  Type Ia supernovae (SNe Ia) are likely the thermonuclear explosions of
carbon-oxygen white-dwarf stars, but their progenitor systems remain elusive. A
few theoretical scenarios for the progenitor systems have been suggested, which
have been shown to agree with some observational properties of SNe Ia. However,
several computational challenges prohibit a robust comparison to the
observations. We focus on the observed $t_0-M_\text{Ni56}$ relation, where
$t_0$ (the $\gamma$-rays' escape time from the ejecta) is positively correlated
with $M_\text{Ni56}$ (the synthesized $^{56}$Ni mass). Comparing to the
$t_0-M_\text{Ni56}$ relation bypasses the need for radiation transfer
calculations, as the value of $t_0$ can be directly inferred from the ejecta.
We show that all known SNe Ia models fail to reproduce the observed
$t_0-M_\text{Ni56}$ correlation.
