Early observations from the James Webb Space Telescope (JWST) have revealed an overabundance of massive high-redshift galaxies, raising the question of whether this points to new physics beyond ΛCDM, or an enhanced formation efficiency of massive stars. We revisit this issue going beyond earlier analyses based on direct comparisons to theoretical bounds at a fixed cosmology, by performing a full Bayesian analysis of the most extreme galaxies in the CEERS imaging and FRESCO spectroscopic samples, jointly constraining cosmological parameters and the baryon-to-star conversion efficiency ε. We do so not only within the spatially flat ΛCDM model, but also in models where the dark energy equation of state w and/or the spatial curvature parameter ΩK are allowed to vary, carefully discussing the impact of both w and ΩK on the cumulative comoving stellar mass density. Within the flat ΛCDM model, once cosmological parameters are marginalized over, the CEERS sample provides a weak 2σ lower limit of ε 0. 07, compatible with astrophysical expectations. In contrast, the FRESCO sample requires ε 0. 5 at 2σ, with values ε 0. 2 disfavored at >5σ. These results do not qualitatively change when we allow w and/or ΩK to vary, with no evidence for deviations from w=-1 or ΩK=0. Our results therefore suggest that the origin of the ``JWST tension'' is unlikely to be cosmological, but lies in the astrophysics of galaxy formation.
Comini et al. (2026) studied this question.