Radiative Transfer in Type Ia Supernovae

Some white dwarf stars die in a thermonuclear runaway leading to complete stellar disruption within seconds -- a Type~Ia supernova. The material ejected in that explosion will shine brightly for months, powered by the decay of freshly synthesized radioactive isotopes. Multi-physics hydrodynamical codes are now simulating the first violent seconds of the event, and a treatment of the subsequent radiation transport is needed to calculate predictions of the observable light curves, spectral evolution, and spectropolarization. Here I discuss Monte Carlo techniques for addressing multi-group time-dependent radiative transfer in 3-dimensional, rapidly expanding plasmas, where the densities are low and non-LTE effects can be important. I compare our model calculations directly to astronomical observations, and discuss how the simulations are helping us understand the progenitors and explosion mechanism of Type~Ia supernovae, as well as refining their applicability as probes of cosmological expansion.