New Hubble Space Telescope Discoveries of Type Ia Supernovae at z ≥ 1: Narrowing Constraints on the Early Behavior of Dark Energy^*

We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to trace the history of cosmic expansion over the last 10 billion yr. These objects, which include 13 spectroscopically confirmed SNe Ia at z ≥ 1, were discovered during 14 epochs of reimaging of the GOODS fields North and South over 2 yr with the Advanced Camera for Surveys on HST. Together with a recalibration of our previous HST-discovered SNe Ia, the full sample of 23 SNe Ia at z ≥ 1 provides the highest redshift sample known. Combining these data with previous SN Ia data sets, we measured H(z) at discrete, uncorrelated epochs, reducing the uncertainty of H(z > 1) from 50% to under 20%, strengthening the evidence for a cosmic jerk—the transition from deceleration in the past to acceleration in the present. The unique leverage of the HST high-redshift SNe Ia provides the first meaningful constraint on the dark energy equation-of-state parameter at z ≥ 1. The result remains consistent with a cosmological constant [w(z) = −1] and rules out rapidly evolving dark energy (dw/dz ≫ 1). The defining property of dark energy, its negative pressure, appears to be present at z > 1, in the epoch preceding acceleration, with ~98% confidence in our primary fit. Moreover, the z > 1 sample-averaged spectral energy distribution is consistent with that of the typical SN Ia over the last 10 Gyr, indicating that any spectral evolution of the properties of SNe Ia with redshift is still below our detection threshold.