Tom Shanks, Lucy Hogarth, Nigel Metcalfe, Durham University, UK)

There has been much discussion of the tension between the values of Hubble's Constant $H_0$ implied by the distance scale and fits to the microwave background's primordial power spectrum. While the latter is fitted by standard cosmological models with $H_0=67.4\pm0.5$kms$^{-1}$Mpc$^{-1}$, the distance scale gives $H_0=73.45\pm1.66$kms$^{-1}$Mpc$^{-1}$, an $\approx10$%, $\approx3.5\sigma$ discrepancy. Here we first show that GAIA parallax distances of Milky Way Cepheids may be between $\approx7-18$% larger than previously estimated, with the potential to produce a corresponding reduction in the value of $H_0$. Then we show that the existence of an $\approx150$h$^{-1}$Mpc `Local Hole' in the galaxy distribution around our position implies an outflow of $\approx500$kms$^{-1}$ averaging over direction. Accounting for this in the recession velocities of SNIa standard candles out to $z\approx0.1$ reduces $H_0$ by a further $\approx1.8$%, while maintaining reasonable consistency with the supernova Hubble diagram. Combining this result with even an $\approx7$% increase in the Cepheid distance scale due to GAIA implies an $\approx9$% reduction in the value of the Hubble Constant, decreasing from $H_0\approx73.45$ to 67.6 kms$^{-1}$Mpc$^{-1}$. This would leave the distance scale and Planck Cosmic Microwave Background values entirely consistent, thus potentially relieving the previous $H_0$ tension.

Adam G. Riess, Stefano Casertano, D'Arcy Kenworthy, Dan Scolnic, Lucas Macri

Shanks et al. (2018) arXiv:1810.02595 make two claims that they argue bring the local measurement and early Universe prediction of H0 into agreement: A) they claim that Gaia DR2 parallax measurements show the geometric calibration of the Cepheid distance scale used to measure H0 to be grossly in error and B) that we live near the middle of an enormous void, further biasing the local measurement of the Hubble constant. We show that the first claim is caused by five erroneous uses of the data: in decreasing order of importance: 1) the use of a distance indicator, main sequence fitting of cluster stars, which is unrelated to the calibration of Cepheids and therefore has no bearing on current measurements of H0; 2) the use of Gaia data for Cepheids that fully saturate the detector, producing unreliable parallaxes; 3) the use of a fixed parallax offset which is known to depend on source magnitude and color but which is derived for sources with extremely different colors and magnitudes; 4) ignoring the uncertainty in this offset; and 5) ignoring the other geometric sources of Cepheid calibration, the distance of the LMC from detached eclipsing binaries and the masers in NGC 4258, which are independent of Milky Way parallaxes. Just resolving the first two of these issues by not using unrelated or saturated data leads to no inconsistency between Gaia parallaxes and the current Cepheid distance scale. The second claim can be refuted 6) because of the increase in chi-squared that the alleged void would entail in SN measurements in the Hubble flow, and 7) because it would represent a 6 sigma fluctuation of cosmic variance between the local and globally measured expansion, requiring us to live in an exceedingly special location.

Tom Shanks, Lucy Hogarth, Nigel Metcalfe, Durham University, England)

Riess et al (2018c) have claimed there exist seven problems in the analyses presented by Shanks et al (2018) where we argue that there is enough uncertainty in Cepheid distances and local peculiar velocity fields to explain the current tension in $H_0$. Here, we take each of the Riess et al (2018c) points in turn and suggest that either they do not apply or that the necessary caveats are already made by Shanks et al (2018). We conclude that the main point to be inferred from our analyses still stands which is that previous claims by Riess et al (2018b) that Gaia parallaxes confirm their Cepheid scale are, at best, premature in advance of further improvements in the Gaia astrometric solution.