ALMA continues to expand its power and capabilities by linking with other millimetre-wavelength telescopes in Europe and North America in a series of very long baseline interferometry (VLBI) observations. In VLBI, data from two or more telescopes are combined to form a single, virtual telescope that spans the geographic distance between them. This way, it is possible to form an Earth-sized telescope with extraordinarily fine resolution. In particular, VLBI at mm wavelenghts (mm-VLBI) offers the highest achievable spatial resolution in astronomy, of the order of microarcseconds.
A beamformer for ALMA has been developed that can aggregate the entire collecting area of the array into a single, very large aperture (equivalent to a 84-m diameter) telescope. Joint VLBI observations that include ALMA as a phased array (p-ALMA) with other telescopes worldwide are now possible (GMVA and EHT). The inclusion of p-ALMA into a global VLBI network brings a transformational leap in capabilities, including unprecedented sensitivity at very high angular resolution (sources one order of magnitude weaker are becoming accessible to μas-resolution studies) and greatly improving image fidelity for bright objects thanks to the north-south baseline (high-dynamic range imaging).
p-ALMA observations can address a wide range of research topics, that include: imaging the event horizon of a black hole, tests of general relativity near black holes, accretion/outflow processes around supermassive black holes, jet launch and collimation from AGN and microquasars, pulsar and magnetar emission processes, the chemical history of the universe and the evolution of fundamental constants across cosmic time, masers in young and evolved stars, astrometry, etc.