Cosmological Results from the RAISIN Survey: Using Type Ia Supernovae in
  the Near Infrared as a Novel Path to Measure the Dark Energy Equation of
  State
  Type Ia supernovae (SN Ia) are more nearly standard candles when measured in
the near-infrared (NIR) than in the optical. With this motivation, from
2012-2017 we embarked on the RAISIN program with the Hubble Space Telescope
(HST) to obtain rest-frame NIR light curves for a cosmologically distant sample
of 37 SN Ia ($0.2<z<0.7$) discovered by Pan-STARRS and the Dark Energy Survey.
By comparing higher-$z$ HST data with 43 SN Ia at $z<0.1$ observed in the NIR
by the Carnegie Supernova Project, we construct a Hubble diagram exclusively
from NIR observations to pursue a unique avenue to constrain the dark energy
equation of state parameter, $w$. We analyze the dependence of the full set of
Hubble residuals on the SN Ia host galaxy mass and find Hubble residual steps
of size $\sim$0.08-0.12 mag with 2- to 3-$\sigma$ significance depending on the
method and step location used. Combining our NIR sample with Cosmic Microwave
Background (CMB) constraints, we find $1+w=-0.04\pm0.12$
(statistical$+$systematic errors). The largest systematic errors are the
redshift-dependent SN selection biases and the properties of the NIR mass step.
We also use these data to measure ${\rm H}_0=75.4\pm 2.4~{\rm
km~s^{-1}~Mpc^{-1}}$ from stars with geometric distance calibration in the
hosts of 8 SNe Ia observed in the NIR versus ${\rm H}_0=65.9\pm3.4~{\rm
km~s^{-1}~Mpc^{-1}}$ using an inverse distance ladder approach tied to Planck.
Using optical data we find $1+w=-0.01\pm0.09$ and with optical and NIR data
combined, we find $1+w=0.03\pm0.08$; these shifts of up to $\sim$0.07 in $w$
could point to inconsistency in the optical versus NIR SN models. There will be
many opportunities to improve this NIR measurement and better understand
systematic uncertainties through larger low-$z$ samples, new light-curve
models, calibration improvements, and eventually building high-$z$ samples from
the Roman Space Telescope.
