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We intend to study a new class of cosmological models in $f(R, T)$ modified theories of gravity, hence define the cosmological constant $\Lambda$ as a function of the trace of the stress energy-momentum-tensor $T$ and the Ricci scalar $R$, and name such a model "$\Lambda(R, T)$ gravity" where we have specified a certain form of $\Lambda(R, T)$. $\Lambda(R, T)$ is also defined in the perfect fluid and dust case. Some physical and geometric properties of the model are also discussed. The pressure, density and energy conditions are studied both when $\Lambda$ is a positive constant and when $\Lambda=\Lambda(t)$, i.e a function of cosmological time, t. We study the behaviour of some cosmological quantities such as Hubble and deceleration parameters. The model is innovative in the sense that it has been described in terms of both $R$ and $T$ and display a better understanding of the cosmological observations., Comment: 10 pages, 10 figures

It is well known that traversable wormholes are valid solutions of the Einstein field equations, but these structures can only be maintained by violating the null energy condition. In this paper, we have obtained such wormhole solutions in an isothermal galactic halo, as well as in a void. We have shown that the null energy condition is violated, with the help of a suitable redshift function obtained from flat galactic rotation curves., Comment: 7 pages, 10 figures. Accepted in Mod.Phys.Lett.A

In this paper we solve Friedmann equations by considering a universal media as a non-perfect fluid with bulk viscosity and is described by a general "gamma law" equation of state of the form $p= (\gamma -1) \rho + \Lambda(t)$, where the adiabatic parameter $\gamma$ varies with scale factor $R$ of the metric and $\Lambda$ is the time dependent cosmological constant. A unified description of the early evolution of the universe is presented by assuming the bulk viscosity and cosmological parameter in a linear combination of two terms of the form: $\Lambda(t)=\Lambda_{0} + \Lambda_{1}\frac{\dot{R}}{R}$ and $\zeta = \zeta_{0} + \zeta_{1} \frac{\dot{R}}{R}$, where $\Lambda_{0},\;\Lambda_{1},\, \zeta_{0}$ and $ \zeta_{1}$ are constants, in which an inflationary phase is followed by the radiation dominated phase. For this general gamma law equation of state, an entirely integrable dynamical equation to the scale factor $R$ is obtained along with its exact solutions. In this framework we demonstrate that the model can be used to explain the dark energy dominant universe and for a special choice of the parameters we can explain the accelerating expansion of the universe also for two different phases, viz. combination of dark energy and dark matter phase as well as unified dark energy phase. A special physical check has been performed through sound speed constraint to validate the model. At last we obtain a scaling relation between the Hubble parameter with redshift., Comment: 19 pages, 3 figures

In this work we consider the Kantowski-Sachs (KS) universe in the framework of varying speed of light theory. We present the general solutions of the gravitational field equations with variable speed of light $c(t)$, gravitational coupling parameter $G(t)$ and the decaying vacuum energy $\Lambda(t)$ for the KS model. In the limiting case for the equation of state (EOS) parameter $\gamma=2$ (stiff fluid with $p=\rho c^2$) and $\gamma =1$ (dust with $p=0$), exact solutions of the field equations are obtained. The numerical solutions are also presented for both the cases. Moreover, it is shown that in the limiting case of large time, the mean anisotropy parameter tends to zero for $\gamma=2$ and $\gamma=1$. Thus the time variation of the fundamental constants provides an effective mechanism for the isotropization of the KS universe., Comment: 7 pages, 2 figures

We consider the brane Kantowski-Sachs universe when bulk space is five dimensional anti-de Sitter obeying a linear equation of state of the form $p=(\gamma -1)\rho +p_{0}$, where $ \gamma,\:p_{0}$ are a parameters. In this framework we discuss the brane Kantowski-Sachs cosmology and compare it with standard Kantowski-Sachs cosmology in Einstein theory for some specific matter choice. For several specific choice of relation between energy and pressure, the behavior of the scale factors at early time is found. It is shown that the brane Kantowski-Scahs cosmology may quickly become isotropic for some choices of matter. We also discuss the universe fate of Kantowski-Sachs cosmological model by means of the rip or singularity in both the theories., Comment: 5 pages, no figures. arXiv admin note: text overlap with arXiv:hep-th/0201188 by other authors

This paper discusses a new wormhole solution that admits conformal motion, given a noncommutative-geometry background. After a discussion of the wormhole geometry and the energy conditions, the analysis proceeds with the calculation of the active gravitational mass, a discussion of the TOV equation describing the equilibrium conditions, as well as the nature of the total gravitational energy. The wormhole spacetime is not asymptotically flat and is therefore cut off at some radial distance and joined to an exterior vacuum solution., Comment: Latex, 7 pages, 8 figures

This paper has been withdrawn by arXiv administrators because of an inappropriate amount of overlap with hep-th/0505013., Comment: This paper has been withdrawn by arXiv administrators

In this paper we have considered the multidimensional cosmological implications of a decay law for $\Lambda$ term that is proportional to $\beta \frac{\ddot {a}}{a}$, where $\beta$ is a constant and $a$ is the scale factor of RW-space time. We discuss the cosmological consequences of a model for the vanishing pressure for the case $k=0$. It has been observed that such models are compatible with the result of recent observations and cosmological term $\Lambda$ gradually reduces as the universe expands. In this model $\Lambda$ varies as the inverse square of time, which matches its natural units. The proper distance, the luminosity distance-redshift, the angular diameter distance-redshift, and look back time-redshift for the model are presented in the frame work of higher dimensional space time. The model of the Freese {\it et al.} ({\it Nucl. Phys. B} {\bf 287}, 797 (1987)) for $n=2$ is retrieved for the particular choice of $A_{0}$ and also Einstein-de Sitter model is obtained for $A_{0} = {2/3}$. This work has thus generalized to higher dimensions the well-know result in four dimensional space time. It is found that there may be significant difference in principle at least, from the analogous situation in four dimensional space time., Comment: 10 pages, no figure, to be appear in IJMPD

In this paper the observational consequence of the cosmological models and the expression for the neoclassical tests, luminosity distance, angular diameter distance and look back time are analyzed in the framework of Lyra geometry. It is interesting to note that the space time of the universe is not only free of Big Bang singularity but also exhibits acceleration during its evolution., Comment: 8 pages, no figure, submitted to Lett. Math. Phys

In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW) cosmological models by considering three different forms of variable $\Lambda$: $\Lambda\sim(\frac{\dot{a}}{a})^2$,$\Lambda\sim(\frac{\ddot{a}} {a})$ and $\Lambda \sim \rho$. It is found that, the connecting free parameters of the models with cosmic matter and vacuum energy density parameters are equivalent, in the context of higher dimensional space time. The expression for the look back time, luminosity distance and angular diameter distance are also derived. This work has thus generalized to higher dimensions the well-known results in four dimensional space time. It is found that there may be significant difference in principle at least, from the analogous situation in four dimensional space time., Comment: 16 pages, no figure

Implications of cosmological model with a cosmological term of the form $\Lambda = \beta \frac{\ddot {a}}{a}$, where $\beta$ is a constant, are analyzed in multidimensional space time. The proper distance, the luminosity distance-redshift, the angular diameter distance-redshift, and look back time-redshift for the model are presented. It has been shown that such models are found to be compatible with the recent observations. This work has thus generalized to higher dimensions the well-know result in four dimensional space time. It is found that there may be significant difference in principle at least,from the analogous situation in four dimensional space time., Comment: 11 pages, no figure

An expression is derived where the mass is connected to an integral over the pressure of gravitating matter in the frame work of five dimensional(5D) space-time., Comment: 5 pages Latex, No figure

Lee. et.al. (1976) analysed the bimetric theory with the help of parameterized post Newtonian (PPN) formalism. They found that the post Newtonian limit of the theory is identical with that of general theory of relativity except for their PPN parameter $\alpha_{2}$, on the basis of cosmological considerations. In the present paper it is pointed out that feasibility of such considerations are doubtful in five dimensional bimetric theory of relativity. As the universe is unique and is governed by physical laws, many different cosmologies are possible. Examples are given for some possible cosmological models, which are different, that those of Lee. et.al. This work is an extension in five dimension of a similar one obtained earlier by Rosen (1977) for four dimensional space-time., Comment: Latex 8 pages, no figure

Exact solution are obtained for a homogeneous spacially isotropic cosmological model in a matter free space with or without cosmological consant for a n-dimensional Kaluza-Klein type of metric in the rest mass varying theory of gravity proposed by Wesson[1983]. The behavior of the model is discussed., Comment: Latex 4 pages, no figure

A static conformally flat spherically symmetric perfect fluid cosmological model in Lyra geometry is investigatd., Comment: 4 pages latex, no figure

We study the Robertson-Walker type model in the Lyttleton-Bondi universe in five dimensional general theory of relativity. Some exact and physical properties of solutions are discussed., Comment: latex 4 pages, no figure

Higher dimensional solutions are obtained for a homogeneous, spatially isotropic cosmological model in Wesson theory of gravitation. Some cosmological parameter are also calculated for this model., Comment: latex 4 pages, no figure

A new class of non-static higher dimensional vacuum solutions in space-time -mass (STM) theory of gravity is found. This solution represent expanding universe without big bang singularity and the higher dimension of these models shrinks as they expands., Comment: latex 4 pages, no figure