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In this paper we have considered a model of modified Chaplygin gas and its role in accelerating phase of the universe. We have assumed that the equation of state of this modified model is valid from the radiation era to ΛCDM model. We have used recently developed statefinder parameters in characterizing different phase of the universe diagrammatically.

Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of state ρ = (1 + ω)λ .

The phenomenological parametrizations of dark-energy (DE) equation of state can be very helpful, since they allow for the investigation of its cosmological behavior despite the fact that its underlying theory is unknown. However, although there has been a large amount of research on DE parametrizations which involve two or more free parameters, the one-parameter parametrizations seem to be underestimated. We perform a detailed observational confrontation of five one-parameter DE models, with observational data from cosmic microwave background (CMB), Joint light-curve analysis sample from Supernovae Type Ia observations (JLA), baryon acoustic oscillations (BAO) distance measurements, and cosmic chronometers (CC). We find that all models favor a phantom DE equation of state at present time, while they lead to H0 values in perfect agreement with its direct measurements and therefore they offer an alleviation to the H0-tension. Finally, performing a Bayesian analysis we show that although ΛCDM cosmology is still favored, one-parameter DE models have similar or better efficiency in fitting the data comparing to two-parameter DE parametrizations, and thus they deserve a thorough investigation.

Recently, a generalized gravity theory was proposed by Harko etal where the Lagrangian density is an arbitrary function of the Ricci scalar R and the trace of the stress-energy tensor T, known as F(R,T) gravity. In their derivation of the field equations, they have not considered conservation of the stress-energy tensor. In the present work, we have shown that a part of the arbitrary function f(R,T) can be determined if we take into account of the conservation of stress-energy tensor, although the form of the field equations remain similar. For homogeneous and isotropic model of the universe the field equations are solved and corresponding cosmological aspects has been discussed. Finally, we have studied the energy conditions in this modified gravity theory both generally and a particular case of perfect fluid with constant equation of state.

A model of an emergent universe is formulated using the mechanism of particle creation. Here the universe is considered as a non-equilibrium thermodynamical system with dissipation due to particle creation mechanism. The universe is chosen as spatially flat FRW space-time and the cosmic substratum is chosen as perfect fluid with barotropic equation of state.Both first and second order deviations from equilibrium prescription is considered and it is found that the scenario of emergent universe is possible in both the cases.

The paper deals with the mechanism of particle creation in the framework of irreversible thermodynamics. The second order non-equilibrium thermodynamical prescription of Israel and Stewart has been presented with particle creation rate, treated as the dissipative effect.In the background of a flat FRW model, we assume the non-equilibrium thermodynamical process to be isentropic so that the entropy per particle does not change and consequently the dissipative pressure can be expressed linearly in terms of the particle creation rate. Here the dissipative pressure behaves as a dynamical variable having a non-linear inhomogeneous evolution equation and the entropy flow vector satisfies the second law of thermodynamics.Further, using the Friedmann equations and by proper choice of the particle creation rate as a function of the Hubble parameter, it is possible to show (separately) a transition from the inflationary phase to the radiation era and also from matter dominated era to late time acceleration. Also, in analogy to analytic continuation, it is possible to show a continuous cosmic evolution from inflation to late time acceleration by adjusting the parameters. It is found that in the de Sitter phase, the comoving entropy increases exponentially with time, keeping entropy per particle unchanged. Subsequently, the above cosmological scenarios has been described from field theoretic point of view by introducing a scalar field having self interacting potential. Finally, we make an attempt to show the cosmological phenomenon of particle creation as Hawking radiation, particularly during the inflationary era.

A model of an emergent universe is obtained in brane world. Here the bulk energy is in the form of cosmological constant, while the brane consists of a fluid satisfying an equation of state of the form p b = 1 3 ρ b , which is effectively a radiation equation of state at high energies. It is shown that with the positive bulk cosmological constant, one of our models represents an emergent universe.The search for singularity free inflationary models in the context of Classical General Relativity has recently led to the development of emergent universes.Harrison [1] obtained a model of the closed universe containing radiation, which approaches the state of an Einstein static model asymptotically, i.e. as t → −∞. This kind of model has so far been discovered subsequently by several workers in the recent past such as that of Ellis and Maartens [2], Ellis et al. [3]. They obtained closed universes with a minimally coupled scalar field φ with a special form for self interacting potential and possibly some ordinary matter with equation of state p = ωρ where − 1 3 ≤ ω ≤ 1. However, exact analytic solutions were not presented in these models, although their behaviour alike that of an emergent universe was highlighted. An emergent universe is a model universe in which there is no timelike singularity, is ever existing and having almost static behaviour in the infinite past (t → −∞) as is mentioned earlier. The model eventually evolves into an inflationary stage. In fact, the emergent universe scenario can be said to be a modern version and extension of the original Lemaitre-Eddington universe. Mukherjee et al. [4] obtained solutions for Starobinsky model for flat FRW space time and studied the features of

The paper deals with a theoretical model for interacting dark energy. The interaction between the cold dark matter (dust) and the dark energy has been assumed to be non-gravitational in nature. Exact analytic cosmological solutions are obtained both for constant and variable equation of state for dark energy. It is found that, for very small value of the coupling parameter (in the interaction term), the model asymptotically extends up to ΛCDM, while the model can enter into the phantom domain asymptotically, if the coupling parameter is not so small. Both the solutions are then analyzed with 194 Supernovae Type Ia data. The best fit parameters are shown with 1σ and 2σ confidence intervals. Finally, we have discussed the cosmographic parameters for both the cases.PACS numbers: 95.35.+d, 95.36.+x, 98.80.Es.

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