Type Ia supernova problem

The Type Ia supernova problem is the lack of a (complete) consensus regarding the mechanism that triggers Type Ia supernovae. The fact that they are thermonuclear explosions (powered by fusion) and that a white dwarf (WD) is the supernova progenitor is accepted, but the mechanism of triggering them and the explanations of all their spectral energy distribution/light curve characteristics are not. Triggering mechanisms are generally classified as a single degenerate model (often SD) or a double degenerate model (DD), for models involving, respectively, one or two white dwarfs. In the former case, it is presumed a giant star is interacting with the white dwarf, and in the latter case, it is presumed two white dwarfs interact, e.g., collide through orbital decay (e.g., through gravitational waves) or through the more complex interactions of a multiple star system. It is also theorized that both SD- and DD-model triggering may occur. In all these cases, it is presumed the mass is enlarged until the heats it to the point that internal helium burning begins, but various theories exist regarding more-specific details of the progenitors and the circumstances that trigger the supernova.

Even presuming an answer to the question of progenitors and the triggering circumstance, details of the actual ignition are not established: a large thermonuclear explosion requires sufficient fuel to remain intact in proximity for enough to burn in the very short time of such a large explosion, which analytical models suggest requires special circumstances, regarding which there are theories. One dimensional models have proved insufficient to convincingly fill in the model-gaps and the development of a well-accepted tractable three dimensional model likely requires some particular combination of techniques (using appropriate subgrid-scale physics and/or zooming in on key locations) that has yet to be found.