Dark-Energy Dynamics Required to Solve the Cosmic Coincidence

Dynamic dark energy (DDE) models are often designed to solve the cosmic coincidence (why, just now, is the dark energy density $\rho_{de}$, the same order of magnitude as the matter density $\rho_m$?) by guaranteeing $\rho_{de} \sim \rho_m$ for significant fractions of the age of the universe. This typically entails ad-hoc tracking or oscillatory behaviour in the model. However, such behaviour is neither sufficient nor necessary to solve the coincidence problem. What must be shown is that a significant fraction of observers see $\rho_{de} \sim \rho_m$. Precisely when, and for how long, must a DDE model have $\rho_{de} \sim \rho_{m}$ in order to solve the coincidence? We explore the coincidence problem in dynamic dark energy models using the temporal distribution of terrestrial-planet-bound observers. We find that any dark energy model fitting current observational constraints on $\rho_{de}$ and the equation of state parameters $w_0$ and $w_a$, does have $\rho_{de} \sim \rho_m$ for a large fraction of observers in the universe. This demotivates DDE models specifically designed to solve the coincidence using long or repeated periods of $\rho_{de} \sim \rho_m$.Comment: 16 pages, 8 figures, Submitted to Phys. Rev.

Reconstructing the properties of dark energy from recent observations

We explore the properties of dark energy from recent observational data, including the Gold Sne Ia, the baryonic acoustic oscillation peak from SDSS, the CMB shift parameter from WMAP3, the X-ray gas mass fraction in cluster and the Hubble parameter versus redshift. The $\Lambda CDM$ model with curvature and two parameterized dark energy models are studied. For the $\Lambda CDM$ model, we find that the flat universe is consistent with observations at the $1\sigma$ confidence level and a closed universe is slightly favored by these data. For two parameterized dark energy models, with the prior given on the present matter density, $\Omega_{m0}$, with $\Omega_{m0}=0.24$, $\Omega_{m0}=0.28$ and $\Omega_{m0}=0.32$, our result seems to suggest that the trend of $\Omega_{m0}$ dependence for an evolving dark energy from a combination of the observational data sets is model-dependent.Comment: 16 pages, 15 figures, To appear in JCA

Bayesian evidence and model selection approach for time-dependent dark energy

We use parameterized post-Friedmann (PPF) description for dark energy and apply ellipsoidal nested sampling to perform the Bayesian model selection method on different time-dependent dark energy models using a combination of $Planck$ and data based on distance measurements, namely baryon acoustic oscillations and supernovae luminosity distance. Models with two and three free parameters described in terms of linear scale factor $a$, or scaled in units of e-folding $\ln a$ are considered. Our results show that parameterizing dark energy in terms of $\ln a$ provides better constraints on the free parameters than polynomial expressions. In general, two free-parameter models are adequate to describe the dynamics of the dark energy compared to their three free-parameter generalizations. According to the Bayesian evidence, determining the strength of support for cosmological constant $\Lambda$ over polynomial dark energy models remains inconclusive. Furthermore, considering the $R$ statistic as the tension metric shows that one of the polynomial models gives rise to a tension between $Planck$ and distance measurements data sets. The preference for the logarithmic equation of state over $\Lambda$ is inconclusive, and the strength of support for $\rm \Lambda$CDM over the oscillating model is moderate.Comment: Accepted for publication in MNRAS. 8 pages, 4 figure