‘Echo Mapping’ in Faraway Galaxies Could Measure Vast Cosmic Distances – Jet Propulsion Laboratory

Matter swirling around supermassive black holes creates bursts of light that “echo” in nearby dust clouds. These traveling signals could serve as a new cosmic yardstick.
When you look up at the night sky, how do you know whether
the specks of light that you see are bright and far away, or relatively faint
and close by? One way to find out is to compare how much light the object actually
emits with how bright it appears. The difference between its true luminosity
and its apparent brightness …

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