

It provides a detailed analysis of the $A_L$ lensing anomaly, highlighting its complex dependence on data selection and foregrounds, and emphasizes the need for robustness checks in cosmological modeling.

Findings

$A_L$ varies with ecliptic latitude, showing a $2.9\sigma$ preference for $A_L>1$ near the ecliptic.

217 GHz data show a stronger preference for $A_L>1$ than lower frequencies.

Removing 217 GHz data reduces the anomaly to $1.8\sigma$, indicating its significant influence.

Abstract

We revisit the lensing anomaly in the Planck 2018 temperature (TT) data and examine its robustness to frequency selection and additional sky masking. Our main findings are: (1) The phenomenological lensing amplitude parameter, $A_L$, varies with ecliptic latitude, with a $2.9\sigma$ preference for $A_L>1$ near the ecliptic, and $1.0\sigma$ preference near the ecliptic poles, compared to $2.5\sigma$ on the original masks. This behavior is largely or solely from 217 GHz and suggestive of some non-random effect given the Planck scan strategy. (2) The 217 GHz TT data also show a stronger preference for $A_L>1$ than the lower frequencies. The shifts in $A_L$ from 217 GHz with additional Galactic dust masking are too large to be explained solely by statistical fluctuations, indicating some connection with the foreground treatment. Overall, the Planck $A_L$ anomaly does not have a single simple cause. Removing the 217 GHz TT data leaves a $1.8\sigma$ preference for $A_L>1$. The low-multipole ($\ell<30$) TT data contribute to the preference for $A_L>1$ through correlations with $\Lambda$CDM parameters. The 100 and 143 GHz data at $\ell\geq30$ prefer $A_L>1$ at $1.3\sigma$, and this appears robust to the masking tests we performed. The lensing anomaly may impact fits to alternative cosmological models. Marginalizing over $A_L$, optionally applied only to Planck TT spectra, can check this. Models proposed to address cosmological tensions should be robust to removal of the Planck 217 GHz TT data.