Hierarchical assembly is ΛCDM's signature process: galaxies grow by merging, and merger activity should rise steeply into the past at every observable epoch. The measured record disagrees with itself and, in its most carefully measured ranges, with the model. Spectroscopic close-pair studies (MUSE deep fields, Ventou et al. 2017) found the major merger fraction rising only to z of 2 to 3 and then declining at higher redshift; morphological surveys with JWST (Ferreira et al. 2024) find disk-dominated, undisturbed galaxies persisting as the majority population to z of 6 and beyond, with disturbed merger-signature fractions far below hierarchical expectations. Meanwhile the newest JWST pair-fraction analysis (Duan et al. 2025, arXiv:2407.09472) reports pair fractions rising to z of 8 then flattening, with merger rates plateauing at about 6 per Gyr to z of 11.5.
The internal contradiction among probes is itself the problem: pair counts, morphology, and kinematics should tell one story if merging proceeds as simulated, and they do not. Where the model expects merger-disrupted morphologies to dominate early, JWST sees dynamically settled disks at z of 4 to 8; where it expects pair fractions to climb steeply, spectroscopic samples found decline; and reconciling the measures requires merger observability timescales tuned per epoch. The deeper structural issue: if mergers drive mass growth, the quiet morphologies and cold kinematics of early massive galaxies leave the model explaining how its central process operates without leaving its fingerprints.
The standing is contested across methods, with the resolution carrying high stakes: either merger statistics confirm hierarchical assembly's tempo, or the early universe assembled its galaxies largely without the process the model is named for. Wide JWST and Euclid samples with uniform pair and morphology pipelines will arbitrate.