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 3C 84 ( 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 .