Callum Witten, Jake S. Bennett, Pascal A. Oesch, Seunghwan Lim, Chamilla Terp, Jakob M. Helton, Kasper E. Heintz, Romain A. Meyer, William McClymont, Thomas Herard-Demanche, Emma Giovinazzo

JWST has revealed an overabundance of apparent (`Coma'-like) cluster progenitors at $z>5$ that is $\sim$ two (300) times in excess of, and in $2.3σ$ ($4σ$) tension with, the number density of such objects at present day and theoretical predictions from $Λ$CDM. We present an analysis of protocluster candidates between $5<z<9$ from the literature, and in the TNG-Cluster and TNG300 simulations, aimed at resolving this tension. We first identify an inconsistency in how protocluster candidates are characterised: observational halo masses are estimated by summing the stellar mass over very large apertures ($\gtrsim 50\, R_{\rm vir}$), but are compared to the halo mass within the virial radius from simulations. In addition, these halo masses are commonly compared to the backward-tracked halo mass evolution of clusters, whereas a forward-tracked evolution of massive high-redshift haloes is more appropriate. Correcting these inconsistencies, while accounting for the merging of coincident protoclusters and the fluctuations associated with cosmic variance, entirely alleviates the tension. Ultimately, we find that $64\%$ of the protocluster candidates are instead likely to be proto-groups (i.e., $M_{200c}<10^{14}\, {\rm M_{\odot}}$) and none appear likely to become Coma-like clusters (i.e., $M_{200c}>10^{15}\, {\rm M_{\odot}}$) by $z=0$. Although some of these overdensities may not become clusters, they still trace extreme nodes of the cosmic web that may host early environmental effects, drive the first ionised regions, and contribute significantly to the cosmic star-formation rate density. These results demonstrate the need for more careful comparisons between observations and simulations of high-redshift protoclusters, and that improved selection criteria, potentially using the summation of the halo mass on Lagrangian scales, are vital for high-redshift protocluster science.