The Cosmic Rush Hour: Rapid Formation of Bright, Massive, Disky, Star-Forming Galaxies as Signatures of Early-Universe Physics

Abstract Early JWST observations have revealed a high-redshift universe more vibrant than predicted by canonical galaxy-formation models within ΛCDM, showing an excess of ultraviolet(UV)-bright, massive, and morphologically mature galaxies. Departures from ΛCDM prior to recombination can imprint signatures on non-linear structure formation at high redshift. In this paper, we investigate one such scenario – Early Dark Energy, originally proposed to resolve the Hubble tension – and its implications for these high-redshift challenges. We present the first large-scale cosmological hydrodynamic simulations of these models. Modifications to the pre-recombination expansion history accelerate early structure formation and produce UV luminosity and stellar mass functions in excellent agreement with JWST measurements, requiring essentially no additional calibrations. Predictions converge to ΛCDM at lower redshifts (z ≲ 3), thereby preserving all successes of ΛCDM. This model also accelerates the emergence of stellar and gaseous disks, increasing their number densities by ~0.5 dex at z ≃ 6–7, primarily due to the higher abundance of massive galaxies. Taken together, these results demonstrate how early-universe physics can simultaneously reconcile multiple high-redshift challenges and the Hubble tension while retaining the core achievements of ΛCDM. This opens a pathway to constraining a broad class of beyond-ΛCDM models with forthcoming observations.