Can deflagration-detonation transitions occur in type Ia supernovae?

The mechanism for a deflagration-detonation transition (DDT) by turbulent preconditioning, which has previously been used to explain the possible occurrence of delayed detonations in Type Ia supernova explosions, is argued here to be conceptually inconsistent. It relies crucially on diffusive heat losses of the burned material on macroscopic scales. Regardless of the amplitude of the turbulent velocity fluctuations, the typical gradient scale for the temperature fluctuations is shown to be of the order of the laminar flame width or smaller rather than the homogeneity scale required for DDT, which is larger by more than a factor of 1000. Furthermore, thermonuclear flames cannot be fully quenched in regions that are much larger than the laminar flame width as a consequence of their simple "chemistry." Possible alternative explosion scenarios are briefly discussed.