Rest, ArminScolnic, D.Foley, R. J.Huber, M. E.Chornock, R.Narayan, GauthamTonry, J. L.Berger, EdoSoderberg, AliciaStubbs, ChristopherRiess, A.Kirshner, RobertSmartt, S. J.Schlafly, E.Rodney, S.Botticella, M. T.Brout, D.Challis, P.Czekala, IanDrout, Maria RebeccaHudson, M. J.Kotak, R.Leibler, C.Lunnan, RMarion, G. H.McCrum, M.Milisavljevic, D.Pastorello, A.Sanders, Nathan EdwardSmith, K.Stafford, E.Thilker, D.Valenti, S.Wood-Vasey, W. M.Zheng, Z.Burgett, W. S.Chambers, K. C.Denneau, L.Draper, P. W.Flewelling, H.Hodapp, K. W.Kaiser, N.Kudritzki, R.-P.Magnier, E. A.Metcalfe, N.Price, P. A.Sweeney, W.Wainscoat, R.Waters, C.2015-07-022014Rest, A., D. Scolnic, R. J. Foley, M. E. Huber, R. Chornock, G. Narayan, J. L. Tonry, et al. 2014. “Cosmological Constraints from Measurements of Type Ia Supernovae Discovered during the First 1.5 yr of the Pan-STARRS1 Survey.” The Astrophysical Journal 795, no. 1: 44. doi:10.1088/0004-637x/795/1/441538-4357http://nrs.harvard.edu/urn-3:HUL.InstRepos:16953381We present griz P1 light curves of 146 spectroscopically confirmed Type Ia supernovae (SNe Ia; 0.03 < z < 0.65) discovered during the first 1.5 yr of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We find that the systematic uncertainties in the photometric system are currently 1.2% without accounting for the uncertainty in the Hubble Space Telescope Calspec definition of the AB system. A Hubble diagram is constructed with a subset of 113 out of 146 SNe Ia that pass our light curve quality cuts. The cosmological fit to 310 SNe Ia (113 PS1 SNe Ia + 222 light curves from 197 low-z SNe Ia), using only supernovae (SNe) and assuming a constant dark energy equation of state and flatness, yields $w=-1.120^{+0.360}_{-0.206}\hbox{(Stat)} ^{+0.269}_{-0.291}\hbox{(Sys)}$. When combined with BAO+CMB(Planck)+H 0, the analysis yields $\Omega _{\rm M}=0.280^{+0.013}_{-0.012}$ and $w=-1.166^{+0.072}_{-0.069}$ including all identified systematics. The value of w is inconsistent with the cosmological constant value of –1 at the 2.3σ level. Tension endures after removing either the baryon acoustic oscillation (BAO) or the H 0 constraint, though it is strongest when including the H 0 constraint. If we include WMAP9 cosmic microwave background (CMB) constraints instead of those from Planck, we find $w=-1.124^{+0.083}_{-0.065}$, which diminishes the discord to <2σ. We cannot conclude whether the tension with flat ΛCDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 SN sample with ~three times as many SNe should provide more conclusive results.en-UScosmological parameterscosmology: observationsdark energysupernovae: generalJournal Article2015-07-0210.1088/0004-637x/795/1/44