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Type Ia supernovae (SNe Ia) are more precise standardizable candles whenmeasured in the near-infrared (NIR) than in the optical. With this motivation,from 2012-2017 we embarked on the RAISIN program with the Hubble SpaceTelescope (HST) to obtain rest-frame NIR light curves for a cosmologicallydistant sample of 37 SN Ia ($0.2 \lesssim z \lesssim 0.6$) discovered byPan-STARRS and the Dark Energy Survey. By comparing higher-$z$ HST data with 42SN Ia at $z<0.1$ observed in the NIR by the Carnegie Supernova Project, weconstruct a Hubble diagram from NIR observations (with only time of maximumlight and some selection cuts from optical data) to pursue a unique avenue toconstrain the dark energy equation of state parameter, $w$. We analyze thedependence of the full set of Hubble residuals on the SN Ia host galaxy massand find Hubble residual steps of size $\sim$0.06-0.1~mag with 1.5- to2.5-$\sigma$ significance depending on the method and step location. Combiningour NIR sample with CMB constraints, we find $1+w=-0.17\pm0.12$ (stat$+$syst).The largest systematic errors are the redshift-dependent SN selection biasesand the properties of the NIR mass step. We also use these data to measure$H_0=75.9\pm 2.2$ km s$^{-1}$ Mpc$^{-1}$ from stars with geometric distancecalibration in the hosts of 8 SNe Ia observed in the NIR versus$H_0=71.2\pm3.8$ km s$^{-1}$ Mpc$^{-1}$ using an inverse distance ladderapproach tied to Planck. Using optical data we find $1+w=-0.10\pm0.09$ and withoptical and NIR data combined, we find $1+w=-0.06\pm0.07$; these shifts of upto 0.11 in $w$ could point to inconsistency in optical versus NIR SN models.There will be many opportunities to improve this NIR measurement and betterunderstand systematic uncertainties through larger low-$z$ samples, newlight-curve models, calibration improvements, and by building high-$z$ samplesfrom the Roman Space Telescope.

the astrophysical journal

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