2018ApJ...863..185D

Very long baseline interferometry (VLBI) at millimeter (mm) wavelengths is being employed to resolve event horizon-scale structure of the environment surrounding the Milky Way black hole at an angular resolution of a few tens of microarcseconds. The same approach could also resolve the orbital separation of a population of massive black hole binaries (MBHBs). Modeling the inspiral of binaries due to gravitational-wave emission and gas and requiring binary orbital periods of less than 10 yr, we estimate that there may exist ∼100 resolvable MBHBs that are bright enough to be observed by mm-wavelength VLBI instruments over the entire sky at redshifts z <= 0.5. We propose to search for these resolvable MBHBs by identifying binaries with the required orbital separations from periodic quasar light curves identified in optical and near-IR surveys. These periodic-light-curve candidates can be followed up with radio observations to determine their promise for observation with VLBI at mm wavelengths. The VLBI observations over the timescale of a binary orbit can allow unprecedented precision in the measurement of the binary mass, to within 30%. In combination with an independent binary mass measurement, VLBI observation would allow a novel O (10 % ) measurement of the Hubble constant, independent from those currently proposed and employed.