Type Ia Supernova Discoveries at z>1 From the Hubble Space Telescope: Evidence for Past Deceleration and Constraints on Dark Energy Evolution

TL;DR

Riess et al. present 16 Type Ia supernovae at $z>1$ discovered with HST/ACS GOODS, extending the Hubble diagram to $z>1.5$ and providing robust evidence for a past decelerating phase followed by recent acceleration. Using a kinematic expansion history and the MLCS2k2 light-curve fitter (with UV extension) alongside cross-checks with BATM, they constrain $\Omega_M \approx 0.29$ and $\Omega_\Lambda \approx 0.71$ in a flat universe, and find the dark-energy equation of state to be $w \approx -1$ with limited room for rapid evolution. Joint analysis with external constraints tightens the $w_0$–$w'$ plane, yielding $w_0$ near -1 and modest or no evolution ($w'$ small), thus favoring a cosmological constant while ruling out simple monotonic dimming scenarios. The study also assesses SN utility and lensing, finding no significant evolution or selection biases and confirming that lensing does not substantially bias the SN distances; these results strengthen the standard cosmological model and inform prospects for future high-redshift SN surveys.

Abstract

We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these and 170 previous SNe Ia are provided. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7 (chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant. When combined with external flat-Universe constraints we find w=-1.02 + 0.13 - 0.19 (and $<-0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc^2. Joint constraints on both the recent equation of state of dark energy, $w_0$, and its time evolution, dw/dz, are a factor of ~8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.

Figures (1)

• Figure 1: Figure 1. See attached jpeg image of SN hosts.