Cosmic ruler and black holes

ASTRONOMERS have a new gadget in their cosmic toolbox that is capable of measuring distances to very faraway objects. The method uses active galactic nuclei, the bright, violent regions at the centers of galaxies, to gauge distances farther than existing cosmic yardsticks can reach.Having such an odometer is crucial for understanding how space, time and matter behave at the scale of the whole universe, and could help solve mysteries such as the nature of the dark energy that is accelerating the expansion of the universe.For four decades, astronomers have been trying to turn these luminous beacons into cosmic mile markers. Now, scientists at the University of Copenhagen's Dark Cosmology Centre and their collaborators think they've got it worked out. 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. Astronomers call objects whose properties can be used to predict their actual brightness standard candles.'It's the radius-luminosity relationship that allows us to assume that these active galactic nuclei are standard candles,' says postdoctoral fellow Kelly Denney of the Dark Cosmology Centre, an author of the study, which will appear in the Astrophysical Journal.
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