Longer Cosmic Ruler Based On Black Holes - Science News: The brightness of an active nucleus is tightly related to the radius of a region of hot gases surrounding the central black hole. When scientists determine that radius, they can predict how intrinsically bright the nucleus should be — and compare that value to how bright it appears, which depends on distance...
A technique called reverberation mapping measures how long it takes photons being kicked out of the black hole’s immediate neighborhood to reappear after they’ve traversed the hot, gassy maelstrom surrounding the black hole. Because light travels at a constant speed, astronomers can determine the gassy region’s radius. Then, the luminosity of the active galactic nucleus can be calculated.
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