Cosmic variance of the local Hubble flow in large-scale cosmological
  simulations
  The increasing precision in the determination of the Hubble parameter has
reached a per cent level at which large-scale cosmic flows induced by
inhomogeneities of the matter distribution become non-negligible. Here we use
large-scale cosmological N-body simulations to study statistical properties of
the local Hubble parameter as measured by local observers. We show that the
distribution of the local Hubble parameter depends not only on the scale of
inhomogeneities, but also on how one defines the positions of observers in the
cosmic web and what reference frame is used. Observers located in random dark
matter haloes measure on average lower expansion rates than those at random
positions in space or in the centres of cosmic voids, and this effect is
stronger from the halo rest frames compared to the CMB rest frame. We compare
the predictions for the local Hubble parameter with observational constraints
based on type Ia supernovae (SNIa) and CMB observations. Due to cosmic
variance, for observers located in random haloes we show that the Hubble
constant determined from nearby SNIa may differ from that measured from the CMB
by 0.8 per cent at 1sigma statistical significance. This scatter is too small
to significantly alleviate a recently claimed discrepancy between current
measurements assuming a flat LCDM model. However, for observers located in the
centres of the largest voids permitted by the standard LCDM model, we find that
Hubble constant measurements from SNIa would be biased high by 5 per cent,
rendering this tension inexistent in this extreme case.
