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.

In the present work we discuss a third alternative to explain the latest observational data concerning the accelerating Universe and its different stages. The particle creation mechanism in the framework of non-equilibrium thermodynamics is considered as a basic cosmic mechanism acting on the flat FRW geometry. By assuming that the gravitationally induced particle production occurs under "adiabatic" conditions, the deceleration parameter is expressed in terms of the particle creation rate which is chosen as a truncated power series of the Hubble parameter. The model shows the evolution of the Universe starting from inflation to the present late time acceleration and it also predicts future decelerating stage.Comment: 9 pages, 3 figure

The paper deals with universal thermodynamics for FRW model of the universe bounded by apparent (or event) horizon. Assuming Hawking temperature on the horizon, the unified first law is examined on the horizon for different gravity theories. The results show that equilibrium configuration is preserved with a modification to Bekenstein entropy on the horizon.It is well known today that recent observational predictions [1] divide the physicists into two groups. The first group has been trying to explain this late time acceleration within the frame work of standard cosmology, assuming the existence of an exotic matter with negative pressure (called dark energy (DE)). But till now the nature of DE is completely unknown to us and is an unresolved problem in modern theoretical physics (see [2], [3] and references therein). On the otherhand, the second group is of the opinion of a modified gravity theory-a modification of Einstein's general relativity. A common and widely used modified theory is f (R)-gravity theory where the Lagrangian density R (the Ricci scalar) in the Einstein-Hilbert action is replaced by an arbitrary function of R i.e., f (R) ([4] for a review and references therein). Also there are other modified gravity theories namely Scalar Tensor Theory, Brane world scenario, f (G), f (R, G) and f (T ) gravity theories, where T is the usual torsion scalar, G = R µγρσ R µγρσ − 4R µγ R µγ + R 2 is the Gauss Bonnet invariant term and R µγρσ and R µγ are the usual Riemann curvature tensor and Ricci tensor respectively. These modified theories [5-10] are considered as gravitational alternatives for DE and may serve as dark matter [11].Further inspection of a gravity theory from thermodynamical view point is also an interesting issue in modern theoretical physics. The deep connection between gravity and thermodynamics is strongly believed due to Ads/CFT correspondence [12] and black hole thermodynamics [13]. This belief was put one step forward by the seminal works of Jacobson [14] and Padmanabhan [15]. By introducing the local Rindler horizon and assuming the Clausius relation δQ = T dS for all local Rindler causal horizons through each spacetime point, Jacobson deduced the Einstein's field equations from the proportionality of entropy (S) to the horizon area (A). Here δQ stands for the variation of the heat flow and T is the Unruh temperature measured by an accelerated observer just inside the horizon. Although Jacobson derived the equivalence along null directions but it is speculated that the results may also be true along any other direction in the tangent to the space-time. Padmanabhan, in the reverse way, showed that field equations in Einstein gravity as well as in Lanczos-Lovelock gravity for a spherically symmetric spacetime can be expressed in the form of thermodynamic identity: dE = T dS − P dV . In this derivation, the modified terms could emerge in quantum pictures and hence one may think that thermodynamics can profile gravity beyond the classical level.Alternatively, relevant to universal thermodyn...

Recent observational evidences claim an accelerating expansion of the universe at present epoch. It is commonly incorporated in standard cosmology by the introduction of an exotic matter (that violates the strong energy condition) known as dark energy (DE). As event horizon exists for accelerating universe so there has been a lot of work on universal thermodynamics (i.e., thermodynamics of the universe bounded by apparent or event horizon). Recently, thermodynamical equilibrium has been examined for both the horizons. In the present work we show that universal thermodynamics with event horizon is favored by DE from the point of view of equilibrium thermodynamical prescription.

In this article we have used the recently introduced redefined Hawking temperature on the event horizon and investigated whether the generalised second law of thermodynamics (GSLT) and thermodynamic equilibrium holds for both the event and the apparent horizons. Here we have considered FRW universe and examined the GSLT and thermodynamic equilibrium with three examples. Finally, we have concluded that from the thermodynamic viewpoint, the universe bounded by the event horizon is more realistic than that by the apparent horizon at least for some examples.

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The present work deals with a detailed study of universal thermodynamics in different modified gravity theories. The validity of the generalized second law of thermodynamics (GSLT) and thermodynamical equilibrium (TE) of the Universe bounded by a horizon (apparent/event) in f(R)-gravity, Einstein-Gauss-Bonnet gravity, RS-II brane scenario and DGP brane model has been investigated. In the perspective of recent observational evidences, the matter in the Universe is chosen as interacting holographic dark energy model. The entropy on the horizons are evaluated from the validity of the unified first law and as a result there is a correction (in integral form) to the usual Bekenstein entropy. The other thermodynamical parameter namely temperature on the horizon is chosen as the recently introduced corrected Hawking temperature. The above thermodynamical analysis is done for homogeneous and isotropic flat FLRW model of the Universe. The restrictions for the validity of GSLT and the TE are presented in tabular form for each gravity theory. Finally, due to complicated expressions, the validity of GSLT and TE are also examined from graphical representation, using three Planck data sets.

We propose a novel equation of state (EoS) which explains the evolutionary history of a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe. The uniqueness of this EoS lies in the fact that it incorporates the Lambert W function in a special fashion. It is explicitly demonstrated that with observationally relevant values of the unknown parameters ϑ1 and ϑ2, all the evolutionary phases of the Universe can be reproduced. Moreover, it also shows that the initial singularity is unavoidable and asserts that the late-time acceleration of the Universe would continue forever.