Can standard cosmological models explain the observed Abell cluster bulk flow?

Lauer and Postman (LP) observed that all Abell clusters with redshifts less than 15,000 km s^-1 appear to be participating in a bulk flow of 689 km s^-1 with respect to the cosmic microwave background. We find this result difficult to reconcile with all popular models for large-scale structure formation that assume Gaussian initial conditions. This conclusion is based on Monte Carlo realizations of the LP data, drawn from large particle-mesh N-body simulations for six different models of the initial power spectrum (standard, tilted, and Ω₀ = 0.3 cold dark matter, hot dark matter, and two variants of the primordial baryon isocurvature model). We have taken special care to treat properly the longest-wavelength components of the power spectra. The simulations are sampled, "observed," and analyzed as identically as possible to the LP cluster sample. Large-scale bulk flows as measured from clusters in the simulations are in excellent agreement with those measured from the grid: the clusters do not exhibit any strong velocity bias on large scales. Bulk flows with amplitude as large as that reported by LP are not uncommon in the Monte Carlo data sets; the distribution of measured bulk flows before error bias subtraction is roughly Maxwellian, with a peak around 400 km s^-1 However the χ² of the observed bulk flow, taking into account the anisotropy of the error ellipsoid, is much more difficult to match in the simulations. The models examined are ruled out at confidence levels between 94% and 98%.