  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
micro-arcseconds. 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 years, 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. 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 (~10%) measurement of the Hubble constant, independent from
those currently proposed and employed.
