What fraction of Type Ia supernovae come from two white dwarfs?
In plain words
A white dwarf may explode after taking matter from an ordinary companion star, or after merging or interacting with a second white dwarf. Astronomers still do not know which route makes most of the observed explosions.
Precise statement
For normal SNe Ia in Milky Way-like galaxies, determine the fraction $f_{\mathrm{DD}}$ produced by double-degenerate systems (two white dwarfs) versus single-degenerate systems (white dwarf plus nondegenerate donor). The answer must be consistent with the delay-time distribution (approximately proportional to $t^{-1}$), the absence of stripped hydrogen in nebular spectra (no detection in 111 events, Tucker et al. 2020, with limits well below the $0.1\ \text{to}\ 0.5\,M_{\mathrm{sun}}$ expected from a hydrogen-rich donor), early light-curve excesses and the measured rate per unit stellar mass.
What would settle it
A population synthesis calibrated on the delay-time distribution combined with complete nebular-spectrum hydrogen limits and companion searches across a volume-limited SN Ia sample.
Status in the literature
Unverified note
Most evidence through 2025 favours double-degenerate systems for the majority, but single-degenerate contributions to subclasses remain under debate.