Using variability and VLBI to measure cosmological distances
  In this paper, we propose a new approach to determining cosmological
distances to active galactic nuclei (AGN) via light travel-time arguments,
which can be extended from nearby sources to very high redshift sources. The
key assumption is that the variability seen in AGN is constrained by the speed
of light and therefore provides an estimate of the linear size of an emitting
region. This can then be compared with the angular size measured with very long
baseline interferometry (VLBI) in order to derive a distance. We demonstrate
this approach on a specific well studied low redshift (z = 0.0178) source 3C84
(NGC 1275), which is the bright radio core of the Perseus Cluster. We derive an
angular diameter distance including statistical errors of $D_{A} =
72^{+5}_{-6}$ Mpc for this source, which is consistent with other distance
measurements at this redshift. Possible sources of systematic errors and ways
to correct for them are discussed.
