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We investigate constraints on early dark energy (EDE) using ACT DR4, SPT-3G2018, Planck polarization, and restricted Planck temperature data (at$\ell<650$), finding a $3.3\sigma$ preference ($\Delta\chi^2=-16.2$ for 3additional degrees of freedom) for EDE over $\Lambda$CDM. The EDE contributes amaximum fractional energy density of $f_{\rm EDE}(z_c)=0.163^{+0.047}_{-0.04}$at a redshift $z_c=3357\pm200$ and leads to a CMB inferred value of the Hubbleconstant $H_0=74.2^{+1.9}_{-2.1}$ km/s/Mpc. We find that Planck and ACT DR4data provide the majority of the improvement in $\chi^2$, and that theinclusion of SPT-3G pulls the posterior of $f_{\rm EDE}(z_c)$ away from$\Lambda$CDM. This is the first time that a moderate preference for EDE hasbeen reported for these three combined CMB data sets. We find that includingmeasurements of supernovae luminosity distances and the baryon acousticoscillation standard ruler only minimally affects the preference ($3.0\sigma$),while measurements that probe the clustering of matter at late times - thelensing potential power spectrum from Planck and $f \sigma_8$ from BOSS -decrease the significance of the preference to 2.6$\sigma$. Conversely, addinga prior on the $H_0$ value as reported by the SH0ES collaboration increases thepreference to the $4-5\sigma$ level. In the absence of this prior, theinclusion of Planck TT data at $\ell>1300$ reduces the preference from$3.0\sigma$ to $2.3\sigma$ and the constraint on $f_{\rm EDE}(z_c)$ becomescompatible with $\Lambda$CDM at $1\sigma$. We explore whether systematic errorsin the Planck polarization data may affect our conclusions and find thatchanging the TE polarization efficiencies significantly reduces the Planckpreference for EDE. More work will be necessary to establish whether thesehints for EDE within CMB data alone are the sole results of systematic errorsor an opening to new physics.

Hints of Early Dark Energy in Planck, SPT, and ACT data: new physics or systematics?