Tension and Systematics in the Gold06 SnIa Dataset

The Gold06 SnIa dataset recently released in astro-ph/0611572 consists of five distinct subsets defined by the group or instrument that discovered and analyzed the corresponding data. These subsets are: the SNLS subset (47 SnIa), the HST subset (30 SnIa), the HZSST subset (41 SnIa), the SCP subset (26 SnIa) and the Low Redshift (LR) subset (38 SnIa). These subsets sum up to the 182 SnIa of the Gold06 dataset. We use Monte-Carlo simulations to study the statistical consistency of each one of the above subsets with the full Gold06 dataset. In particular, we compare the best fit $w(z)$ parameters (w_0,w_1) obtained by subtracting each one of the above subsets from the Gold06 dataset (subset truncation), with the corresponding best fit parameters (w^r_0,w^r_1) obtained by subtracting the same number of randomly selected SnIa from the same redshift range of the Gold06 dataset (random truncation). We find that the probability for (w^r_0,w^r_1)=(w_0,w_1) is large for the Gold06 minus SCP (Gold06-SCP) truncation but is less than 5% for the Gold06-SNLS, Gold06-HZSST and Gold06-HST truncations. This result implies that the Gold06 dataset is not statistically homogeneous. By comparing the values of the best fit (w_0,w_1) for each subset truncation we find that the tension among subsets is such that the SNLS and HST subsets are statistically consistent with each other and `pull' towards LCDM (w_0=-1,w_1=0) while the HZSST subset is statistically distinct and strongly `pulls' towards a varying w(z) crossing the line $w=-1$ from below (w_00). We also isolate six SnIa that are mostly responsible for this behavior of the HZSST subset.Comment: 10 pages, 6 Figures. References added. The mathematica files with the numerical analysis of the paper may be found at http://leandros.physics.uoi.gr/gold06/gold06.ht

Is the CMB shift parameter connected with the growth of cosmological perturbations?

We verify numerically that in the context of general relativity (GR), flat models which have the same $\Omega_{\rm m}$ and CMB shift parameter $R$ but different $H(a)$ and $w(a)$ also have very similar (within less than 8%) growth of perturbations even though the dark energy density evolution is quite different. This provides a direct connection between geometrical and dynamical tests of dark energy and may be used as a cosmological test of general relativity.Comment: 6 pages, 3 figures, Accepted for publication in MNRA

Observational Constraints on Phantom Crossing DGP Gravity

We study the observational constraints on the Phantom Crossing DGP model. We demonstrate that the crossing of the phantom divide does not occur within the framework of the original Dvali-Gabadadze-Porrati (DGP) model or the DGP model developed by Dvali and Turner. By extending their model in the framework of an extra dimension scenario, we study a model that realizes crossing of the phantom divide. We investigate the cosmological constraints obtained from the recent observational data of Type Ia Supernovae, Cosmic Microwave Background anisotropies, and Baryon Acoustic Oscillations. The best fit values of the parameters with 1$\sigma$ (68%) errors for the Phantom Crossing DGP model are $\Omega_{m,0}=0.27^{+0.02}_{-0.02}$, $\beta=0.54^{+0.24}_{-0.30}$. We find that the Phantom Crossing DGP model is more compatible with the observations than the original DGP model or the DGP model developed by Dvali and Turner. Our model can realize late-time acceleration of the universe, similar to that of $\Lambda$CDM model, without dark energy due to the effect of DGP gravity. In our model, crossing of the phantom divide occurs at a redshift of $z \sim 0.2$.Comment: 17 pages, 9 figures, 1 table, Accepted for publication in International Journal of Modern Physics

Testing LCDM with the Growth Function \delta(a): Current Constraints

We have compiled a dataset consisting of 22 datapoints at a redshift range (0.15,3.8) which can be used to constrain the linear perturbation growth rate f=\frac{d\ln\delta}{d\ln a}. Five of these data-points constrain directly the growth rate f through either redshift distortions or change of the power spectrum with redshift. The rest of the datapoints constrain f indirectly through the rms mass fluctuation \sigma_8(z) inferred from Ly-\alpha at various redshifts. Our analysis tests the consistency of the LCDM model and leads to a constraint of the Wang-Steinhardt growth index \gamma (defined from f=\Omega_m^\gamma) as \gamma=0.67^{+0.20}_{-0.17}. This result is clearly consistent at $1\sigma$ with the value \gamma={6/11}=0.55 predicted by LCDM. A first order expansion of the index \gamma in redshift space leads to similar results.We also apply our analysis on a new null test of LCDM which is similar to the one recently proposed by Chiba and Nakamura (arXiv:0708.3877) but does not involve derivatives of the expansion rate $H(z)$. This also leads to the fact that LCDM provides an excellent fit to the current linear growth data.Comment: 7 pages, 4 figures. Added comments on the data of Table I (after eq. (2.16)). Corrected a typo on eq. (2.15). The mathematica files with the numerical analysis of this study may be found at http://nesseris.physics.uoi.gr/growth/growth.ht

Cosmic Acceleration Data and Bulk-Brane Energy Exchange

We consider a braneworld model with bulk-brane energy exchange. This allows for crossing of the w=-1 phantom divide line without introducing phantom energy with quantum instabilities. We use the latest SnIa data included in the Gold06 dataset to provide an estimate of the preferred parameter values of this braneworld model. We use three fitting approaches which provide best fit parameter values and hint towards a bulk energy component that behaves like relativistic matter which is propagating in the bulk and is moving at a speed v along the fifth dimension, while the bulk-brane energy exchange component corresponds to negative pressure and signifies energy flowing from the bulk into the brane. We find that the best fit effective equation of state parameter $w_{eff}$ marginally crosses the phantom divide line w=-1. Thus, we have demonstrated both the ability of this class of braneworld models to provide crossing of the phantom divide and also that cosmological data hint towards natural values for the model parameters.Comment: 12 pages, 2 figures, added comments, references update

Viable f(T) models are practically indistinguishable from LCDM

We investigate the cosmological predictions of several $f(T)$ models, with up to two parameters, at both the background and the perturbation levels. Using current cosmological observations (geometric supernovae type Ia, cosmic microwave background and baryonic acoustic oscillation and dynamical growth data) we impose constraints on the distortion parameter, which quantifies the deviation of these models from the concordance $\Lambda$ cosmology at the background level. In addition we constrain the growth index $\gamma$ predicted in the context of these models using the latest perturbation growth data in the context of three parametrizations for $\gamma$. The evolution of the best fit effective Newton constant, which incorporates the $f(T)$-gravity effects, is also obtained along with the corresponding $1\sigma$ error regions. We show that all the viable parameter sectors of the $f(T)$ gravity models considered practically reduce these models to $\Lambda$CDM. Thus, the degrees of freedom that open up to $\Lambda$CDM in the context of $f(T)$ gravity models are not utilized by the cosmological data leading to an overall disfavor of these models.Comment: 16 pages, 9 figures, changes match published versio

Duality invariance and cosmological dynamics

A duality transformation that interrelates expanding and contracting cosmological models is shown to single out a duality invariant, interacting two-component description of any irrotational, geodesic and shearfree cosmic medium with vanishing three curvature scalar. We apply this feature to a system of matter and radiation, to a mixture of dark matter and dark energy, to minimal and conformal scalar fields, and to an enlarged Chaplygin gas model of the cosmic substratum. We extend the concept of duality transformations to cosmological perturbations and demonstrate the invariance of adiabatic pressure perturbations under these transformations.Comment: 14 pages, Discussion enlarged, accepted for publication in IJMP