Proper Motions Of VLBI Lenses, Inertial Frames and The Evolution of   Peculiar Velocities
  Precise determinations of the image positions in quad gravitational lenses using VLBI can be used to measure the transverse velocity of the lens galaxy and the observer. The typical proper motions are $\mu$as yr$^{-1}$, so the time scale to measure the motion is ten years. By measuring the dipole of the proper motions in an ensemble of lenses we can set limits on the deviation of the inertial frame defined by the lenses from that defined by the CMB dipole and estimate the Hubble constant. The residual proper motions after subtracting the dipole probe the evolution of peculiar velocities with redshift and can be used to estimate the density parameter $\Omega_0$. For $N$ lenses, VLBI measurement accuracies of $\sigma_\theta$, and a baseline of $T$ years, we estimate that the 2$\sigma$ limit on the rms peculiar velocity of the lens galaxies is $3100 (\sigma_\theta/10\mu\{as})({yrs}/T)/N^{1/2} \kms$, and that the time required for the 2--$\sigma$ limit to reach the level of the local rms peculiar velocity $v_{0,rms}$ is approximately $10 N^{-1/2} (v_{0,rms}/600\kms)(\sigma_\theta/10\mu as)$ years. For a ten year baseline and $N=10$ lenses we expect the 1$\sigma$ limit on the misalignment with the CMB dipole to be $\Delta \theta=20^{\circ}$ or equivalently to obtain an upper limit of $\Delta H_0 /H_0 < 0.34 $. 
C.S Kochanek, T.S. Kolatt, M. Bartelmann