There seems to be countless black holes in the universe – space-time rabbit holes that forever swallow anything passing nearby. The most massive black holes, weighing millions or billions of times as much as our Sun, lurk in the centers of galaxies. When these sleeping dragons gobble up anything passing nearby they can blaze forth as bright lighthouses called active galactic nuclei. Other black holes do not pull in surrounding material constantly, but in fits and bursts, making their brightness flicker.

This behavior was used by astronomers to go black hole hunting. One of the best hunting grounds is the Hubble Ultra Deep Field – which unveiled faint galaxies that existed not long after the big bang. The Hubble Ultra Deep Field photo was revealed in 2004. A team of astronomers from Stockholm University looked at later images of the Hubble Ultra Deep Field and found changes in brightness among some galaxies. These changes are attributed to black hole variability – like flickering holiday lights. The result is that they found more black holes in the early universe than has previously been reported.

The relationship between early galaxies and massive black holes is a chicken-and-egg dilemma for cosmologists. What came first? Revisiting the extraordinary Hubble Ultra Deep Field offers new clues.