Impact of Peculiar Velocities on Measurements of \(H_0\)

Since the Hubble constant measures the expansion rate of the Universe, any local motions that deviate from that expansion have the potential to bias measurements of \(H_0\) . In this chapter we explore the nature of such “peculiar velocities” and the impact they can have on cosmological inferences. We first summarise the theoretical background of peculiar velocities and the different types of motions to be considered—from monopoles, through dipoles and bulk flows, to higher-order motion. We then discuss how peculiar velocities can be directly measured, or the peculiar-velocity-field predicted using the density distribution. Finally we discuss the ways in which peculiar velocities can be taken into account to ensure \(H_0\) measurements are robust, by including in the analysis random uncertainties, correlated uncertainties, and/or direct peculiar velocity corrections. We also discuss the magnitude of expected peculiar velocity biases; based on theoretical predictions and observational evidence, it is unlikely that peculiar velocity contamination could explain the Hubble tension.