  Type Ia supernovae are generally thought to be due to the thermonuclear
explosions of carbon-oxygen white dwarfs with masses near the Chandrasekhar
mass. This scenario, however, has two long-standing problems. First, the
explosions do not naturally produce the correct mix of elements, but have to be
finely tuned to proceed from sub-sonic deflagration to super-sonic detonation.
Second, population models and observations give formation rates of
near-Chandrasekhar white dwarfs that are far too small. Here, we suggest that
type Ia supernovae instead result from mergers of roughly equal-mass
carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass
remnants. Numerical studies of such mergers have shown that the remnants
consist of rapidly rotating cores that contain most of the mass and are hottest
in the center, surrounded by dense, small disks. We argue that the disks
accrete quickly, and that the resulting compressional heating likely leads to
central carbon ignition. This ignition occurs at densities for which pure
detonations lead to events similar to type Ia supernovae. With this merger
scenario, we can understand the type Ia rates, and have plausible reasons for
the observed range in luminosity and for the bias of more luminous supernovae
towards younger populations. We speculate that explosions of white dwarfs
slowly brought to the Chandrasekhar limit---which should also occur---are
responsible for some of the "atypical" type Ia supernovae.
