Constraining H0 Via Extragalactic Parallax
  We examine the prospects for measurement of the Hubble parameter $H_0$ via
observation of the secular parallax of other galaxies due to our own motion
relative to the cosmic microwave background rest frame. Peculiar velocities
make distance measurements to individual galaxies highly uncertain, but a
survey sampling many galaxies can still yield a precise $H_0$ measurement.
  We use both a Fisher information formalism and simulations to forecast errors
in $H_0$ from such surveys, marginalizing over the unknown peculiar velocities.
The optimum survey observes $\sim 10^2$ galaxies within a redshift
$z_\mathrm{max}=0.05$. The required errors on proper motion are comparable to
those that can be achieved by Gaia and future astrometric instruments. A
measurement of $H_0$ via parallax has the potential to shed light on the
tension between different measurements of $H_0$.
