Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies are Inconsistent with the Lyman-α Forest

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate, $H_0$. 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, $n_s$. Here, we present the first constraints on axion-like EDE using data from the Lyman-$\alpha$ 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 Ly$\alpha$ 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 Ly$\alpha$ 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, $f_{\rm EDE}$, from 0.07 to 0.03 and constrains $H_0=67.9_{-0.4}^{+0.4}$ km/s/Mpc (68% CL), with maximum a posteriori value $H_0=67.9$ km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Ly$\alpha$ data. Our Ly$\alpha$-based EDE constraints yield $H_0$ values that are in $>4\sigma$ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Ly$\alpha$ forest data for $n_s$ values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Ly$\alpha$ forest severely constrains canonical EDE models that could resolve the Hubble tension.