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The early-science observations made by the James Webb Space Telescope (JWST) have revealed an excess of ultramassive galaxy candidates that appear to challenge the standard cosmological model (ΛCDM). Here, we argue that any modifications to ΛCDM that can produce such ultramassive galaxies in the early Universe would also affect the UV galaxy luminosity function (UV LF) inferred from the Hubble Space Telescope (HST). The UV LF covers the same redshifts (z≈7–10) and host-halo masses (Mh≈1010–1012M⊙) as the JWST candidates, but tracks star-formation rate rather than stellar mass. We consider beyond-ΛCDM power-spectrum enhancements and show that any departure large enough to reproduce the abundance of ultramassive JWST candidates is in conflict with the HST data. Our analysis, therefore, severely disfavors a cosmological explanation for the JWST abundance problem. Looking ahead, we determine the maximum allowable stellar-mass function and provide projections for the high-z UV LF given our constraints on cosmology from current HST data.Received 23 May 2023Revised 2 October 2023Accepted 25 October 2023DOI:https://doi.org/10.1103/PhysRevLett.132.061002© 2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDark matterFormation & evolution of stars & galaxiesLarge scale structure of the UniversePhysical SystemsGalaxiesGravitation, Cosmology & Astrophysics
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