Narrow Results

We study the possibly existing anisotropy in the accelerating expansion Universe with various supernovae data, the baryon acoustic oscillation and the observational Hubble data. We present combined results from these probes, deriving constraints on the equation of state (EoS), $w$, of dark energy (DE) and its energy density in the Universe. We fit the cosmological parameters simultaneously employing the maximum likelihood analysis. By combining data and considering anisotropy effects, we find that the EoS of DE are $w(0.01\le z\le 1.75)=-0.998\pm 0.056$, $w(0<z<1)=-1.086\pm 0.065$, $w(0.15<z<1.1)=-1.096\pm 0.123$ and $w(0.01<z<2.3)=-1.0426\pm 0.054$ within $1{\sigma }^{\prime }$ confidence level. Finally, introducing an anisotropy appears to improve the fit to observations with respect to that provided by an isotropic $w$CDM model.

We demonstrate that the FRW model filled with dark energy fluid, satisfying the general form of the equation of state, p = w(z)ρ, can be reduced to the one-dimensional Hamiltonian flow. On the other hand we show that potential of this system can be reconstructed from the supernovae type Ia (SNIa) data. We show that the V-reconstruction method can be used to differentiate between dark energy models.

We investigate the possibility that part of the dark matter is not made out of the usual cold dark matter (CDM) dust-like particles, but is in the form of a fluid of strings with barotropic factor w_s = -1/3 of cosmic origin. To this aim, we split the dark matter density parameter into two terms and investigate the dynamics of a spatially flat universe filled with baryons, CDM, a fluid of strings and dark energy, modeling the latter as a cosmological constant or a negative pressure fluid with a constant equation of state w < 0. To test the viability of the models and to constrain their parameters, we use the Type Ia supernovae Hubble diagram and data on the gas mass fraction in galaxy clusters. We also discuss the weak field limit of a model comprising a significant fraction of dark matter in the form of a fluid of strings and show that this mechanism makes it possible to reduce the need for the elusive and up to now undetected CDM. We finally find that a model comprising both a cosmological constant and a fluid of strings fits the data very well and eliminates the need for phantom dark energy, thus representing a viable candidate for alleviating some of the problems plaguing the dark side of the universe.

Few statements in cosmology can be made without assuming a cosmological model within which to interpret data. Statements about cosmic acceleration are no exception to this rule, and the inferred positive volume acceleration of our universe often quoted in the literature is valid in the context of the standard Friedmann–Lemaître–Robertson–Walker (FLRW) class of spacetimes. Using the Joint Light-curve Analysis (JLA) catalogue of supernovae Type Ia (SNIa), we examine the fit of a class of exact scaling solutions with dynamical spatial curvature formulated in the framework of a scalar averaging scheme for relativistic inhomogeneous spacetimes. In these models, global volume acceleration may emerge as a result of the nonlocal variance between expansion rates of clusters and voids, the latter gaining volume dominance in the late-epoch universe. We find best-fit parameters for a scaling model of backreaction that are reasonably consistent with previously found constraints from SNIa, CMB, and baryon acoustic oscillations data. The quality of fit of the scaling solutions is indistinguishable from that of the ΛCDM model and the timescape cosmology from an Akaike Information Criterion (AIC) perspective. This indicates that a broad class of models can account for the $z<\sim 1$ expansion history.