Hints of early dark energy in Planck, SPT, and ACT data: New physics or systematics?

We investigate constraints on early dark energy (EDE) using ACT DR4, SPT-3G 2018, Planck polarization, and restricted Planck temperature data (ate < 650), finding a 3.3 sigma preference (delta chi(2 )= -16.2 for three additional degrees of freedom) for EDE over ACDM. The EDE contributes a maximum fractional energy density of f(EDE)(z(c)) = 0.163(-0.04)( +0.047) at a redshift z(c) = 3357 +/- 200 and leads to a CMB inferred value of the Hubble constant H-0 = 74.2(-2.1)(+1.9 ) km/s/Mpc. We find that Planck and ACT DR4 data provide the majority of the improvement in chi(2), and that the inclusion of SPT-3G pulls the posterior of f(EDE)(zc) away from ACDM. This is the first time that a moderate preference for EDE has been reported for these combined CMB datasets including Planck polarization. We find that including measurements of supernovae luminosity distances and the baryon acoustic oscillation standard ruler only minimally affects the preference (3.0 sigma), while measurements that probe the clustering of matter at late times & mdash;the lensing potential power spectrum from Planck and f sigma(8) from BOSS & mdash;decrease the significance of the preference to 2.6 sigma. Conversely, adding a prior on the H-0 value as reported by the SH0ES collaboration increases the preference to the 4 - 5 sigma level. In the absence of this prior, the inclusion of Planck TT data at e > 1300 reduces the preference from 3.0 sigma to 2.3 sigma and the constraint on f(EDE)(z(c)) becomes compatible with ACDM at 1 sigma. We explore whether systematic errors in the Planck polarization data may affect our conclusions and find that changing the TE polarization efficiencies significantly reduces the Planck preference for EDE. More work will be necessary to establish whether these hints for EDE within CMB data alone are the sole results of systematic errors or an opening to new physics.