The impact of peculiar velocities on supernova cosmology
  We study correlated fluctuations of Type Ia supernova observables due to
peculiar velocities of both the observer and the supernova host galaxies, and
their impact on cosmological parameter estimation. We aim to demonstrate using
the CosmicFlows-3 dataset that at low redshifts the corrections for peculiar
velocities in the JLA catalogue have been systematically underestimated. By
querying a horizon-size N-body simulation we find that compared to a randomly
placed observer, an observer in an environment like our local Universe will see
2-8 times stronger correlations between supernovae in the JLA catalogue. Hence
the covariances usually employed underestimate the effects of coherent motion
of the supernova host galaxies. Contrary to previous studies which asserted
that peculiar velocities have negligible effect on cosmological parameter
estimation, we find that when such correlations are taken into account the JLA
data favour significantly smaller values of the dark energy density than in the
standard $\Lambda$CDM model. A joint fit to simultaneously determine the
cosmological parameters and the bulk flow indicates that the latter is around
250 km/s even beyond 200/h Mpc. The local bulk flow is thus an essential
nuisance parameter to be included in analyses of the supernova Hubble diagram.
