The case for dynamical dark energy revisited

TL;DR

The paper investigates whether dark energy is dynamical by performing a model-independent reconstruction of the expansion history $H(z)$ in a flat universe, using a flexible ansatz to relate $H(z)$, $ ho_{DE}$, and the equation of state $w(z)$. By fitting to the Riess et al. Gold supernova sample and incorporating CMB information, the authors find that, without priors on $\,Ω_{0m}$ and $h$, evolving dark energy ($w_0<-1$ today) provides a better fit than ΛCDM, with a transition from deceleration to acceleration around $z_T≈0.39$. When ΛCDM-based CMB priors are imposed, the inferred evolution weakens, yielding $z_T≈0.57$ and $w_0$ closer to −1, implying a closer alignment with ΛCDM. The results show strong dependence on the SN data sampling and priors, highlighting the need for more high-redshift data and independent constraints on $\,Ω_{0m}$ and $h$ to robustly determine the nature of dark energy.

Abstract

We investigate the behaviour of dark energy using the recently released supernova data of Riess et al ~(2004) and a model independent parameterization for dark energy (DE). We find that, if no priors are imposed on $Ω_{0m}$ and $h$, DE which evolves with time provides a better fit to the SNe data than $Λ$CDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe+CMB, the best-fit DE model has $w_0 < -1$ at the present epoch and the transition from deceleration to acceleration occurs at $z_T = 0.39 \pm 0.03$. However, DE evolution becomes weaker if the $Λ$CDM based CMB results $Ω_{0m} = 0.27 \pm 0.04$, $h = 0.71 \pm 0.06$ are incorporated in the analysis. In this case, $z_T = 0.57 \pm 0.07$. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.

Figures (8)

• Figure 1: $1\sigma, \ 2\sigma, \ 3\sigma$ confidence levels in the $w_0-w_1$ space for the ansatz (\ref{['eq:state1']}) for $\Omega_{0 \rm m}=0.3$, using different subsets of data from riess04. The filled circle represents the $\Lambda$CDM point.
• Figure 2: The logarithmic variation of dark energy density $\rho_{\rm DE}(z)/\rho_{0{\rm c}}$ (where $\rho_{0{\rm c}}=3 H_0^2/8 \pi G$ is the present critical energy density) with redshift for $\Omega_{0 \rm m}=0.3$ using different subsets of data from riess04, for the ansatz (\ref{['eq:taylor']}). In each panel, the thick solid line shows the best-fit, the light grey contour represents the $1\sigma$ confidence level, and the dark grey contour represents the $2\sigma$ confidence level around the best-fit. The dotted line denotes matter density $\Omega_{0 \rm m} (1+z)^3$, and the dashed horizontal line denotes $\Lambda$CDM.
• Figure 3: The variation of equation of state of dark energy $w(z)$ with redshift for $\Omega_{0 \rm m}=0.3$ using different subsets of data from riess04, for the ansatz (\ref{['eq:taylor']}). In each panel, the thick solid line shows the best-fit, the light grey contour represents the $1\sigma$ confidence level, and the dark grey contour represents the $2\sigma$ confidence level around the best-fit. The dashed horizontal line denotes $\Lambda$CDM.
• Figure 4: The ($A_1,A_2$) parameter space for the ansatz (\ref{['eq:taylor']}) for different values of $\Omega_{0 \rm m}$, using the 'Gold' sample of SNe from riess04. The star in each panel marks the best-fit point, and the solid contours around it mark the $1\sigma, 2\sigma, 3\sigma$ confidence levels around it. The filled circle represents the $\Lambda$CDM point. The corresponding $\chi^2$ for the best-fit points are given in table \ref{['tab:chi']}.
• Figure 5: The logarithmic variation of dark energy density $\rho_{\rm DE}(z)/\rho_{0{\rm c}}$ (where $\rho_{0{\rm c}}=3 H_0^2/8 \pi G$ is the present critical energy density) with redshift for different values of $\Omega_{0 \rm m}$, using the 'Gold' sample of SNe from riess04. The reconstruction is done using the polynomial fit to dark energy, ansatz (\ref{['eq:taylor']}). In each panel, the thick solid line shows the best-fit, the light grey contour represents the $1\sigma$ confidence level, and the dark grey contour represents the $2\sigma$ confidence level around the best-fit. The dotted line denotes matter density $\Omega_{0 \rm m} (1+z)^3$, and the dashed horizontal line denotes $\Lambda$CDM.