D14 Cosmological Black Holes: Image of the Event Horizon

Topic

The central figure in this model is the supermassive black hole—specifically, the event horizon that defines it—as tested by two completely different instruments.

A global network of radio telescopes, synchronized to function as a single interferometer the size of the entire Earth, achieves an angular resolution fine enough to image the shadow cast by the event horizons of two distinct supermassive black holes onto the hot material surrounding them.

A second instrument—an infrared interferometer combining four large telescopes—achieves sufficient resolution to track the orbits of individual stars around the black hole at the center of our own galaxy; this allows for testing general relativity in the strong-field regime and directly measuring the mass of that central black hole.

The most remarkable aspect of the first instrument is that it effectively creates an Earth-sized telescope by correlating—using supercomputers—signals recorded independently at stations scattered across the globe; a signal emitted tens of millions of years ago reaches these various stations with a time lag of mere nanoseconds—a lag that can be calculated based on Earth's geometry.

The most remarkable feature of the second instrument is its ability to detect oscillations from a "hot spot" orbiting at a significant fraction of the speed of light, thereby verifying—for the first time and at the scale of an actual black hole—the relativistic precession predicted by general relativity. Thus, from the event horizon that opens this discussion to the confirmed relativistic precession that concludes it—taking in the global radio telescope network and the infrared interferometer along the way—cosmological black holes subject a single horizon (invisible by definition) to two independent tests: one that directly images it, and another that measures, orbit by orbit, how general relativity shapes its immediate surroundings.

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