Gaia Cepheid parallaxes and ‘Local Hole’ relieve H0 tension

ABSTRACT 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.