A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon.
Realizing this goal would open a new window on the study of general relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of an event horizon, and fundamental black hole physics. Steady long-term progress on improving the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made it extremely likely that this goal will be achieved within the next decade.
Key science results so far:
SAGITTARIUS A [Sgr A] is a black hole: Material is falling into the Sgr A* system. If Sgr A* had a surface no bigger than the size measured by the EHT, it would be a very bright source at infrared wavelengths. However, Sgr A* is a faint infrared source, indicating that energy is disappearing through an event horizon, the existence of which defines a black hole.
The accretion disc in Sgr A* is inclined to the line of sight: The material orbiting and falling into the black hole is confined to a region called the accretion disk. The size obtained from the EHT indicates that we are viewing this disk closer to edge-on than face-on. EHT data also place constraints on other parameters, such as the spin of the black hole.
The millimetre emission in Sgr A* is offset from the black hole: As an object gets close to a black hole, it appears bigger than it actually is. The apparent size measured by the EHT is smaller than the minimum size allowed by gravitational lensing. The emission must therefore be offset from the black hole. Models indicate that we are seeing the portion of the accretion disk that is moving toward us.
The variability in Sgr A* occurs near the black hole: The emission we detect from Sgr A* changes in brightness, but its apparent size does not change. While the mechanism that produces this variability is not well understood, EHT data indicate that the changes occur in the accretion flow very near the black hole.
This new 1.3 mm VERY LARGE BASE-LINE [VLBL] detection confirms that short-wavelength VLBI of Sgr A* can and will be used to directly probe the event horizon of this black hole candidate: in short, Sgr A* is the right object, VLBI is the right technique, and this decade is the right time.
Over the next decade, the group proposes to combine existing and planned millimetre/submillimetre facilities into a high-sensitivity, high angular resolution Event Horizon Telescope that will bring us as close to the edge of black hole as we will ever come. This effort will include development and deployment of submillimetre dual-polarization receivers, highly stable frequency standards to enable VLBI at 230-450 GHz, higher-bandwidth VLBI back ends and recorders, as well as commissioning of new submillimetre VLBI sites.