Galaxy disks in the early universe are too calm. ALMA kinematics of [C II] and CO in galaxies at z of 4 to 5 reveal regularly rotating disks with rotation-to-dispersion ratios V/sigma of 7 to 10, comparable to local spirals: dynamically cold systems within 1.5 billion years of the Big Bang (Rizzo et al. 2020, 2021; Lelli et al. 2021; Roman-Oliveira et al. 2023, who find regular rotation and low turbulence across a diverse z of 4.5 sample). Some ALMA analyses counter that the population-level kinematics remain consistent with turbulent disks with dispersions 4 to 5 times local values (arXiv:2304.00036), and the choice of kinematic tracer shifts the answer, but the existence of a population of genuinely cold, settled, fast-rotating disks at z of 4 to 5 has survived scrutiny and grown with JWST stellar-kinematic confirmations.
ΛCDM forms early disks hot: rapid accretion, mergers every few hundred Myr, and violent disk instabilities should maintain high gas dispersions, with simulations predicting V/sigma of 1 to 3 at these epochs and dynamically cold disks emerging only after z of 1 to 2 once merger rates subside. Each cold disk at z of 4 to 5 therefore requires the model to have locally suspended its own assembly process: quiet merger histories, finely regulated feedback, and stable gas transport, arranged within the universe's most violent epoch. Simulations can produce rare cold outliers, but the observed incidence among massive star-forming galaxies keeps exceeding them.
The standing is a maturing crisis in disk formation timing: kinematic tracer systematics are real but cannot erase the cold population, and the same surveys keep finding the disks earlier. ALMA wide samples and JWST integral-field kinematics are now measuring the cold-disk fraction as a function of epoch, the curve that any formation theory must hit.