Kavli Affiliate: David N. Spergel
| First 5 Authors: J. Colin Hill, Erminia Calabrese, Simone Aiola, Nicholas Battaglia, Boris Bolliet
| Summary:
The early dark energy (EDE) scenario aims to increase the value of the Hubble
constant ($H_0$) inferred from cosmic microwave background (CMB) data over that
found in $Lambda$CDM, via the introduction of a new form of energy density in
the early universe. The EDE component briefly accelerates cosmic expansion just
prior to recombination, which reduces the physical size of the sound horizon
imprinted in the CMB. Previous work has found that non-zero EDE is not
preferred by Planck CMB power spectrum data alone, which yield a 95% confidence
level (CL) upper limit $f_{rm EDE} < 0.087$ on the maximal fractional
contribution of the EDE field to the cosmic energy budget. In this paper, we
fit the EDE model to CMB data from the Atacama Cosmology Telescope (ACT) Data
Release 4. We find that a combination of ACT, large-scale Planck TT (similar to
WMAP), Planck CMB lensing, and BAO data prefers the existence of EDE at
$>99.7$% CL: $f_{rm EDE} = 0.091^{+0.020}_{-0.036}$, with $H_0 =
70.9^{+1.0}_{-2.0}$ km/s/Mpc (both 68% CL). From a model-selection standpoint,
we find that EDE is favored over $Lambda$CDM by these data at roughly
$3sigma$ significance. In contrast, a joint analysis of the full Planck and
ACT data yields no evidence for EDE, as previously found for Planck alone. We
show that the preference for EDE in ACT alone is driven by its TE and EE power
spectrum data. The tight constraint on EDE from Planck alone is driven by its
high-$ell$ TT power spectrum data. Understanding whether these differing
constraints are physical in nature, due to systematics, or simply a rare
statistical fluctuation is of high priority. The best-fit EDE models to ACT and
Planck exhibit coherent differences across a wide range of multipoles in TE and
EE, indicating that a powerful test of this scenario is anticipated with
near-future data from ACT and other ground-based experiments.
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