No evidence for bulk velocity from type Ia supernovae

Abstract

We revisit the effect of peculiar velocities on low-redshift type Ia\nsupernovae. Velocities introduce an additional guaranteed source of\ncorrelations between supernova magnitudes that should be considered in all\nanalyses of nearby supernova samples but has largely been neglected in the\npast. Applying a likelihood analysis to the latest compilation of nearby\nsupernovae, we find no evidence for the presence of these correlations,\nalthough, given the significant noise, the data is also consistent with the\ncorrelations predicted for the standard $\\Lambda$CDM model. We then consider\nthe dipolar component of the velocity correlations -- the frequently studied\n"bulk velocity" -- and explicitly demonstrate that including the velocity\ncorrelations in the data covariance matrix is crucial for drawing correct and\nunambiguous conclusions about the bulk flow. In particular, current supernova\ndata is consistent with no excess bulk flow on top of what is expected in\n$\\Lambda$CDM and effectively captured by the covariance. We further clarify the\nnature of the apparent bulk flow that is inferred when the velocity covariance\nis ignored. We show that a significant fraction of this quantity is expected to\nbe noise bias due to uncertainties in supernova magnitudes and not any physical\npeculiar motion.\n