Impact of CMB low-$\ell$ EE polarization data on dark energy parameterizations

Abstract

Measurement of the optical depth to reionization ($τ_\mathrm{reio}$) is largely driven by the large-scale EE polarization mode of CMB ($\ell<30$). Removing the low-$\ell$ EE data potentially alleviates various cosmological tensions. In this work, we study the effect of the low-$\ell$ EE polarization mode on the CPL, JBP and BA dark energy parameterizations using CMB data from Planck and ACT DR6, combined with DESI BAO and PantheonPlus compilation of Type Ia supernovae. We find that excluding low-$\ell$ EE data shifts $τ_\mathrm{reio}$ and $A_s$ to higher values through the unbroken $A_s-τ_\mathrm{reio}$ degeneracy with a $\sim(1.4-1.8)σ$ shift in $A_s$ for $Λ$CDM and JBP, and a milder $\sim 1σ$ shift for CPL and BA. The equation of state (EOS) for all three parameterizations move towards the quintessence regime ($w(z) > -1$) upon exclusion of low-$\ell$ EE data, driven primarily by the strengthening of the $w_a-A_s$ and the $w_a-τ_\mathrm{reio}$ correlations, with a small effect from the correlations of $w_0$ with $A_s$ and $τ_\mathrm{reio}$. The most prominent effect occurs in JBP, where the EOS lies entirely within the quintessence regime at $1σ$ when excluding low-$\ell$ EE data. Model comparison through AIC shows positive evidence in favor of CPL and BA and weak evidence in favor of JBP, robust to the inclusion of low-$\ell$ EE data and CMB data used. DIC model comparison shows strong (positive) evidence in favor of CPL and BA when including (excluding) low-$\ell$ EE data. For JBP, ACT DR6 yields positive (weak) evidence when including (excluding) low-$\ell$ EE data, while Planck weakly favors $Λ$CDM regardless of low-$\ell$ EE inclusion.