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Physical Review D


We investigate constraints on early dark energy (EDE) using ACT DR4, SPT-3G 2018, Planck polarization, and restricted Planck temperature data (at ℓσ preference (Δχ2= −16.2 for three additional degrees of freedom) for EDE over Λ CDM. The EDE contributes a maximum fractional energy density of fEDE(zc) = 0.163(+0.047)/(−0.04) at a redshift zc = 3357±200 and leads to a CMB inferred value of the Hubble constant H0 = 74.2(+1.9)/(−2.1) km/s/Mpc. We find that Planck and ACT DR4 data provide the majority of the improvement in χ2, and that the inclusion of SPT-3G pulls the posterior of fEDE(zc) away from Λ CDM. 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 (3.0σ), while measurements that probe the clustering of matter at late times—the lensing potential power spectrum from Planck and 8 from BOSS—decrease the significance of the preference to 2.6σ. Conversely, adding a prior on the H0 value as reported by the SH0ES collaboration increases the preference to the 4−5σ level. In the absence of this prior, the inclusion of Planck TT data at ℓ > 1300 reduces the preference from 3.0σ to 2.3σ and the constraint on fEDE(zc) becomes compatible with Λ CDM at 1σ. 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.


This work is freely available courtesy of the American Physical Society.

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