Generalized second law of thermodynamics with corrected entropy in tachyon cosmology

This work is to study the generalized second law (GSL) of thermodynamics in tachyon cosmology where the tachyon field is coupled to the matter Lagrangian while the boundary of universe is assumed to be a dynamical apparent horizon. The two logarithmic and power law corrected entropy on the apparent horizon is also discussed and the conditions for validity of GSL in both scenarios are investigated by using observational data of Sne Ia. In comparison to other research works, since the model is constrained by observational data, the conditions obtained for the dimensionless constant parameter, α in both logarithmic and power law entropy correction of GSL are (physically) meaningful and realistic. The model also predicts an accelerating universe with no phantom crossing in the past or future.     

Phase transition in Schwarzschild-de Sitter spacetime

Using a static massive spherically symmetric scalar field coupled to gravity in the Schwarzschild-de Sitter (SdS) background, first we consider some asymptotic solutions near horizon and their local equations of state (E.O.S.) on them. We show that near cosmological and event horizons our scalar field behaves as a dust. At the next step near two pure de Sitter or Schwarzschild horizons we obtain a coupling dependent pressure to energy density ratio. In the case of a minimally coupling this ratio is ?1 which springs to the mind thermodynamical behavior of dark energy. If having a negative pressure behavior near these horizons we concluded that the coupling constant must be ξ<¼. Therefore we derive a new constraint on the value of our coupling ξ. These two different behaviors of unique matter in the distinct regions of spacetime at present era can be interpreted as a phase transition from dark matter to dark energy in the cosmic scales and construct a unified scenario.     

The generalized second law of thermodynamics for the interacting in f(T) gravity

We study the validity of the generalized second law (GSL) of gravitational thermodynamics in a non-flat FRW universe containing the interacting in f(T) gravity. We consider that the boundary of the universe to be confined by the dynamical apparent horizon in FRW universe. In general, we discuss the effective equation of state, deceleration parameter and GLS in this framewok. Also, we find that the interacting-term Q modifies these quantities and in particular, the evolution of the total entropy, results in an increases on the GLS of thermodynamic, by a factor $4\pi R_{A}^{3} Q/3$ . By using a viable f(T) gravity with an exponential dependence on the torsion, we develop a model where the interaction term is related to the total energy density of matter. Here, we find that a crossing of phantom divide line is possible for the interacting-f(T) model.     

Nonlinear electrodynamics in f(T) gravity and generalized second law of thermodynamics

In this paper, we study the nonlinear electrodynamics in the framework of f(T) gravity for FRW universe along with dust matter, magnetic and torsion contributions. We evaluate the equation of state and deceleration parameters to explore the accelerated expansion of the universe. The validity of generalized second law of thermodynamics for Hubble and event horizons is also investigated in this scenario. For this purpose, we assume polelike and power-law forms of scale factor and construct f(T) models. The graphical behavior of the cosmological parameters versus smaller values of redshift z represent the accelerated expansion of the universe. It turns out that the generalized second law of thermodynamics holds for all values of z with Hubble and event horizons in polelike scale factor whereas for power-law form, it holds in a specific range of z for both horizons.     

Entropy and statefinder diagnosis in chameleon cosmology

In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models with bouncing behavior such as chameleon cosmology where the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. From a thermodynamic point of view, to link between thermodynamic and geometric parameters in cosmological models, we introduce “entropy rate of change multiplied by the temperature” as a model independent thermodynamic state parameter together with the well known {r,s} statefinder to differentiate the dark energy models.     

Cosmology of f(T) gravity in a holographic dark energy and nonisotropic background

It is shown that the acceleration of the universe can be understood by considering a f(T) gravity models. Modified teleparallel gravity theory with the torsion scalar has recently gained a lot of attention as a possible explanation of dark energy. For these f(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. We consider spatially homogenous and anisotropic Bianchi type I universe in the context of f(T) gravity. The de Sitter, power-law and general exponential solutions are assumed for the scale factor in each spatial direction and the corresponding cosmological models are reconstructed. We reconstruct f(T) theories from two different holographic dark energy models in different time durations. For the holographic dark energy model, the dark energy dominated era with new setting up is chosen for reconstruction, and the Ricci dark energy model, radiation, matter and dark energy dominated time durations are all investigated. Finally we have obtained a modified gravity action consistent with the holographic dark energy scenario.     

General class of Bianchi cosmological models with Λ in creation-field cosmology

The solutions of Einstein’s equations with cosmological constant (Λ) in the presence of a creation field have been obtained for general class of anisotropic cosmological models. We have obtained the cosmological solutions for two different scenarios of average scale factor. In first case, we have discussed three different types of physically viable cosmological solutions of average scale factor for the general class of Bianchi cosmological models by using a special law for deceleration parameter which is linear in time with a negative slope. In second case, we have discussed another three different forms of cosmological solutions by using the average scale factor in three different scenarios like Intermediate scenario, Logamediate scenario and Emergent scenario. All physical parameters are calculated and discussed in each physical viable cosmological model. We examine the nature of creation field and cosmological constant is dominated the early Universe but they do not survive for long time and finally tends to zero for large cosmic time t. We have also discussed the all energy conditions in each cases.     

Entropy of viscous Universe models

The cosmological event horizon entropy and the apparent horizon entropy of the ΛCDM and the Bianchi type I Universe model with viscosity has been calculated numerically, and analytically in the large time limit. It is shown that for these Universe models the cosmological event horizon entropy increases with time and for large times it approaches a finite maximum value. The effect of viscosity upon the entropy is also studied and we have found that its role is to decrease the entropy. The bigger the viscosity coefficient is the less the entropy will be. Furthermore, the radiation entropy for the ΛCDM Universe model with and without viscosity is investigated, and together with the cosmological event horizon entropy are used to examine the validity of the generalized second law of thermodynamics, which states that the total rate of change of entropy of the Universe is never negative, in this Universe model.     

Bianchi type VI h perfect fluid cosmological model in f(R,T) theory

In this paper, we have investigated Bianchi type VI _h cosmological model filled with perfect fluid in the framework of f(R,T) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor proposed by Harko et al. (Phys. Rev. D 84:024020, 2011). We have obtained the cosmological models by solving the field equations. Some physical behaviors of the model are also studied.     

Behaviour of physical parameters in extended gravity with hyperbolic function

In this paper, we have constructed the cosmological model of the universe in f(R, T) theory of gravity in a Bianchi type \(\mathrm{VI}_h\) universe for the functional f(R, T) in the form \(f(R,T)=\mu R+\mu T\), where R and T are respectively Ricci scalar and trace of energy momentum tensor and \(\mu \) is a constant. We have made use of the hyperbolic scale factor to find the physical parameters and metric potentials defined in the space-time. The physical parameters are constrained from different representative values to build up a realistic cosmological model aligned with the observational behaviour. The state finder diagnostic pair is found to be in the acceptable range. The energy conditions of the model are also studied.     

Quantum statistical entropy of Schwarzchild-de Sitter spacetime

Using the quantum statistical method, we calculate quantum statistical entropy between the black hole horizon and the cosmological horizon in Schwarzchild spacetime and derive the expression of quantum statistical entropy in de Sitter spacetime. Under the Unruh-Verlinde temperature of Schwarzchild-de Sitter spacetime in the entropic force views, we obtain the expression of quantum statistical entropy in de Sitter spacetime. It is shown that in de Sitter spacetime quantum statistical entropy is the sum of thermodynamic entropy corresponding black hole horizon and the one corresponding cosmological horizon. And the correction term of de Sitter spacetime entropy is obtained. Therefore, it is confirmed that the black hole entropy is the entropy of quantum field outside the black hole horizon. The entropy of de Sitter spacetime is the entropy of quantum field between the black hole horizon and the cosmological horizon.     

Vaidya black hole in non-stationary de Sitter space: Hawking’s temperature

In this paper we present a class of non-stationary solutions of Einstein’s field equations describing embedded Vaidya-de Sitter black holes with a cosmological variable function Λ(u). The Vaidya-de Sitter black hole is interpreted as the radiating Vaidya black hole is embedded into the non-stationary de Sitter space with variable Λ(u). The energy-momentum tensor of the Vaidya-de Sitter black hole is expressed as the sum of the energy-momentum tensors of the Vaidya null fluid and that of the non-stationary de Sitter field, and satisfies the energy conservation law. We study the energy conditions (like weak, strong and dominant conditions) for the energy-momentum tensor. We find the violation of the strong energy condition due to the negative pressure and leading to a repulsive gravitational force of the matter field associated with Λ(u) in the space-time. We also find that the time-like vector field for an observer in the Vaidya-de Sitter space is expanding, accelerating, shearing and non-rotating. It is also found that the space-time geometry of non-stationary Vaidya-de Sitter solution with variable Λ(u) is Petrov type D in the classification of space-times. We also find the Vaidya-de Sitter black hole radiating with a thermal temperature proportional to the surface gravity and entropy also proportional to the area of the cosmological black hole horizon.     

A new class of Bianchi cosmological models in f(R,T) gravity

The new class of cosmological model of the early Universe is considered with f(R,T) modified theories of gravity (Harko et al. in Phys. Rev. D 84:024020, 2011). The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail. We have also discussed the well-known astrophysical phenomena, namely the Hubble parameter H(z), luminosity distance (d _L ) and distance modulus μ(z) with redshift.     

Bianchi type V viscous fluid cosmological models in presence of decaying vacuum energy

Bianchi type V viscous fluid cosmological model for barotropic fluid distribution with varying cosmological term Λ is investigated. We have examined a cosmological scenario proposing a variation law for Hubble parameter H in the background of homogeneous, anisotropic Bianchi type V space-time. The model isotropizes asymptotically and the presence of shear viscosity accelerates the isotropization. The model describes a unified expansion history of the universe indicating initial decelerating expansion and late time accelerating phase. Cosmological consequences of the model are also discussed.     

Hawking radiation and entropy of Kerr-Newman black hole

The entropy correction of Kerr-Newman black hole is investigated using the Hamilton-Jacobi method beyond semiclassical approximation. To get entropy correction, the inverse of the sum of square of event horizon (r ₊) and the square of rotational parameter a of the black hole is taken as the proportionality parameter for quantum corrections of the action I _i to the semiclassical action I ₀. It has been shown that as quantum effects are taken into account the corrections to the Bekenstein-Hawking entropy of the stationary black hole include a logarithmic term and an inverse area term beyond the semiclassical approximation.     

Variable Chaplygin gas cosmologies in f(R,T) gravity with particle creation

A flat FLRW (Friedmann–Lemaitre–Robertson–Walker) cosmological model with perfect fluid comprising of variable Chaplygin gas (VCG) has been studied in the context of f(R, T) gravity with particle creation. The solutions of the modified field equations are obtained through three different considered form of scale factors. The effective pressure is negative throughout the evolution of universe, which leads to accelerated expansion of the universe. In addition to that we have also discussed the importance of particle creation pressure on the cosmological parameters, energy conditions and state-finder diagnostic parameters. It is noticed that the time evolution of source function yields almost constant particle production at late times.     

Five dimensional universe model with variable cosmological term Λ and a big bounce

We assume the four dimensional induced matter of the 5D Ricci flat bouncing cosmological solution contains a perfect fluid. The big bounce singularity of simple 5D cosmological model is studied with the cosmological term Λ=α ρ and Λ=β H ² where α and β are constants and ρ and H are respectively energy density and Hubble parameter. This big bounce singularity is found to be an event horizon at which the scale factor and mass density of the universe are finite, while the pressure is infinite.      

Reconstruction of f(R), f(T) and f(\mathcal{G}) models inspired by variable deceleration parameter

We study an special law for the deceleration parameter, recently proposed by Akarsu and Dereli, in the context of f(R), f(T) and $f(\mathcal{G})$ theories of modified gravity. This law covers the law of Berman for obtaining exact cosmological models to account for the current acceleration of the universe, and also gives the opportunity to generalize many of the dark energy models having better consistency with the cosmological observations. Our aim is to reconstruct the f(R), f(T) and $f(\mathcal{G})$ models inspired by this law of variable deceleration parameter. Such models may then exhibit better consistency with the cosmological observations.