  Type Ia Supernovae (SNe Ia) are standardizable candles that allow us to
measure the recent expansion rate of the Universe. Due to uncertainties in
progenitor physics, potential astrophysical dependencies may bias cosmological
measurements if not properly accounted for. The dependency of the intrinsic
luminosity of SNe Ia with their host-galaxy environment is often used to
standardize SNe Ia luminosity and is commonly parameterized as a step function.
This functional form implicitly assumes two-populations of SNe Ia. In the
literature, multiple environmental indicators have been considered, finding
different, sometimes incompatible, step function amplitudes. We compare these
indicators in the context of a two-populations model, based on their ability to
distinguish the two populations. We show that local H$\alpha$-based specific
star formation rate (lsSFR) and global stellar mass are better tracers than,
for instance, host galaxy morphology. We show that tracer accuracy can explain
the discrepancy between the observed SNe Ia step amplitudes found in the
literature. Using lsSFR or global mass to distinguish the two populations can
explain all other observations, though lsSFR is favoured. As lsSFR is strongly
connected to age, our results favour a prompt and delayed population model. In
any case, there exists two populations that differ in standardized magnitude by
at least $0.121\pm0.010\,\mathrm{mag}$.

Cosmology and Nongalactic Astrophysics