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Register a new userProbing the dynamical behavior of dark energy
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We investigate dynamical behavior of the equation of state of dark energy $w_{de}$ by employing the linear-spline method in the region of low redshifts from observational data (SnIa, BAO, CMB and 12 $H(z)$ data). The redshift is binned and $w_{de}$ is approximated by a linear expansion of redshift in each bin. We leave the divided points of redshift bins as free parameters of the model, the best-fitted values of divided points will represent the turning positions of $w_{de}$ where $w_{de}$ changes its evolving direction significantly (if there exist such turnings in our considered region). These turning points are natural divided points of redshift bins, and $w_{de}$ between two nearby divided points can be well approximated by a linear expansion of redshift. We find two turning points of $w_{de}$ in $zin(0,1.8)$ and one turning point in $zin (0,0.9)$, and $w_{de}(z)$ could be oscillating around $w=-1$. Moreover, we find that there is a $2sigma$ deviation of $w_{de}$ from -1 around $z=0.9$ in both correlated and uncorrelated estimates.
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We use the Constitution supernova, the baryon acoustic oscillation, the cosmic microwave background, and the Hubble parameter data to analyze the evolution property of dark energy. We obtain different results when we fit different baryon acoustic oscillation data combined with the Constitution supernova data to the Chevallier-Polarski-Linder model. We find that the difference stems from the different values of $Omega_{m0}$. We also fit the observational data to the model independent piecewise constant parametrization. Four redshift bins with boundaries at $z=0.22$, 0.53, 0.85 and 1.8 were chosen for the piecewise constant parametrization of the equation of state parameter $w(z)$ of dark energy. We find no significant evidence for evolving $w(z)$. With the addition of the Hubble parameter, the constraint on the equation of state parameter at high redshift isimproved by 70%. The marginalization of the nuisance parameter connected to the supernova distance modulus is discussed.
We consider the models of vacuum energy interacting with cold dark matter in this study, in which the coupling can change sigh during the cosmological evolution. We parameterize the running coupling $b$ by the form $b(a)=b_0a+b_e(1-a)$, where at the early-time the coupling is given by a constant $b_{e}$ and today the coupling is described by another constant $b_{0}$. We explore six specific models with (i) $Q(a)=b(a)H_0rho_0$, (ii) $Q(a)=b(a)H_0rho_{rm de}$, (iii) $Q(a)=b(a)H_0rho_{rm c}$, (iv) $Q(a)=b(a)Hrho_0$, (v) $Q(a)=b(a)Hrho_{rm de}$, and (vi) $Q(a)=b(a)Hrho_{rm c}$. The current observational data sets we use to constrain the models include the JLA compilation of type Ia supernova data, the Planck 2015 distance priors data of cosmic microwave background observation, the baryon acoustic oscillations measurements, and the Hubble constant direct measurement. We find that, for all the models, we have $b_0<0$ and $b_e>0$ at around the 1$sigma$ level, and $b_0$ and $b_e$ are in extremely strong anti-correlation. Our results show that the coupling changes sign during the evolution at about the 1$sigma$ level, i.e., the energy transfer is from dark matter to dark energy when dark matter dominates the universe and the energy transfer is from dark energy to dark matter when dark energy dominates the universe.
We consider the consequences of having no prior knowledge of the true dark energy model for the interpretation of cosmological observations. The magnitude of redshift-space distortions and weak-lensing shear is determined by the metric on the geodesics of which galaxies and light propagate. We show that, given precise enough observations, we can use these data to completely reconstruct the metric on our past lightcone and therefore to measure the scale- and time-dependence of the anisotropic stress and the evolution of the gravitational potentials in a model-independent manner. Since both dark matter and dark energy affect the visible sector only through the gravitational field they produce, they are inseparable without a model for dark energy: galaxy bias cannot be measured and therefore the distribution of dark matter determined; the peculiar velocity of dark matter can be identified with that of the galaxies only when the equivalence principle holds. Given these limitations, we show how one can nonetheless build tests for classes of dark energy models which depend on making measurements at multiple scales at a particular redshift. They are null tests on the model-independent observables, do not require modeling evolution in time and do not require any parametrization of the free functions of these models, such as the sound speed. We show how one can rule out or constrain the whole class of the most-general scalar-tensor theories even without assuming the quasi-static limit.
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 $Omega_{0m}$ and $h$, DE which evolves with time provides a better fit to the SNe data than $Lambda$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 $Lambda$CDM based CMB results $Omega_{0m} = 0.27 pm 0.04$, $h = 0.71 pm 0.06$ 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.
We determine constraints on spatially-flat tilted dynamical dark energy XCDM and $phi$CDM inflation models by analyzing Planck 2015 cosmic microwave background (CMB) anisotropy data and baryon acoustic oscillation (BAO) distance measurements. XCDM is a simple and widely used but physically inconsistent parameterization of dynamical dark energy, while the $phi$CDM model is a physically consistent one in which a scalar field $phi$ with an inverse power-law potential energy density powers the currently accelerating cosmological expansion. Both these models have one additional parameter compared to standard $Lambda$CDM and both better fit the TT + lowP + lensing + BAO data than does the standard tilted flat-$Lambda$CDM model, with $Delta chi^2 = -1.26 (-1.60)$ for the XCDM ($phi$CDM) model relative to the $Lambda$CDM model. While this is a 1.1$sigma$ (1.3$sigma$) improvement over standard $Lambda$CDM and so not significant, dynamical dark energy models cannot be ruled out. In addition, both dynamical dark energy models reduce the tension between the Planck 2015 CMB anisotropy and the weak lensing $sigma_8$ constraints.
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