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

TL;DR

This paper investigates the tension between local and cosmic microwave background measurements of Hubble's Constant, suggesting Gaia systematics and local galaxy distribution inhomogeneities could reconcile the differences.

Contribution

It demonstrates that Gaia parallax systematics and local galaxy inhomogeneities can significantly reduce the H_0 tension, aligning local measurements with Planck results.

Findings

Gaia parallaxes imply longer Cepheid distances by 12-15%.

Local galaxy inhomogeneity causes a 1.8% reduction in H_0.

Combined effects reduce H_0 estimate by about 7%."

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.