Stress Testing $Λ$CDM with High-redshift Galaxy Candidates

Early data from JWST have revealed a bevy of high-redshift galaxy candidates with unexpectedly high stellar masses. An immediate concern is the consistency of these candidates with galaxy formation in the standard cosmological model. In the $\Lambda$CDM paradigm, the stellar mass ($M_\star$) of a galaxy is limited by the available baryonic reservoir of its host dark matter halo. The mass function of dark matter halos therefore imposes an absolute upper limit on the number density $n(>M_\star,z)$ and stellar mass density $\rho_{\star}(>M_\star,z)$ of galaxies more massive than $M_\star$ at any epoch $z$. Here I show that the most massive galaxy candidates in JWST observations at $z\sim 7-10$ lie at the very edge of these limits, indicating an important unresolved issue with the properties of galaxies derived from the observations, how galaxies form at early times in $\Lambda$CDM, or within this standard cosmology itself.