The missing parents of cosmology's favorite standard candles.

Type-Ia supernovae (SNe Ia) are thermonuclear bombs in which about onesolar mass of carbon and oxygen are burned into iron-peak elements.The fuel is apparently a "white dwarf" stellar remnant. SNe Ia becamepopular about 15 years ago, when it became clear that they can serveas excellent cosmological distance indicators. In 1998, they providedthe first evidence that the cosmic expansion is accelerating under theinfluence of an enigmatic "dark energy", culminating in the 2011 NobelPrize in Physics. However, despite their confident use for cosmology,a major embarrassment remains: no one knows, based on direct evidence,what exactly is exploding. Two scenarios have been on the table for along time for explaining how a white dwarf can ignite and explode as aSN Ia. In the "single-degenerate" picture, a white dwarf accretesmatter from a companion "normal" star (i.e. a star with a classicalequation of state) , until approaching the Chandrasekhar limit andigniting. In the "double-degenerate" picture, a close white-dwarfbinary loses energy and angular momentum to gravitational waves, untilthe two white dwarfs merge, thus starting the ignition and thethermonuclear runaway. However, both scenarios have theoretical andobservational problems, and little or no direct evidence to supportthem. Measurement of SN Ia rates, as a function of cosmic time andenvironment, can shed light on this problem. I will show how,recently, many different measurements are converging toward a singleSN Ia "delay-time distribution". This is is the SN Ia rate, as afunction of time, that would follow a hypothetical short burst of starformation, i.e., it is the Green's function of SNe Ia. The emergingfunction is remarkably similar to what one expects from white dwarfmergers, based directly on the fundamentals of gravitational waveemission.