In this paper we investigate how observational effects could possibly bias
cosmological inferences from peculiar velocity measurements. Specifically, we
look at how bulk flow measurements are compared with theoretical predictions.
Usually bulk flow calculations try to approximate the flow that would occur in
a sphere around the observer. Using the Horizon Run 2 simulation we show that
the traditional methods for bulk flow estimation can overestimate the magnitude
of the bulk flow for two reasons: when the survey geometry is not spherical
(the data do not cover the whole sky), and when the observations undersample
the velocity distributions. Our results may explain why several bulk flow
measurements found bulk flow velocities that seem larger than those expected in
standard {\Lambda}CDM cosmologies. We recommend a different approach when
comparing bulk flows to cosmological models, in which the theoretical
prediction for each bulk flow measurement is calculated specifically for the
geometry and sampling rate of that survey. This means that bulk flow values
will not be comparable between surveys, but instead they are comparable with
cosmological models, which is the more important measure.Comment: 11 pages, 5 figures. Accepted for publication in MNRA
