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The current study examines the realisation of cosmic bounce in two situations involving two distinct scale factor selections, one of which is a scale factor already developed for bouncing and the other of which is a scale factor created by truncating a series expansion of a de Sitter scale factor. Generalized Chaplygin gas (GCG) is assumed to be the background fluid in both situations. When the scale factor is set to the first kind, the pre-bounce scenario's GCG energy density decreases due to contraction, reaches its lowest point at t = 0 during the bounce, and then rises as a result of expansion following the bounce. However, it is noted that the truncation has an impact on the density evolution from pre-bounce in the other scale factor scenario. The influence of bulk viscosity is shown in all circumstances, in addition to the influence of non-viscosity, and the test for stability makes use of the squared speed of sound. At the turn-around places, the null energy criterion is also violated. The final stage of the study includes a cosmographic analysis and a demonstration of the Hubble flow dynamics. In conclusion, we find that inflationary cosmology can also be realized with GCG as the background fluid for two-scale factor options. When the equivalent cosmic parameter is examined for pre-bounce and post-bounce scenarios, a symmetry is frequently seen. The symmetry occurs near the point of bouncing or turning. [ABSTRACT FROM AUTHOR]

In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified f (T) gravity framework under the purview of viscous cosmological settings. We also took non-viscous cases into account. We studied the equation of state (EoS) parameter under various circumstances and judged the stability of the models through the sign of the squared speed of sound. We observed the mgCG behaving like avoidance of Big Rip in the presence of bulk viscosity at the turnaround point and in non-viscous cases, a phantom-like behavior appears. The turnaround point equation of state parameter crosses the phantom boundary, violating NEC. The role of the mgCG's model parameters was also investigated before and after the bounce. A Hubble flow dynamics was carried out and, it was revealed that mgCG is capable of realizing an inflationary phase as well as an exit from inflation. An f (T) gravitational paradigm was also considered, where the mgCG density was reconstructed in the presence of bulk viscosity. The role of the parameters associated with the bouncing scale factor, describing how fast the bounce takes place, was also studied in this framework. Finally, the reconstructed mgCG turned out to be stable against small perturbations irrespective of the presence of bulk viscosity and modified gravity scenario. Finally, the reconstruction scheme was assessed using statistical analysis, Shannon entropy. [ABSTRACT FROM AUTHOR]

In this work, we have studied various cosmological parameters in the presence of viscous new Tsallis holographic dark energies for interacting scenarios in the framework of Chern–Simons modified gravity. The bulk viscosity has been considered with the bulk viscous pressure chosen in the form Π = − 3 H ξ 0 + ξ 1 H + ξ 2 (Ḣ + H 2) . Hubble parameter H has been obtained from the above choice of scale factor and in this viscous scenario, the effective pressure has been obtained in Chern–Simons framework whose field equation, cosmological consequences have been investigated. It has been observed that for the interaction scenario in the presence of bulk viscosity the EoS parameter is staying above − 1 , which indicates quintessence behavior. Hence, for the universe filled with a bulk viscous fluid can have the possibility of avoidance of big-rip, although the earlier transition from quintessence to phantom is not avoidable. [ABSTRACT FROM AUTHOR]

In the framework of Saez–Ballester (SB) theory, the current study proposes a reconstruction scheme for Tsallis and Kaniadakis holographic dark energy, particular cases of a more generalised Nojiri–Odintsov holographic dark energy (Nojiri and Odintsov, 2006; The European Physical Journal C , 2017, 77 , pp.1-8.), with Hubble horizon and future event horizons serving as the universe's respective trapping horizons. The Hubble parameter was rebuilt by placing a constraint on the metric potential inside the conservation equation. As a result, the holographic dark energy of Tsallis and Kaniadakis are recreated, and the cosmology and thermodynamics are examined. Lastly, an interacting van der Waals (VDW) coupled fluid model was examined in the context of bulk viscosity, and the behaviour of the equation of state (EoS) parameter was examined. • In the framework of Saez–Ballester (SB) theory the study proposes a reconstruction scheme for Tsallis and Kaniadakis HDE. • H is reconstructed by constraining the metric potential inside the conservation equation. • Interacting van der Waals (VDW) coupled fluid model is considered in bulk viscous framework. [ABSTRACT FROM AUTHOR]

The present study reports a reconstruction scheme for f(T) gravity under the assumption that the density due to the torsion T evolves like the variable‐generalized Chaplygin gas (VGCG), a phenomenological candidate of dark energy. This reconstruction has been demonstrated in viscous as well as non‐viscous cases. The state‐finder trajectory {r − s} has been found to attain the ΛCDM fixed point under this reconstruction scenario. The reconstructed equation of state parameter (EoS) has been studied for their phantom/quintom behavior. The effect of bulk viscous pressure has been found to become prominent in the late stage. The generalized second law (GSL) of thermodynamics has been studied for Bekeinstein ‐ Hawking entropy area relation in both non‐viscous and viscous cases. The GSL is found to be valid in both cases. [ABSTRACT FROM AUTHOR]

The work presented in this paper reports a rigorous study of the reconstruction of the modified gravity in the framework of the scalar field models of dark energy and holographic Ricci dark energy, a generalized version of the holographic dark energy presented in S. Nojiri and S.D. Odintsov. Gen. Relativ. Gravitation, 38, 1285 (2006). The tachyon and quintessence scalar fields have been considered and the cosmology associated with the presence of bulk viscosity has been studied. In the first part of our study, we have demonstrated the behaviour of the bulk viscosity coefficient in the framework of the reconstructed tachyon scalar field model of dark energy. The scale factor is chosen in the form a(t) = a0tβ, where β > 0. Two scalar field models, namely, tachyon and quintessence, have been considered in the framework of the modified field equations through incorporation of the bulk viscous pressure. The reconstructed density and pressure of the scalar field models have been explored for the cosmological consequences in the presence of bulk viscosity. The behaviour of the effective equation of state parameters has been investigated. Finally, we have reconstructed f(T) gravity in the presence of holographic Ricci dark energy and a transition of the effective equation of state parameter from quintessence to phantom has been observed. [ABSTRACT FROM AUTHOR]

Motivated by the work of Nojiri et al. [S. Nojiri, S. D. Odintsov and E. N. Saridakis, Holographic inflation, Phys. Lett. B 797 (2019) 134829], this study reports a model of inflation under the consideration that the inflationary regime is originated by a type of holographic energy density. The infrared cutoff has been selected based on the modified holographic model that is a particular case of Nojiri–Odintsov holographic dark energy [S. Nojiri and S. D. Odintsov, Unifying phantom inflation with late-time acceleration: Scalar phantom–non-phantom transition model and generalized holographic dark energy, Gen. Relativ. Gravit. 38 (2006) 1285] that unifies phantom inflation with the acceleration of the universe on late time. On getting an analytical solution for Hubble parameter we considered the presence of bulk viscosity and the effective equation of state parameter appeared to be consistent with inflationary scenario with some constraints. It has also being observed that in the inflationary scenario the contribution of bulk viscosity is not of much significance and its influence is increasing with the evolution of the universe. Inflationary observables have been computed for the model and the slow-roll parameters have been computed. Finally, it has been observed that the trajectories in n s − r are compatible with the observational bound found by Planck. It has been concluded that the tensor to scalar ratio for this model can explain the primordial fluctuation in the early universe as well. [ABSTRACT FROM AUTHOR]

This study reports a reconstruction scheme of a Dark Energy (DE) model with higher-order derivative of Hubble parameter, which is a particular case of Nojiri–Odintsov holographic DE (HDE) [S. Nojiri and S. D. Odintsov, Gen. Relativ. Gravit. 38 (2006) 1285.] that unifies phantom inflation with the acceleration of the universe on late-time. The reconstruction has been carried out in the presence of bulk viscosity, where the bulk-viscous pressure has been taken as a function of Hubble parameter. Ranges of cosmic time t have been derived for quintessence, cosmological constant and phantom behaviour of the equation-of-state (EoS) parameter. In the viscous scenario, the reconstruction has been carried out in an interacting and noninteracting situations and in both the cases stability against small perturbations has been observed. Finally, the slow roll parameters have been studied and a scope of exit from inflation has been observed. Also, the availability of quasi-exponential expansion has been demonstrated for interacting viscous scenario and a study through tensor-to-scalar ratio has ensured consistency of the model with the observational bound by Planck. Alongwith primordial fluctuations, the interacting scenario has been found to generate strong dissipative regime. [ABSTRACT FROM AUTHOR]

Work reported in this study demonstrates the reconstruction schemes for the f (G) gravity in the framework of bulk viscosity and holographic background evolution by considering the universe filled by the viscous fluid that is just special class of more general fluids as described in Nojiri and Odintsov [Inhomogeneous equation of state of the universe: Phantom era, future singularity, and crossing the phantom barrier, Phys. Rev. D72 (2005) 023003]. The bulk viscous pressure has been considered as Π = − 3 H ξ , with ξ = ξ 0 + ξ 1 H + ξ 2 (Ḣ + H 2). Considering the scale factor in power law form and taking holographic dark energy (HDE) with density ρ Λ = 3 c 3 M p 2 L − 2 and generalized extended holographic dark energy (EGHRDE) with density ρ Λ = 3 c 2 (α Ḣ + β H 2) , a specific case of Nojiri–Odintsov holographic DE ([Unifying phantom inflation with late-time acceleration: Scalar phantom–non-phantom transition model and generalized holographic dark energy, Gen. Relativ. Gravit.38 (2006) 1285]) we have derived solutions for f (G) and the subsequent effective equation of state parameters have been found to behave like quintom irrespective of the choice of c 2 . Finally, considering f (G) as quintessence scalar field we have explored the possibility of quasi-exponential expansion and warm inflation. [ABSTRACT FROM AUTHOR]

The research presented in the current article explores the potential for inflation realization in a cosmic environment with bulk viscosity using a tachyon scalar field. The present study rewrites the entropy conservation equation to take bulk viscosity into account. The work's initial phase examined the behaviour of the equation of state parameter for logamediate, intermediate, and emergent scenarios within a framework of viscous cosmology having the viscosity coefficient in linear and polynomial forms. The study then focused on the relative dominance of the scalar field and potential for the tachyon scalar field in the presence of bulk viscosity for the different choices of scale factor and came to the conclusion that the prerequisite for the realization of inflation is met in the framework of bulk viscosity. • The study investigates the likelihood of inflation. • A cosmological background with bulk viscosity is taken into account. • The EoS parameter is investigated for various scale factor options. • Entropy conservation is rephrased with tachyon. • The inflation realization for viscous tachyon scalar field is examined. [ABSTRACT FROM AUTHOR]

The present study reports a reconstruction scheme for gravity considering the scale factor in the power law form. The equation of state parameter has been studied for this reconstructed model along with the deceleration parameter and the statefinder pair . The statefinder trajectory has been found to interpolate between dust and CDM phase of the universe. Cosmological evolution of primordial perturbations has been studied through scalar metric fluctuations and finally the reconstructed model has been tested for its consistency with the generic expansion of the universe. [ABSTRACT FROM AUTHOR]

The present paper reports a study on the bouncing behavior of the viscous modified Chaplygin gas (MCG) in Einstein as well as modified gravity framework. For a bouncing scale factor proposed by Cai et al., Class. Quantum Grav. 28 (2011) 215011, we have studied the cosmology of MCG in presence of bulk viscosity. In Einstein gravity framework, we have studied the equation of state parameter and it has been found to cross indicating the end of the early accelerated expansion and it has also been observed that for flat FRW universe the presence of bulk viscosity induces the crossing of phantom boundary. Role of the model parameters of the MCG has also been investigated before and after the bounce. A Hubble flow dynamics has been carried out and, it was revealed that MCG is capable of realizing inflationary phase as well as an exit from inflation. A gravitational paradigm has also been considered, where the MCG density has been reconstructed in presence of bulk viscosity. Role of of the bouncing scale factor, describing how fast the bounce takes place, has also been studied in this framework. [ABSTRACT FROM AUTHOR]

The present paper reports a study on modified Chaplygin gas (MCG)-based reconstruction scheme for extended holographic Ricci dark energy (EHRDE) in the presence of viscous type dissipative term. The dissipative effect has been described by using Eckart approach. Under the assumption that the universe is filled with MCG-EHRDE under the influence of bulk viscosity we have studied the cosmological dynamics, where the bulk viscosity coefficient has been chosen in a particular time varying form , where and are constant coefficients and is the Hubble parameter. Furthermore, we have reconstructed the potential and dynamics of viscous MCG-EHRDE as scalar field. Thereafter we have studied the statefinder trajectories to discern its departure from cold dark matter (CDM) and finally investigated validity of the generalized second law (GSL) of thermodynamics considering event horizon as the enveloping horizon of the universe. [ABSTRACT FROM AUTHOR]

In this paper, we report a study on the viscous extended holographic Ricci dark energy (EHRDE) model under the assumption of existence of bulk viscosity in the linear barotropic fluid and the EHRDE in the framework of standard Eckart theory of relativistic irreversible thermodynamics and it has been observed that the non-equilibrium bulk viscous pressure is significantly smaller than the local equilibrium pressure. We have studied the equation of state (EoS) parameter and observed that the EoS behaves like 'quintom' and is consistent with the constraints set by observational data sets from SNLS3, BAO and Planck + WMAP9 + WiggleZ measurements in [S. Kumar and L. Xu, Phys. Lett. B 737, 244 (2014)]. Analysis of statefinder parameters has shown the possibility of attainment of Lambda cold dark matter (CDM) phase under current model and at the same time it has been pointed out that the redshift z = 0, i.e. the current universe, the statefinder pair is different from that of CDM and the CDM can be attained in a later stage of the universe. An analysis of stability has shown that although the viscous EHRDE along with viscous barotropic is classically unstable in the present epoch, it can lead to a stable universe in very late stage. Considering an universe enveloped by event horizon, we have observed validity of generalized second law (GSL) of thermodynamics. [ABSTRACT FROM AUTHOR]

Motivated by the work of Saridakis (Phys. Rev. D102, 123525 (2020)), the present study reports the cosmological consequences of Barrow holographic dark energy (HDE) and its thermodynamics. The literature demonstrates that dark energy (DE) may result from electroweak symmetry breaking that triggers a phase transition from early inflation to late-time acceleration. In the present study, we incorporated viscosity in the Barrow HDE. A reconstruction scheme is presented for the parameters associated with Barrow holographic dark energy under the purview of viscous cosmology. The equation of state (EoS) parameter is reconstructed in this scenario and quintessence behaviour is observed. Considering Barrow HDE as a specific case of Nojiri–Odintsov (NO) HDE, we have observed quintom behaviour of the EoS parameter and for some values of n the EoS has been observed to be very close to − 1 for the current universe. The generalised second law of thermodynamics has come out to be valid in all the scenarios under consideration. Physical viability of considering Barrow HDE as a specific case of NO HDE is demonstrated in this study. Finally, it has been observed that the model under consideration is very close to Λ CDM and cannot go beyond it. [ABSTRACT FROM AUTHOR]

• A reconstruction of f (R) gravity based on holographic Ricci dark energy is reported in presence of bulk viscosity. • Reconstructed EoS parameter w R shows quintom behaviour in non-viscous and quintessence in viscous case. • Generalized second law is found to be valid under this reconstruction based on the choice of a model parameter. • Stability of the reconstructed f (R) model is tested using squared speed of sound. The present paper reports a study of the reconstruction of f (R) gravity from holographic Ricci dark energy, a specific case of Nojiri–Odintsov holographic DE (ref Nojiri and Odinstov, 2005a), in the presence of bulk viscosity. The reconstructed equation of state (EoS) parameter w R was found to have a transition from quintessence to phantom in the absence of the bulk viscosity. Thus, the behaviour of w R for this reconstruction scheme was identified as "quintom" in the absence of the bulk viscosity. However, in the case of the bulk viscous scenario w R behaves like "quintessence". Furthermore, the generalized second law of thermodynamics was tested in the case of the reconstructed f (R) gravity. Finally, the stability of this model against small perturbations was demonstrated with the aid of squared speed of sound. [ABSTRACT FROM AUTHOR]

From the hydrodynamic viewpoint, the inclusion of viscosity in the theory of cosmic fluid may appear as an abstraction. Using modern observational results for the Hubble parameter and standard Friedmann formalism, we may have a description of the universe up to the inflationary era, or we may be able to analyse the probable ultimate fate of the universe. Inspired by this bulk viscosity formalism reviewed in Brevik et al., (2017) in this work, we report a study of the DBI essence scalar field model of dark energy (DE) interacting with pressureless dark matter (DM) in the presence of bulk viscous pressure of the form ξ = ξ 0 + ξ 1 H + ξ 2 (H ̇ + H 2). We choose the scale factor in three forms: emergent, logamediate, and intermediate. We choose the interaction term Q in the form Q = 3 H δ ρ Λ , where ρ Λ denotes the density of the scalar field model of DE. We present a detailed cosmology of the interacting DE and observe a quintom behaviour of the equation of state (EoS). We conduct a statefinder analysis to attain Λ CDM fixed point by the interacting DE. Subsequently, we report a reconstruction scheme for f (T) gravity. Finally, we conduct thermodynamic analysis where we consider entropy's logarithmic and power-law corrections. • DBI essence scalar field model interacting with pressureless dark matter is considered in viscous scenario. • A detailed cosmology of the interacting DE with quintom behaviour of the equation of state (EoS) choosing different scale factors. • A statefinder analysis is carried out to check attainment of Λ CDM fixed point. • A reconstruction scheme for f (T) gravity is presented and its cosmology is demonstrated. • Thermodynamic analysis is carried out with logarithmic and power-law corrections. [ABSTRACT FROM AUTHOR]