Galaxy spins know about the cosmic web in ways ΛCDM struggles to arrange. Observations from SDSS, MaNGA and SAMI integral-field surveys establish that low-mass spiral galaxies tend to spin with their axes aligned parallel to their host filaments while high-mass galaxies spin perpendicular, with a transition mass near 10^10 solar masses; tidal torque theory in ΛCDM can produce qualitatively similar trends in simulations, but the observed alignment amplitudes and their coherence lengths exceed the predictions, part of the broader pattern of spin correlations extending to 30 to 100 Mpc separations that overshoot tidal torque expectations by factors of 10 to 20. The 2021 MeerKAT detection of coherent bulk rotation in an individual filament, solid-body-like at about 110 km/s over megaparsec scales (Tudorache et al.; confirmed in follow-up samples), gave filaments themselves an angular momentum that tidal torquing struggles to supply.
In ΛCDM, spin arises late and locally: protogalaxies acquire angular momentum from tidal torques exerted by neighboring overdensities during collapse, a perturbative mechanism whose correlations decay quickly with distance, predicting weak alignments that mergers further randomize. The observed spin-filament webbing, coherent over tens of megaparsecs, present at high redshift, organized by mass with a clean transition scale, and accompanied by rotating filaments, asks the torque mechanism to do far more than its amplitude allows. Each escalation (resorting to halo spin flips through mergers, anisotropic accretion along filaments) adds machinery without closing the amplitude gap.
The standing is firming as integral-field samples grow: WEAVE, 4MOST, and SKA HI surveys will measure spin-filament statistics across mass scales and redshift, testing whether spin alignment is a weak late acquisition, as torque theory requires, or a strong inherited property of the web itself.