Narrow Results

We show that in the isentropic scenario, the first-order thermodynamical particle creation model gives an emergent universe solution even when the chemical potential is nonzero. However, there exists no emergent universe scenario in the second-order non-equilibrium theory for the particle creation model. We then point out a correspondence between the particle creation model with barotropic equation of state and the equation of state giving rise to an emergent universe without particle creation in spatially flat FRW cosmology.

We study the holographic dark energy (HDE) with adiabatic matter creation process to explain the present-day accelerated expansion of the Universe. The field equations are discussed and solved analytically for various cosmological parameters such as the Hubble parameter, deceleration parameter and effective equation of state parameter in terms of redshift by considering a more general form of the matter creation rate. The viability of such model is checked using the recent data of Type Ia supernovae Pantheon sample, Hubble function $H(z)$ data, joined data of baryon acoustic oscillations/cosmic microwave background and latest local $H_0$ by SH0ES. We achieve the most fitting values of model parameters by performing the statistical analysis of our model with the three different combination of data sets. The HDE model, with IR cutoff as Hubble horizon, in the absence of matter creation does not show phase transition. While the induction of matter creation to the same model shows the recent transition from a decelerating to an accelerated phase. Our outcomes show that the model with matter creation is compatible with the data sets used. We find that the HDE model with matter creation provides the Hubble function which matches sufficiently well with the observed Hubble data. Finally, we perform geometrical and cosmographic analyses to explicate our model and compare it with the other dark energy models like $\mathrm{\Lambda }$CDM model. It is found that the evolution of model starts from Chaplygin gas region in early times, enters into quintessence region in medieval time and finally approaches to $\mathrm{\Lambda }$CDM model.

The effects of the particles creation processes on the evolution of a spatially flat Friedmann–Lemaitre–Robertson–Walker (FLRW) universe are investigated. Our approach is involved in the context of open thermodynamics systems choosing a natural phenomenological ansatz for the particles creation rate. There are reasons to believe that this model gives a simple and appropriate description of the gravitational stability concerning the isotropic model for the Early Universe considered both on a small and large scale. The prelude of our study is that the consideration of the particles production processes are supposed to play an relevant role in the perturbation theory arising in the Early Universe dynamics. We show how perturbations damp forwards in time or remain constant (as in the Cosmological Standard Model) and, differently from the standard analysis, we find instability backward in time from an initial condition on scalar perturbation.

We treat the adiabatic matter creation process in various three-brane models by applying the thermodynamics of open systems. The matter creation rate is found to affect the evolution of the scale factor and energy density of the universe. We find a modification at the early stages of cosmic dynamics. In GB and RS brane worlds, by choosing appropriate parameters we obtain the standard scenario, while the warped DGP model has different solutions. During later stages, since matter creation is negligible, the evolution reduces to FRW expansion, in the RS and GB models.

In this paper, we review modified theories of gravity with a curvature–matter coupling between an arbitrary function of the scalar curvature and the Lagrangian density of matter. This explicit nonminimal coupling induces a nonvanishing covariant derivative of the energy–momentum tensor, that implies nongeodesic motion and consequently leads to the appearance of an extra force. Here, we explore the physical and cosmological implications of the nonconservation of the energy–momentum tensor by using the formalism of irreversible thermodynamics of open systems in the presence of matter creation/annihilation. The particle creation rates, pressure, and the expression of the comoving entropy are obtained in a covariant formulation and discussed in detail. Applied together with the gravitational field equations, the thermodynamics of open systems lead to a generalization of the standard $\mathrm{\Lambda }$CDM cosmological paradigm, in which the particle creation rates and pressures are effectively considered as components of the cosmological fluid energy–momentum tensor. Furthermore, we also briefly present the coupling of curvature to geometry in conformal quadratic Weyl gravity, by assuming a coupling term of the form $L_m{\tilde{R}}^2$, where $L_m$ is the ordinary matter Lagrangian, and $\tilde{R}$ is the Weyl scalar. The coupling explicitly satisfies the requirement of the conformal invariance of the theory. Expressing ${\tilde{R}}^2$ with the use of an auxiliary scalar field and of the Weyl scalar, the gravitational action can be linearized in the Ricci scalar, leading in the Riemann space to a conformally invariant $f(R,L_m)$-type theory, with the matter Lagrangian nonminimally coupled to geometry.

In this paper, we have investigated a very natural question regarding the dynamics of the universe, namely, the possibility of its decelerating phase immediately after the present accelerating phase. To begin with, we have focused on the matter creation theory which is considered to be a viable alternative to dark energy and modified gravity models. Moreover, we have introduced the cosmographic approach which allows us to express the free parameters of a cosmological model in terms of the known cosmographic parameters. Assuming a generalized matter creation rate, we have discussed the theoretical bounds on the model parameters allowing the future deceleration of the universe. Moreover, using the observational bounds on the cosmographic parameters obtained from the low redshifts observational probes, we have also examined the chance of a decelerating phase of the universe. Finally, considering a variety of known cosmological models and parametrizations, we have tested the same possibility. Our analysis shows that the chance of a future decelerating expansion of the universe is highly dependent on the choice of the cosmological models and parametrizations and also on the observational data. Even though the future decelerating expansion is allowed in some cosmological frameworks, but we do not see any strong evidence in favor of this. Perhaps, the future cosmological surveys could offer some more information regarding the fate of the universe.