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

Abstract

There is an $\approx 9\pm 2.5$\% tension between the value of Hubble's Constant, $H_0=67.4\pm 0.5$km\,s${}^{-1}$Mpc${}^{-1}$, implied by the {\it Planck} microwave background power spectrum and that given by the distance scale of $H_0=73.4\pm 1.7$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 $\approx 12-15$\% longer than previously estimated. Similarly, {\it Gaia} also implies $\approx 4.7\pm 1.7$\% longer distances for 46 Cepheids than previous distances on the scale of Riess et al. Then we show that the existence of an $\approx 150$h${}^{-1}$Mpc `Local Hole' in the galaxy distribution implies an outflow of $\approx 500$km\,s${}^{-1}$. Accounting for this in the recession velocities of SNIa standard candles out to $z\approx 0.15$ reduces $H_0$ by a further $\approx 1.8$\%. Combining the above two results would reduce the distance scale $H_0$ estimate by $\approx 7$\% from $H_0\approx 73.4\pm 1.7$ to $\approx 68.9\pm 1.6$ 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.

6 pages, 2 figures, accepted as a Short Communication to MNRAS 2018 December 17