On the Statistical Significance of the Bulk Flow Measured by the PLANCK
  Satellite
  A recent analysis of data collected by the Planck satellite detected a net
dipole at the location of X-ray selected galaxy clusters, corresponding to a
large-scale bulk flow extending at least to $z\sim 0.18$, the median redshift
of the cluster sample. The amplitude of this flow, as measured with Planck, is
consistent with earlier findings based on data from the Wilkinson Microwave
Anisotropy Probe (WMAP). However, the uncertainty assigned to the dipole by the
Planck team is much larger than that found in the WMAP studies, leading the
authors of the Planck study to conclude that the observed bulk flow is not
statistically significant. We here show that two of the three implementations
of random sampling used in the error analysis of the Planck study lead to
systematic overestimates in the uncertainty of the measured dipole. Random
simulations of the sky do not take into account that the actual realization of
the sky leads to filtered data that have a 12% lower root-mean-square
dispersion than the average simulation. Using rotations around the Galactic
pole (the Z axis), increases the uncertainty of the X and Y components of the
dipole and artificially reduces the significance of the dipole detection from
98-99% to less than 90% confidence. When either effect is taken into account,
the corrected errors agree with those obtained using random distributions of
clusters on Planck data, and the resulting statistical significance of the
dipole measured by Planck is consistent with that of the WMAP results.
