Current cosmological data exhibit discordance between indirect and some
direct inferences of the present-day expansion rate, H0. Early dark energy
(EDE), which briefly increases the cosmic expansion rate prior to
recombination, is a leading scenario for resolving this "Hubble tension" while
preserving a good fit to cosmic microwave background (CMB) data. However, this
comes at the cost of changes in parameters that affect structure formation in
the late-time universe, including the spectral index of scalar perturbations,
ns. Here, we present the first constraints on axion-like EDE using data from
the Lyman-α forest, i.e., absorption lines imprinted in background
quasar spectra by neutral hydrogen gas along the line of sight. We consider two
independent measurements of the one-dimensional Lyα forest flux power
spectrum, from the Sloan Digital Sky Survey (SDSS eBOSS) and from the
MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline
dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO)
measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset
reduces the 95% confidence level (CL) upper bound on the maximum fractional
contribution of EDE to the cosmic energy budget, fEDE, from 0.07 to
0.03 and constrains H0=67.9−0.4+0.4 km/s/Mpc (68% CL), with maximum a
posteriori value H0=67.9 km/s/Mpc. Similar results are obtained for the
MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints
yield H0 values that are in >4σ tension with the SH0ES
distance-ladder measurement and are driven by the preference of the Lyα
forest data for ns values lower than those required by EDE cosmologies that
fit Planck CMB data. Taken at face value, the Lyα forest severely
constrains canonical EDE models that could resolve the Hubble tension.Comment: 7+9 pages, 2+9 figures, accepted by Phys. Rev. Let