STUDY OF VISCOUS AND MATTER CREATION EFFECTS IN COSMOLOGY

Abstract

In the thesis, we have studied the effects of bulk viscosity and matter creation on various cosmological models. In chapter 2, we have explored the possibility of bulk viscosity as a possible candidate of dark energy to explain the accelerating Universe.

We have discussed the dissipative processes in the HRDE model within the frame- work of the standard Eckart theory of relativistic thermodynamics. We have observed

that the accelerated expansion may be possible for a non-viscous case but the phase

transition is not possible. For the viscous HRDE model, We have obtained the ex- ponential expansion of the scale factor which gives the time-dependent deceleration

parameter and statefinder pair. It is observed that the model shows the transition from

the decelerated phase to the accelerated phase depending on the values of the vis- cous term and the results show that the recent acceleration is well explained with the

viscous term. In chapter 3, we extend the study of chapter 2 in the framework of modified f(R,T)

gravity theory. We have obtained the exact solutions of the field equations by as- suming the simplest form of f(R,T) = R+λT with constant and variable bulk viscous

coefficients. It is found that the behaviors concerning the cosmic expansion depend

on the coupling parameter of f(R,T) and bulk viscous term. Using statefinder param- eters and Om diagnostic, it has been found that our model shows a similar behavior as

the quintessence model and Chaplygin gas model for different values of the viscosity

coefficient. We have also analyzed the time evolution of the total entropy and gener- alized second law of thermodynamics of the viscous HRDE model in f(R,T) theory

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inside the apparent horizon. Further, chapter 4 deals with the adiabatic matter creation process in the HDE model with the motivation of considering it as an alternative choice to explain the recent accelerating phase of the Universe. We have considered three different forms of matter creation rate to discuss the evolution of the Universe. We have constrained the model parameters through the MCMC method by the use of the EMCEE python package on the latest observational data to discuss various cosmological parameters. We have used cosmographic parameters and Om to discriminate our model with other dark energy models. We have analyzed the model by applying information criterion AIC and BIC based on the penalization associated with the number of parameters. The generalized second law of thermodynamics is found to be valid for this model under certain conditions. In chapter 5, we have explored the effect of bulk viscosity and matter creation in the

HRDE model to observe the current accelerated phase. In the literature, both phe- nomena have been treated as the same cosmological phenomena and some papers

treat both as different phenomena. We have considered these two dissipative phe- nomenons as independent irreversible processes. With the help of best-fitted model

parameters, we have discussed the evolution of various cosmological parameters and also used geometric parameters to distinguish the model from other standard dark energy models. The behavior of energy conditions has also been discussed for the HRDE model. The result shows that the HRDE model with bulk viscosity and matter creation is in good agreement with current observational data. Chapter 6 discusses the matter-dominated model with matter creation cosmology in the FLRW model as an alternative to explain the cosmic acceleration. We have

proposed a new form of matter creation rate, which generalizes some of the previ- ous models in the literature. We have performed the statistical analysis to obtain the

best-fit values of the model parameter by employing the MCMC package EMCEE on a

different combination of publicly available data sets of SNe, OHD, and BAO/CMB. Ex- act solutions of the scale factor and deceleration parameters have been obtained and

discussed their evolution for the best-fit values of model parameter which shows the phase transition from deceleration to recent acceleration. We have also distinguished

this model from other existing dark energy models using two geometrical diagnos- tics: statefinder parameter and Om diagnostic. We also used information criterion AIC

and BIC to compare our model with the standard ΛCDM model. The thermodynamic

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behavior have been discussed for this model by calculating the total entropy for the matter creation.

Table 7.1: Comparison of different models carried out in the thesis with the ΛCDM model Model ztr q0 ωeff(z = 0) t0 (Gyr) ΛCDM 0.66 −0.60 −0.93 13.7 HDE with Γ = 3δH0 0.61 −0.26 −0.50 13.39 HDE with Γ = 3δH0 +3βH 0.92 −0.34 −0.56 14.28 HRDE with ζ = ζ0 +ζ1H, Γ = 3βH 0.68 −0.36 −0.57 13.40 FLRW model with matter creation 0.86 −0.49 −0.62 13.98