GAIA Cepheid parallaxes and 'Local Hole' relieve $H_0$ tension

Abstract

There is an $\approx9\pm2.5$\% tension between the value of Hubble's Constant, $H_0=67.4\pm0.5$km\,s$^{-1}$Mpc$^{-1}$, implied by the {\it Planck} microwave background power spectrum and that given by the distance scale of $H_0=73.4\pm1.7$km\,s$^{-1}$Mpc$^{-1}$. But with a plausible assumption about a {\it Gaia} DR2 parallax systematic offset, we find that {\it Gaia} parallax distances of Milky Way Cepheid calibrators are $\approx12-15$\% longer than previously estimated. Similarly, {\it Gaia} also implies $\approx4.7\pm1.7$\% longer distances for 46 Cepheids than previous distances on the scale of Riess et al. Then we show that the existence of an $\approx150$h$^{-1}$Mpc `Local Hole' in the galaxy distribution implies an outflow of $\approx500$km\,s$^{-1}$. Accounting for this in the recession velocities of SNIa standard candles out to $z\approx0.15$ reduces $H_0$ by a further $\approx1.8$\%. Combining the above two results would reduce the distance scale $H_0$ estimate by $\approx7$\% from $H_0\approx73.4\pm1.7$ to $\approx68.9\pm1.6$ km\,s$^{-1}$Mpc$^{-1}$, in reasonable agreement with the {\it Planck} value. We conclude that the discrepancy between distance scale and {\it Planck} $H_0$ measurements remains unconfirmed due to uncertainties caused by {\it Gaia} systematics and an unexpectedly inhomogeneous local galaxy distribution.