Black holes are perhaps the most exotic and enigmatic objects in the universe. Predicted by Einstein's theory of General Relativity and widely accepted as products of certain examples of massive single-star and binary-star evolution, the most dramatic cases are the "supermassive" black holes found in the centers of essentially all galaxies. Here, accretion has amassed as much matter as exists in 20 billion stars into extraordinarily tiny volumes. Our Galaxy hosts a comparatively modest example, with some 4 million solar masses packed inside a region no larger than Uranus' orbit around the Sun.
Until now, the tiny sizes of black holes coupled with their vast distances have permitted us to view only the gravitational effects of black holes on surrounding stars and gas. However, Dr. S. Doeleman (Smithsonian Astrophysical Observatory) and collaborators are employing the Very Large Baseline Interferometry (VLBI) technique pioneered by radio astronomers to combine signals from several telescopes scattered around the Earth with the goal of imaging the radio emission from just outside a black hole's point of no return - the so-called Schwarzschild radius where the last light emitted by matter flowing into the hole can escape and reach an outside observer.
To effectively combine the signals from widely separated telescopes, each signal must be time-stamped with an extremely precise local reference signal. This proposal seeks funds to acquire ultra-stable frequency references for global VLBI observations at mm-wavelengths aimed at obtaining unprecedented views of the immediate environs of the supermassive black hole at the center of our Milky Way, and possibly also of the much more massive hole that lurks at the center of the nearby giant galaxy M87. Fortunately, such time standards have been developed using hydrogen maser oscillators for applications in geodesy and navigation, and therefore are reasonably affordable and extremely reliable.
Funding for the acquisition of time standards for the global VLBI network is being provided by NSF's Division of Astronomical Sciences through its participation in the Major Research Instrumentation program.
