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

TL;DR

This paper reports the discovery of 21 new SNe Ia with the HST, including 13 at z>1, expanding to 23 SNe Ia at z>1 when combined with reprocessed data, and uses these to constrain the expansion history via $H(z)$ in a largely model-independent way. It employs MLCS2k2 to fit light curves, constructs a SN Ia Hubble diagram, and derives uncorrelated estimates of $H(z)$, showing $H(z)$ is well constrained up to $z\approx1.3$ and providing the first meaningful constraints on dark energy at $z>1$. By exploring both parameterized and non-parametric forms of the dark-energy equation-of-state, the study finds results consistent with a cosmological constant ($w=-1$) while placing meaningful bounds on early-time behavior, with $w(z>1)=-0.8^{+0.6}_{-1.0}$ and $w(z>1)<0$ at 98% confidence when include strong priors. Spectral analysis of the $z>1$ SN sample reveals no evidence for evolution in SN Ia properties relative to the low-redshift population, supporting the reliability of SNe Ia as standard candles across $10$ Gyr of look-back time. The work highlights the crucial role of high-redshift SNe in breaking degeneracies in dark-energy models and emphasizes caution in applying strong high-redshift priors that depend on untested assumptions about $w(z)$ beyond the data.

Abstract

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 years. 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 two years 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. Combined with previous SN Ia datasets, 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.