Undead models and shrouded black holes | Science

Some physical theories take a long time to die, even when evidence piles up against them. Consider the geocentric cosmology that placed the Earth at the center of the universe, or phlogiston, once believed to be a supposed fire-like substance contained within combustible bodies. Another example concerns Active Galactic Nuclei (AGN), and their luminous quasar cousins, which emit huge amounts of visible and ultraviolet (UV) light from a tiny spot in the centers of many galaxies—a feature known as the “Big Blue Bump.” This phenomenon is widely agreed to be caused by the gravitational accretion of surrounding matter onto a central supermassive black hole. In 1973, Shakura and Sunyaev modeled how this accretion could proceed as a thin, opaque, rotating disk that slowly descends under gravity because of its internal viscosity, following the so-called “alpha theory.” The steady gravitational descent heats the gas which then radiates visible-UV light. This model was originally aimed at understanding compact binary star systems but quickly became the standard explanation for AGN too.