COSMO In the literature: contested

Does a local underdensity bias the locally measured Hubble constant?

In plain words

If the Milky Way lies inside a huge region with less matter than average, nearby galaxies recede faster and the measured local expansion rate comes out high. Galaxy counts suggest such a void, while supernova data mostly argue against it.

Precise statement

Keenan, Barger and Cowie (2013, ApJ 775, 62) found the local K-band luminosity density about 1.5 times lower than at $z > 0.1$, suggesting an underdensity to about 300 Mpc. In linear theory $\Delta H/H \sim -f\delta/3$ with growth rate $f \sim 0.5$, so $\delta \sim -0.3$ would raise local h by about 5 percent. Determine the mean matter density contrast $\delta$ within radii 100 to 600 Mpc around the Milky Way with uncertainty below 0.05, and the induced shift in local $h$, from galaxy counts with consistent selection, peculiar velocities and supernova Hubble residuals.

What would settle it

Full-sky galaxy redshift and peculiar-velocity surveys (for example 4MOST, DESI peculiar velocities, WALLABY) combined with supernova Hubble residuals as a function of distance and direction.

Status in the literature

Kenworthy, Scolnic and Riess (2019) found supernova distances inconsistent with such a void at 4 to 5 $\sigma$, while Haslbauer, Banik and Kroupa (2020, MNRAS 499, 2845) argue that the void and the $H0$ tension together conflict with LCDM.