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Supernova Limits on the Cosmic Equation of State

P. M. Garnavich, S. Jha, P. Challis, A. Clocchiatti, A. Diercks, A. V. Filippenko, R. L. Gilliland, C. J. Hogan, R. P. Kirshner, B. Leibundgut, M. M. Phillips, D. Reiss, A. G. Riess, B. P. Schmidt, R. A. Schommer, R. C. Smith, J. Spyromilio, C. Stubbs, N. B. Suntzeff, J. Tonry, S. M. Carroll

TL;DR

This study uses Type Ia supernovae to constrain the cosmic equation of state for an accelerating energy component beyond gravitating matter. By modeling this component as X with density $\Omega_{\rm x}$ and average equation of state $\alpha_{\rm x}$, and imposing the null energy condition, the authors extract tight 95% confidence limits on $\alpha_{\rm x}$ for flat and open geometries. The results favor a vacuum-energy–like component (cosmological constant or quintessence) with $\alpha_{ m x}$ near $-1$, disfavoring topological defects such as domain walls, strings, or textures as the primary driver of acceleration. When combined with CMB constraints on the first acoustic peak, the data indicate a nearly flat Universe with $\Omega_{\rm tot}\approx 0.94\pm0.26$ and a dominant dark energy component, reinforcing the standard cosmological model with a cosmological constant or similar scalar-field dark energy.

Abstract

We use Type Ia supernovae studied by the High-Z Supernova Search Team to constrain the properties of an energy component which may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must be less than -0.55 (95% confidence) for any value of Omega_m and is further limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 . These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova data are consistent with a cosmological constant (alpha_x=-1) or a scalar field which has had, on average, an equation of state parameter similar to the cosmological constant value of -1 over the redshift range of z=1 to the present. Supernova and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda = 0.94 +/- 0.26 (1-sigma).

Supernova Limits on the Cosmic Equation of State

TL;DR

This study uses Type Ia supernovae to constrain the cosmic equation of state for an accelerating energy component beyond gravitating matter. By modeling this component as X with density and average equation of state , and imposing the null energy condition, the authors extract tight 95% confidence limits on for flat and open geometries. The results favor a vacuum-energy–like component (cosmological constant or quintessence) with near , disfavoring topological defects such as domain walls, strings, or textures as the primary driver of acceleration. When combined with CMB constraints on the first acoustic peak, the data indicate a nearly flat Universe with and a dominant dark energy component, reinforcing the standard cosmological model with a cosmological constant or similar scalar-field dark energy.

Abstract

We use Type Ia supernovae studied by the High-Z Supernova Search Team to constrain the properties of an energy component which may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must be less than -0.55 (95% confidence) for any value of Omega_m and is further limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 . These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova data are consistent with a cosmological constant (alpha_x=-1) or a scalar field which has had, on average, an equation of state parameter similar to the cosmological constant value of -1 over the redshift range of z=1 to the present. Supernova and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda = 0.94 +/- 0.26 (1-sigma).

Paper Structure

This paper contains 6 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Analysis
  4. Results
  5. Other Constraints
  6. Conclusions

Figures (3)

  • Figure 1: The joint probability distributions for $\Omega_{\rm m}$ and the density of the unknown component, $\Omega_{\rm x}$, based on the SNIa magnitudes reduced with the MLCS method. Four representative values of the equation of state parameter, $\alpha_{\rm x}$, are shown. See Riess98 for the distribution when $\alpha_{\rm x} = -1$.
  • Figure 2: The joint probability distributions from SNIa for $\Omega_{\rm m}$ and the equation of state parameter, $\alpha_{\rm x}$, assuming a flat spatial geometry ($\Omega_{\rm m} +\Omega_{\rm x}=1$). The top panel uses supernova distances from the MLCS method combined with supernovae reduced using the snapshot method, while the bottom panel is from the $\Delta m_{15}(B)$ technique plus snapshot results. The vertical broken line marks the matter density estimated from galaxy cluster dynamics.
  • Figure 3: The combined constraints from SNIa and the position of the first Doppler peak of the CMB angular power spectrum. The equation of state parameter for the unknown component is $\alpha_{\rm x}=-1$, like that for a cosmological constant. The contours mark the 68%, 95.4%, and 99.7% enclosed probability regions.