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

Tom Shanks, Lucy Hogarth, Nigel Metcalfe

TL;DR

The paper investigates the tension between Planck CMB inferences of H0 and local distance-scale measurements. It leverages Gaia DR2 Cepheid parallaxes (with a quasar-based offset) and a model of a local underdensity (the Local Hole) to reassess the Cepheid calibration and nearby peculiar velocities. The results show Gaia-based Cepheid distances may be longer by about 12–15% for Galactic calibrators and ~4.7% for Riess2018b's sample, while the Local Hole outflow could lower H0 by ~1.8% and a Pantheon SN Ia analysis yields H0 ≈ 72.4 with Ωm ≈ 0.33; the combined effect could pull H0 down to ~68.9, closer to the Planck value. However, these conclusions depend critically on Gaia parallax systematics and the modeling of local density inhomogeneities, so the Planck-distance-scale tension remains unsettled pending improved data and analyses.

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.

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

TL;DR

The paper investigates the tension between Planck CMB inferences of H0 and local distance-scale measurements. It leverages Gaia DR2 Cepheid parallaxes (with a quasar-based offset) and a model of a local underdensity (the Local Hole) to reassess the Cepheid calibration and nearby peculiar velocities. The results show Gaia-based Cepheid distances may be longer by about 12–15% for Galactic calibrators and ~4.7% for Riess2018b's sample, while the Local Hole outflow could lower H0 by ~1.8% and a Pantheon SN Ia analysis yields H0 ≈ 72.4 with Ωm ≈ 0.33; the combined effect could pull H0 down to ~68.9, closer to the Planck value. However, these conclusions depend critically on Gaia parallax systematics and the modeling of local density inhomogeneities, so the Planck-distance-scale tension remains unsettled pending improved data and analyses.

Abstract

There is an \% tension between the value of Hubble's Constant, km\,sMpc, implied by the {\it Planck} microwave background power spectrum and that given by the distance scale of km\,sMpc. 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 \% longer than previously estimated. Similarly, {\it Gaia} also implies \% longer distances for 46 Cepheids than previous distances on the scale of Riess et al. Then we show that the existence of an hMpc `Local Hole' in the galaxy distribution implies an outflow of km\,s. Accounting for this in the recession velocities of SNIa standard candles out to reduces by a further \%. Combining the above two results would reduce the distance scale estimate by \% from to km\,sMpc, in reasonable agreement with the {\it Planck} value. We conclude that the discrepancy between distance scale and {\it Planck} measurements remains unconfirmed due to uncertainties caused by {\it Gaia} systematics and an unexpectedly inhomogeneous local galaxy distribution.

Paper Structure

This paper contains 4 sections, 4 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. New Cepheid parallax distances from Gaia.
  3. $H_0$ and the 'Local Outflow'
  4. Conclusions

Figures (2)

  • Figure 1: Comparison between Cepheid distances based on Gaia parallaxes (assuming the 29 $\mu$as 'quasar' correction), compared to the HST parallaxes used by Riess2018a to help zeropoint their Cepheid scale. Also shown is the same Gaia comparison for Cepheids in open clusters with main sequence fitted distances from Laney1993 and Hoyle2003. Finally, the distance moduli of the 46 Cepheids of Riess2018b are also compared to those from Gaia. The Cepheids $l$ Car, W Sgr, RT Aur and T Mon are also plotted but with brackets because they were left out of numerical comparisons (see text).
  • Figure 2: Outflow peculiar velocity, $\Delta v$ (km s$^{-1}$) inferred via eqs. \ref{['eq:density']} and \ref{['eq:linear']} from the 6dFGS+SDSS galaxy redshift distributions in the 3 sky areas of Whitbourn2014. Here, outflows have positive $\Delta v$.