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We present an analysis of 15 Type Ia supernovae (SNe Ia) at redshift z > 1 (9at 1.5 < z < 2.3) recently discovered in the CANDELS and CLASH Multi-CycleTreasury programs using WFC3 on the Hubble Space Telescope. We combine theseSNe Ia with a new compilation of 1050 SNe Ia, jointly calibrated and correctedfor simulated survey biases to produce accurate distance measurements. Wepresent unbiased constraints on the expansion rate at six redshifts in therange 0.07 < z < 1.5 based only on this combined SN Ia sample. The addedleverage of our new sample at z > 1.5 leads to a factor of ~3 improvement inthe determination of the expansion rate at z = 1.5, reducing its uncertainty to~20%, a measurement of H(z=1.5)/H0=2.67 (+0.83,-0.52). We then demonstrate thatthese six measurements alone provide a nearly identical characterization ofdark energy as the full SN sample, making them an efficient compression of theSN Ia data. The new sample of SNe Ia at z > 1 usefully distinguishes betweenalternative cosmological models and unmodeled evolution of the SN Ia distanceindicators, placing empirical limits on the latter. Finally, employing arealistic simulation of a potential WFIRST SN survey observing strategy, weforecast optimistic future constraints on the expansion rate from SNe Ia.

Type Ia Supernova Distances at z > 1.5 from the Hubble Space Telescope Multi-Cycle Treasury Programs: The Early Expansion Rate