Towards Imaging the Event Horizon in the Galactic Center

Black holes are theoretical predictions of Einstein’s General Theory
of Relativity, where large amounts of matter are concentrated so much
that even light cannot escape its gravitational attraction. Thus,
black holes mark singularities in space and time, which are surrounded
by an event horizon that allows matter and light to go inwards but
never go out again. However, do these supermassive black holes and
their event horizons really exist? The best place to answer this
question is in the center of our own Milky Way.  Here a compact radio
source, Sgr A*, with a mass of 4 Million times the mass of the sun,
seems to mark the central black hole of our Galaxy, providing by the
far the best evidence for the existence of black holes in general. The
nature of the emission process is still under debate, but numerical
magnetohydrodynamic simulations and semi-analytic models seem to
converge towards a combined jet-disk model producing radiation from
radio to X-rays.  Interestingly, radio interferometry and timing
observations are now probing the smallest scales of this object,
directly confirming that the high-frequency radio emission indeed
comes from event-horizon scales. This will allow one to study basic
astrophysical processes such as jet formation and accretion
physics. Using future mm-VLBI experiments, involving, e.g., ALMA, it
should be possible to even image the elusive event horizon of a black
hole for the very first time. This “Event Horizon Telescope” would not
only provide firm evidence for the existence of black holes, but would
also test basic predictions of General Relativity such as the no hair
theorem and variants of GR, making Sgr A* a fundamental laboratory for
precision black hole astrophysics.