The ISW and kinetic Sunyaev-Zel'dovich effects both let cosmologists test the large-scale velocity and potential fields directly: build a template from galaxy surveys, predict where the CMB should carry slight temperature structure from decaying potentials (ISW) or bulk electron motion (kSZ), and cross-correlate (Ho et al. 2009; Sherwin et al. 2012). In ΛCDM both fields are statistically isotropic and linear theory describes them well, so the templates should align with the CMB maps up to noise, identically in every direction.
In practice the directional reconstructions misbehave. ISW and kSZ template fits show anisotropic amplitudes, uneven agreement across the sky, and mismatches between predicted and observed features that persist through different tracer catalogs and reconstruction methods (Keisler and Schmidt 2013; Ruiz-Lapuente et al. 2019). The standard model has no vocabulary for a directional template failure: isotropy is structural, so every mismatch must be assigned to survey footprints, missing tracers, or nonlinear corrections, explanations that have to be re-invented for each new dataset showing the same behavior.
The standing is unsettled and instrumentation is about to outrun the excuses. Simons Observatory and CMB-S4 will deliver kSZ and ISW reconstructions with enough sky and depth to map the template residuals directionally, turning what is now a scattered set of marginal mismatches into either a clean isotropic null or a measured axis structure.