Stellar bars are precision instruments for disk dynamics: they form only in dynamically cold, settled disks, and once formed they exchange angular momentum with everything around them. Surveys traced a bar fraction declining toward higher redshift, but the decline keeps flattening: dynamically mature, long bars are now confirmed at z of 1 to 3, in disks the standard model expects to be too hot and too merger-harassed to host them (Sheth et al. 2008; Melvin et al. 2014), with JWST pushing barred spirals structurally indistinguishable from local analogs back to z near 3.
The model is squeezed from both ends. At high redshift, bars require settled cold disks earlier than hierarchical assembly comfortably provides. At low redshift, simulations tend to overproduce strong bars or need finely tuned disk stability, gas fractions, and merger histories to match the observed evolution (Kraljic et al. 2012). And a third constraint cuts deepest: a bar rotating inside a live cold dark matter halo loses angular momentum to it through dynamical friction and must slow down, yet measured bar pattern speeds are predominantly fast, leaving little room for the halo coupling the particle picture mandates. Mature early bars, tuned late bars, and fast bars everywhere form a pattern the standard framework was not built to host.
The standing tightens with each JWST bar census at higher redshift, while integral-field pattern-speed measurements keep the fast-bar constraint pressing on the halo coupling itself.