Accelerating dark energy models with anisotropic fluid in Bianchi type-VI0VI_{0} space-time

Motivated by the increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, we have discussed some features of the Bianchi type-$VI_{0}$ universes in the presence of a fluid that wields an anisotropic equation of state (EoS) parameter in general relativity. We present two accelerating dark energy (DE) models with an anisotropic fluid in Bianchi type-$VI_{0}$ space-time. To prevail the deterministic solution we choose the scale factor $a(t) = \sqrt{t^{n}e^{t}}$, which yields a time-dependent deceleration parameter (DP), representing a class of models which generate a transition of the universe from the early decelerating phase to the recent accelerating phase. Under the suitable condition, the anisotropic models approach to isotropic scenario. The EoS for dark energy $\omega$ is found to be time-dependent and its existing range for derived models is in good agreement with the recent observations of SNe Ia data (Knop et al. 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. 2004) and latest combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al. 2009; Komatsu et al. 2009). For different values of $n$, we can generate a class of physically viable DE models.The cosmological constant $\Lambda$ is found to be a positive decreasing function of time and it approaches to a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent type Ia supernovae observations. We also observe that our solutions are stable. The physical and geometric aspects of both the models are also discussed in detail.Comment: 22 pages, 8 figures. arXiv admin note: substantial text overlap with arXiv:1010.1121, arXiv:1108.2133, arXiv:1010.236

Magnetized Bianchi Type VI0VI_{0} Barotropic Massive String Universe with Decaying Vacuum Energy Density Λ\Lambda

Bianchi type $VI_{0}$ massive string cosmological models using the technique given by Letelier (1983) with magnetic field are investigated. To get the deterministic models, we assume that the expansion ($\theta$) in the model is proportional to the shear ($\sigma$) and also the fluid obeys the barotropic equation of state. It was found that vacuum energy density $\Lambda \propto \frac{1}{t^{2}}$ which matches with natural units. The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is also discussed.Comment: 14 pages, no figure

Viscous Dark Energy and Phantom Field in An Anisotropic Universe

In this paper we have investigated the general form of viscous and non-viscous dark energy equation of state (EoS) parameter in the scope of anisotropic Bianchi type I space-time. We show that the presence of bulk viscosity causes transition of $\omega^{de}$ from quintessence to phantom but the phantom state is an unstable state (as expected) and EoS of DE tends to $-1$ at late time. Then we show this phantomic description of the viscous dark energy and reconstruct the potential of the phantom scalar field. It is found that bulk viscosity pushes the universe to a darker region. We have also shown that at late time $q\sim-\Omega^{de}$.Comment: 10 page

Crossing the phantom divide line in universal extra dimensions

We investigate the cosmic acceleration and the evolution of dark energy across the cosmological constant boundary in universal extra dimensions UED. We adopt an empirical approach to solve the higher-dimensional cosmological equations so that the deceleration parameter $q$ is consistent with observations. The expressions for the jerk and deceleration parameters are independent of the number of dimensions $n$. The behavior of pressure in $4$D shows a positive-to-negative transition corresponding to the deceleration-to-acceleration cosmic transition. This pressure behavior helps in providing an explanation to the cosmic deceleration-acceleration transition although the reason behind the transition itself remains unknown. In the conventional $4$D cosmology, there is a no-go theorem prevents the EoS parameter of a single perfect fluid in FRW geometry to cross the $\omega=-1$ boundary. The current model includes a single homogenous but anisotropic perfect fluid in a homogenous FRW metric with two different scale factors in the ordinary $4$D and the UED. In contrast to the conventional $4$D cosmology, we have found that the dark energy evolution in UED shows $\omega=-1$ crossing. however, the no-go theorem is still respected in $4$D where the EoS parameter doesn't cross the $\omega=-1$ boundary.Comment: 9 pages, 6 figures, 1 tabl

Tilted Bianchi Type I Cosmological Models Filled with Disordered Radiation in General Relativity Revisited

Tilted Bianchi type I cosmological models filled with disordered radiation in presence of a bulk viscous fluid and heat flow are investigated. The coefficient of bulk viscosity is assumed to be a power function of mass density. Some physical and geometric properties of the models are also discussed.Comment: 12 page

Tilted Bianchi Type V Bulk Viscous Cosmological Models with Varying Λ\Lambda-Term

Conformally flat tilted Bianchi type V cosmological models in presence of a bulk viscous fluid and heat flow are investigated. The coefficient of bulk viscosity is assumed to be a power function of mass density. The cosmological constant is found to be a decreasing function of time, which is supported by results from recent type Ia supernovae observations. Some physical and geometric aspects of the models are also discussed.Comment: 13 pages, no figure

Bulk Viscous cosmological models in Lyra geometry

We have investigated an LRS Bianchi Type I models with bulk viscosity in the cosmological theory based on Lyra's geometry. A new class of exact solutions have been obtained by considering a time-dependent displacement field for a constant value of the deceleration parameter and viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density. The physical behaviour of the models is also discussed.Comment: 18 pages, 7figure

Is Hubble's Expansion due to Dark Energy

{\it The universe is expanding} is known (through Galaxy observations) since 1929 through Hubble's discovery ($V = H D$). Recently in 1999, it is found (through Supernovae observations) that the universe is not simply expanding but is accelerating too. We, however, hardly know only $4\%$ of the universe. The Wilkinson Microwave Anisotropy Probe (WMAP) satellite observational data suggest $73\%$ content of the universe in the form of dark-energy, $23\%$ in the form of non-baryonic dark-matter and the rest $4\%$ in the form of the usual baryonic matter. The acceleration of the universe is ascribed to this dark-energy with bizarre properties (repulsive-gravity). The question is that whether Hubble's expansion is just due to the shock of big-bang & inflation or it is due to the repulsive-gravity of dark-energy? Now, it is believed to be due to dark-energy, say, by re-introducing the once-discarded cosmological-constant $\Lambda$. In the present paper, it is shown that `the formula for acceleration due to dark-energy' is (almost) exactly of same-form as `the acceleration formula from the Hubble's law'. Hence, it is concluded that: yes, `indeed it is the dark-energy responsible for the Hubble's expansion too, in-addition to the current on-going acceleration of the universe'.Comment: 8 pages, 1 figur

Nehomogen cilindrično-simetričan model svemira sa strunama, magnetskim poljem i s promjenljivom kozmološkom konstantom λ

Cylindrically-symmetric inhomogeneous magnetized string cosmological model is investigated with cosmological term Λ varying with time. To get the deterministic solution, it has been assumed that the expansion (θ) in the model is proportional to the eigenvalue σ 1 1 of the shear tensor σ i j . The value of cosmological constant for the model is found to be small and positive which is supported by the results from recent supernovae Ia observations. The physical and geometric properties of the model are also discussed in the presence and absence of magnetic field.Proučavamo nehomogen cilindrično-simetričan model svemira sa strunama, magnetskim poljem i s vremenski promjenljivom kozmološkom konstantom Λ. Radi postizanja rješenja, pretpostavlja se da je širenje (θ) propocionalno svojstvenoj vrijednosti σ 1 1 tenzora posmika σ i j . Nalazi se da je u ovom modelu kozmološka konstanta mala i pozitivna, što je u suglasju s novim opažanjima supernova Ia. Fizička i geometrijska svojstva modela raspravljaju se sa i bez magnetskog polja

Bianchi type-I transit cosmological models with time dependent gravitational and cosmological constants - reexamined

The present study reexamines the recent work of Pradhan et al. (Indian J. Phys. 88: 757, 2014) and obtained general exact solutions of the Einstein's field equations with variable gravitational and cosmological "constants" for a spatially homogeneous and anisotropic Bianchi type-I space-time. To study the transit behaviour of Universe, we consider a law of variation of scale factor $a(t) = \left(t^{k} e^{t}\right)^{\frac{1}{n}}$ which yields a time dependent deceleration parameter $q = - 1 + \frac{nk}{(k + t)^{2}}$, comprising a class of models that depicts a transition of the universe from the early decelerated phase to the recent accelerating phase. We find that the time dependent deceleration parameter is reasonable for the present day Universe and give an appropriate description of the evolution of the universe. For $n = 0.27k$, we obtain $q_{0} = -0.73$ which is similar to observed value of deceleration parameter at present epoch. It is also observed that for $n \geq 2$ and $k = 1$, we obtain a class of transit models of the universe from early decelerating to present accelerating phase. For $k = 0$, the universe has non-singular origin. In these models, we arrive at the decision that, from the structure of the field equations, the behaviour of cosmological and gravitational constants and are related. Taking into consideration the observational data, we conclude that the cosmological constant behaves as a positive decreasing function of time whereas gravitational constant is increasing and tend to a constant value at late time. $H(z)/(1+z)$ data ($32$ points) and model prediction as a function of redshift for different $k$ and $n$ are successfully presented by using recent data (Farooq and Ratra, Astrophys. J. 66: L7, 2013). Some physical and geometric properties of the models are also discussed.Comment: 18 pages, 12 figures. Some figures are changed and some texts were withdrawn. arXiv admin note: text overlap with arXiv:1303.3000; and text overlap with arXiv:gr-qc/0010016 by other author without attributio