Accelerated Structure Formation from Horizon Thermodynamics: Resolving the JWST Early Massive Galaxy Anomaly

The James Webb Space Telescope has revealed galaxies with stellar masses of 109 –1010 M⊙ atredshifts z > 10, within 400 Myr of the Big Bang. In standard ΛCDM cosmology, the free-falltimescale of protogalactic baryonic clouds at these epochs exceeds the age of the universe, and therequired dark matter halos represent > 10σ fluctuations in the primordial density field. We showthat these galaxies form naturally—with no free parameters—in the thermodynamic spacetimeframework of Jacobson (1995) when the Hubble horizon is imposed as a finite outer boundary onthe vacuum mode spectrum. This boundary condition modifies the inertial response of matter ataccelerations below a0 (z) = cH(z), the same mechanism that produces flat galaxy rotation curvesat z = 0 1. Because H(z) is much larger at early times—a0 (z=12) ≈ 26 a0 (z=0)—the modificationextends to all protogalactic scales. Gravitational accelerations in collapsing clouds (g ∼ 10−12 m/s2 )fall 3–4 orders of magnitude below a0 (z), placing them deeply in the modified regime. The resultingcollapse timescales are ∼ 5–18 Myr, compared to > 300 Myr in the standard framework—more thansufficient to assemble the observed galaxies within the available cosmic time. No dark matter, nomodified star formation efficiency, and no new free parameters are required.