Evaluating ΛCDM Predictions for Early Galaxy Formation with Observations from JWST’s SMACS 0723 Field

This paper investigates whether the observed properties of GLASS-z12, a very high-redshift galaxy observed in the JWST SMACS 0723 field, align with predictions made by the Lambda-Cold Dark Matter (CDM) model of cosmology. Using JWST NIRCam imaging products and a spectroscopic redshift of z=12.34, I calculated a lookback time of tL=13.439 ± 0.13 Gyr for GLASS-z12, where the quoted uncertainty reflects sensitivity to the Planck 2018 base-CDM parameters H0 and m (assuming spatial flatness). This implies that the observed light was emitted approximately 352 million years after the Big Bang. Combined with external estimates of its stellar mass (∼ 109 M⊙) and effective radius ∼ 0.5 pkpc, the inferred early formation appears difficult to reconcile with naive expectations from standard hierarchical growth in CDM. Taken together, these results motivate further observational and theoretical work to test whether the apparent tension can be resolved within CDM through parameter choices and astrophysical systematics, or whether extensions such as Early Dark Energy provide a better description of early galaxy formation.