  The early-science observations made by the James Webb Space Telescope (JWST)
have revealed an excess of ultra-massive galaxy candidates that appear to
challenge the standard cosmological model ($\Lambda$CDM). Here, we argue that
any modifications to $\Lambda$CDM that can produce such ultra-massive 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 ($z\approx 7-10$) and host-halo masses $(M_\mathrm{h}\approx
10^{10}-10^{12}\, M_\odot$) as the JWST candidates, but tracks star-formation
rate rather than stellar mass. We consider beyond-$\Lambda$CDM power-spectrum
enhancements and show that any departure large enough to reproduce the
abundance of ultra-massive 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.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies;High Energy Physics - Phenomenology