Cosmology with extragalactic proper motions: harmonic formalism,
  estimators, and forecasts
  We conduct a thorough study into the feasibility of measuring large-scale
correlated proper motions of galaxies with astrometric surveys. We introduce a
harmonic formalism for analysing proper motions and their correlation functions
on the sphere based on spin-weighted spherical harmonics, and study the
statistics of the transverse velocity field induced by large-scale structure.
We use a likelihood formalism to derive optimal estimators for the secular
parallax due to the Solar System's motion relative to distant objects, and
compute the variance and bias due to peculiar velocities and relativistic
aberration. We use a simulated catalogue of galaxy proper motions with radial
distributions and noise properties similar to those expected from Gaia to
forecast the detectability of the proper motion dipole, whose amplitude may be
considered a proxy for the Hubble constant. We find cosmic variance to be the
limiting source of noise for this measurement, forecasting a detectability of
$1$-$2\sigma$ on a single component of the local velocity, increasing to
$2$-$4\sigma$ (equivalent to a 25%-50% measurement of the Hubble constant) if
the CMB dipole is included as prior information. We conduct a thorough study
into the radial dependence of the signal-to-noise, finding that most of the
information comes from galaxies closer than a few hundred Mpc. We forecast that
the amplitude of peculiar transverse velocities can potentially be measured
with 10$\sigma$ significance; such a measurement would offer a unique probe of
cosmic flows and a valuable test of the cosmological model.
