Title: How Long do Quasars Shine?

Luminous quasars are believed to be the progenitors of the supermassive black holes observed ubiquitously at the centers of all massive galaxies. But half a century after their discovery, we are still in the dark about how these black holes actually formed. Our ignorance largely results from the long expected timescale for supermassive black hole growth of 45 million years — far longer than humans have  been conducting astronomical observations.  A holy grail would thus be a direct measurement of the lifetimes of luminous quasars, shedding light on the physical mechanisms responsible for fueling black hole growth, and how the back-reaction of this growth might influence how galaxies form.  I will show how observations of diffuse intergalactic gas in the environs of luminous quasars can be used to chronicle the history of quasar emission on timescales from kiloyears to gigayears. I will also discuss how these same observations can be used to constrain the timing of an important phase transition in our Universe’s history.