New clues to the type-Ia supernova progenitor puzzle

Type-Ia supernovae (SNe Ia) are thermonuclear bombs in which about a
solar mass of carbon and oxygen is burned into iron-peak elements. The
"explosives" are apparently a white dwarf. SNe Ia are excellent
cosmological distance indicators, and as such provided the first
evidence that the cosmic expansion is accelerating.  However, despite
their confident use in cosmology, a major embarrassment remains: no
one knows, based on direct evidence, what exactly is exploding. Two
scenarios have been long considered for explaining how a white dwarf
can ignite and explode as a SN Ia. In the "single-degenerate" picture,
a white dwarf accretes matter from a companion "normal" star (i.e. a
star with a classical equation of state) , until approaching the
Chandrasekhar limit and igniting. In the "double-degenerate" picture,
a close white-dwarf binary loses energy and angular momentum to
gravitational waves, until the two white dwarfs merge, thus starting
the ignition and the thermonuclear runaway. However, both scenarios
have theoretical and observational problems, and little or no direct
evidence to support them. SN Ia rates, as a function of cosmic time
and environment, can provide clues. I will show how many recent
measurements are converging toward a single SN Ia "delay-time
distribution" -- the SN Ia rate, as a function of time, that would
follow a hypothetical short burst of star formation.  The emerging
function is remarkably similar to what one expects from white dwarf
mergers, based directly on the fundamentals of gravitational wave
emission. A measurement of the Galactic binary white dwarf merger rate
also suggests there may indeed be enough such mergers to serve as the
SN Ia progenitors.