Measuring bulk motion of X-ray clusters via the kinematic
  Sunyaev-Zeldovich effect: summarizing the "dark flow" evidence and its
  implications
  In this review we present a comprehensive discussion of peculiar velocity
field measured recently on very large scales with a novel method using X-ray
galaxy clusters as tracers. The measurement is based on the kinematic component
of the Sunyaev-Zeldovich (KSZ) effect produced by Compton scattering of cosmic
microwave background (CMB) photons off the hot intracluster gas, and uses a
large catalog of X-ray selected clusters and all-sky CMB maps obtained with the
WMAP satellite. The method probes the dipole of the CMB temperature field
evaluated at the cluster positions and within the apertures in which the CMB
monopole contribution vanishes, thereby isolating the signal remaining from the
KSZ effect produced by coherently moving clusters. The detection of a highly
significant dipole out to the depth of at least ~ 800 Mpc casts doubt on the
notion that gravitational instability from the observed mass distribution is
the sole -- or even dominant -- cause of the detected motions. Rather it
appears that the flow may extend across the entire observable Universe.
Possible implications include the possibility to constrain the primeval
preinflationary structure of space-time and its landscape, and/or the need for
modifications of presently known physics (e.g. arising from a
higher-dimensional structure of gravity). We review these possibilities in
light of the measurements described here and specifically discuss the prospects
of future measurements and the issues they should resolve. We address the
consistency of these large-scale velocity measurements with those obtained on
smaller scales by studies using galaxies as tracers, and resolve the
discrepancies with two recent claims based on modified CMB analysis schemes.
