2019MNRAS.484L..64S

There is an ≃9 ± 2.5 per cent tension between the value of Hubble's Constant, H₀ = 67.4 ± 0.5 km s^–1 Mpc^–1, implied by the Planck microwave background power spectrum and that given by the distance scale of H₀ = 73.4 ± 1.7 km s^–1Mpc^–1. But with a plausible assumption about a Gaia DR2 parallax systematic offset, we find that Gaia parallax distances of Milky Way Cepheid calibrators are ≃12-15 per cent longer than previously estimated. Similarly, Gaia also implies ≃4.7 ± 1.7 per cent longer distances for 46 Cepheids than previous distances on the scale of Riess et al. Then we show that the existence of an ≃150 h^–1Mpc 'Local Hole' in the galaxy distribution implies an outflow of ≃500 km s^–1. Accounting for this in the recession velocities of SNIa standard candles out to z ≃ 0.15 reduces H₀ by a further ≃1.8 per cent. Combining the above two results would reduce the distance scale H₀ estimate by ≃7 per cent from H₀ ≃ 73.4 ± 1.7 to ≃68.9 ± 1.6 km s^–1Mpc^–1, in reasonable agreement with the Planck value. We conclude that the discrepancy between distance scale and Planck H₀ measurements remains unconfirmed due to uncertainties caused by Gaia systematics and an unexpectedly inhomogeneous local galaxy distribution.