(Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimeter VLBI “Event Horizon Telescope” will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing
Several technological advancements are currently in progress to increase the sensitivity of the millimeter VLBI array. A phased array processor to sum the collecting area on Mauna Kea (Weintroub 2008) has been tested. Similar hardware could be used to increase sensitivity at CARMA, Plateau de Bure, and the Atacama Large Millimeter/ submillimeter Array (ALMA) in Chile. Digital backends (DBEs) have been developed to process 1 GHz of data (4 Gbit s−1 with 2-bit Nyquist sampling), and next-generation DBEs will improve upon this by a factor of four. Mark 5B+ recorders can already record 2 Gbit s−1 data streams (presently requiring two at each site per DBE), and the Mark 5C recorders currently being developed will be able to handle even faster data rates. Cryogenic sapphire oscillators are being examined as a possible frequency standard to supplement or replace hydrogen masers to provide greater phase stability, which may improve coherence at higher frequencies.
Future observations will initially focus on improving sensitivity by observing a wider bandwidth and using phased array processors. Dual polarization observations will become a priority not only for the p2 improvement in sensitivity for total-power observations but also to allow full polarimetric VLBI of Sgr A*. Higher frequency observations, such as in the 345 GHz atmospheric window, will provide even greater spatial resolution in a frequency regime where interstellar scattering and optical depth effects are minimized.
The timing is right to move forward on building an Event Horizon Telescope to produce high-fidelity images of Sgr A* as well as other scientifically compelling sources, such as M87. Receivers currently being produced en masse for ALMA could be procured for other millimeter VLBI stations, in many cases providing substantial improvements in sensitivity. Studies of climate and weather will be necessary to provide information on the astronomical suitability of prospective sites for future telescopes, such as those at the present ALMA Test Facility or additional telescopes constructed specifically for millimeter VLBI (which would mesh well with present ALMA construction). Some existing telescopes will require improvements to their systems, such as increasing the bandwidth of the intermediate frequency signal after mixing. It will also be highly desirable to install permanent VLBI hardware at all sites to allow turnkey VLBI observing in order to maximize the efficiency of VLBI observations in terms of personnel time and transportation costs.
Current 1.3 mm VLBI observations have established that the millimeter emission emanates from a compact region offset from the center of the black hole. These data are already being used to constrain key physical parameters (e.g., spin, inclination, orientation) in models of the emission (e.g., RIAF models). Future additions to the VLBI array would allow the millimeter emission to be imaged directly