The effect of peculiar velocities on supernova cosmology
  We present an analysis of peculiar velocities and their effect on supernova
cosmology. In particular, we study (a) the corrections due to our own motion,
(b) the effects of correlations in peculiar velocities induced by large-scale
structure, and (c) uncertainties arising from a possible local under- or
over-density. For all of these effects we present a case study of their impact
on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey
(SDSS-II SN Survey). Correcting supernova redshifts for the CMB dipole slightly
over-corrects nearby supernovae that share some of our local motion. We show
that while neglecting the CMB dipole would cause a shift in the derived
equation of state of Delta w ~ 0.04 (at fixed matter density) the additional
local-motion correction is currently negligible (Delta w<0.01). We use a
covariance-matrix approach to statistically account for correlated peculiar
velocities. This down-weights nearby supernovae and effectively acts as a
graduated version of the usual sharp low-redshift cut. Neglecting coherent
velocities in the current sample causes a systematic shift of ~2% in the
preferred value of w and will therefore have to be considered carefully when
future surveys aim for percent-level accuracy. Finally, we perform n-body
simulations to estimate the likely magnitude of any local density fluctuation
(monopole) and estimate the impact as a function of the low-redshift cutoff. We
see that for this aspect the low-z cutoff of z=0.02 is well-justified
theoretically, but that living in a putative local density fluctuation leaves
an indelible imprint on the magnitude-redshift relation.
