scholarly journals CROSSING OF THE w = -1 BARRIER IN VISCOUS MODIFIED GRAVITY

2006 ◽  
Vol 15 (05) ◽  
pp. 767-775 ◽  
Author(s):  
IVER BREVIK
Keyword(s):  
Scalar Curvature ◽  
Bulk Viscosity ◽  
Modified Gravity ◽  
Barrier Crossing ◽  
Flat Universe ◽  
Scalar Expansion ◽  
Big Rip

We consider a modified form of gravity in which the action contains a power α of the scalar curvature. It is shown how the presence of a bulk viscosity in a spatially flat universe may drive the cosmic fluid into the phantom region (w < -1) and thus into a Big Rip singularity, even if it lies in the quintessence region (w > -1) in the nonviscous case. The condition for this to occur is that the bulk viscosity contains the power (2α-1) of the scalar expansion. Two specific examples are discussed in detail. The present paper is a generalization of the recent investigation dealing with barrier crossing in Einstein's gravity: I. Brevik and O. Gorbunova, Gen. Relativ. Gravit.37, 2039 (2005).

scholarly journals VISCOUS FRW COSMOLOGY IN MODIFIED GRAVITY

2005 ◽  
Vol 14 (11) ◽  
pp. 1899-1906 ◽  
Author(s):  
I. BREVIK ◽  
O. GORBUNOVA ◽  
Y. A. SHAIDO
Keyword(s):  
Equation Of State ◽  
Bulk Viscosity ◽  
Time Variation ◽  
Modified Gravity ◽  
Equations Of Motion ◽  
Frw Cosmology ◽  
Conservation Of Energy ◽  
Scalar Expansion ◽  
Big Rip ◽  
Energy And Momentum

We discuss a modified form of gravity implying that the action contains a power α of the scalar curvature. Coupling with the cosmic fluid is assumed. As equation of state for the fluid, we take the simplest version where the pressure is proportional to the density. Based upon a natural ansatz for the time variation of the scale factor, we show that the equations of motion are satisfied for a general α. Also the condition of conservation of energy and momentum is satisfied. Moreover, we investigate the case where the fluid is allowed to possess a bulk viscosity, and find the noteworthy fact that consistency of the formalism requires the bulk viscosity to be proportional to the power (2α-1) of the scalar expansion. In Einstein's gravity, where α = 1, this means that the bulk viscosity is proportional to the scalar expansion. This mathematical result is of physical interest; as discussed recently by the authors, there exists in principle a viscosity-driven transition of the fluid from the quintessence region into the phantom region, implying a future Big Rip singularity.

Interacting Holographic Modified Gravity in a Non-flat Universe

2011 ◽  
Vol 50 (10) ◽  
pp. 3275-3283 ◽  
Author(s):  
A. Banijamali ◽  
M. R. Setare ◽  
B. Fazlpour
Keyword(s):  
Modified Gravity ◽  
Flat Universe

scholarly journals Phantom scalar dark energy as modified gravity: Understanding the origin of the Big Rip singularity

2007 ◽  
Vol 646 (2-3) ◽  
pp. 105-111 ◽  
Author(s):  
F. Briscese ◽  
E. Elizalde ◽  
S. Nojiri ◽  
S.D. Odintsov
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Big Rip ◽  
Phantom Scalar

A study on the bouncing behavior of modified Chaplygin gas in presence of bulk viscosity and its consequences in the modified gravity framework

2017 ◽  
Vol 14 (12) ◽  
pp. 1750181 ◽  
Author(s):  
Surajit Chattopadhyay
Keyword(s):  
Bulk Viscosity ◽  
Modified Gravity ◽  
State Parameter ◽  
Chaplygin Gas ◽  
Scale Factor ◽  
Accelerated Expansion ◽  
Model Parameters ◽  
Modified Chaplygin Gas ◽  
Frw Universe

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 [Formula: see text] 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 [Formula: see text] gravitational paradigm has also been considered, where the MCG density has been reconstructed in presence of bulk viscosity. Role of [Formula: see text] of the bouncing scale factor, describing how fast the bounce takes place, has also been studied in this framework.

scholarly journals KERR–NEWMAN BLACK HOLES IN f(R) THEORIES

2013 ◽  
Vol 11 (01) ◽  
pp. 1450001 ◽  
Author(s):  
J. A. R. CEMBRANOS ◽  
A. DE LA CRUZ-DOMBRIZ ◽  
P. JIMENO ROMERO
Keyword(s):  
Black Holes ◽  
Scalar Curvature ◽  
Modified Gravity ◽  
Constant Scalar Curvature ◽  
Field Equations ◽  
Metric Tensor ◽  
Modified Gravity Theories ◽  
Astrophysical Objects ◽  
Newman Black Hole ◽  
The Rich

In the context of f(R) modified gravity theories, we study the Kerr–Newman black hole solutions. We study nonzero constant scalar curvature solutions and discuss the metric tensor that satisfies the modified field equations. We conclude that, in the absence of a cosmological constant, the black holes (BHs) existence is determined by the sign of a parameter h dependent of the mass, the charge, the spin and the scalar curvature. Different values of this parameter lead to diverse astrophysical objects, such as extremal and marginal extremal BHs. Thermodynamics of BHs are then studied, as well as their local and global stability. We analyze these features in a large variety of f(R) models. We remark the main differences with respect to general relativity and show the rich thermodynamical phenomenology that characterizes this framework.

Possible fates for the accelerating Universe

2006 ◽  
Vol 84 (6-7) ◽  
pp. 583-589
Author(s):  
V Faraoni
Keyword(s):  
Stability Analysis ◽  
Modified Gravity ◽  
Accelerating Universe ◽  
Late Time ◽  
De Sitter ◽  
Recent Proposal ◽  
Time Dynamics ◽  
Big Rip ◽  
Exponential Expansion ◽  
General Potential

The accelerating Universe may end in eternal de Sitter expansion, in a Big Rip, or in super-exponential expansion. We discuss a gauge-independent stability analysis of de Sitter space in scalar–tensor and in modified gravity, the late-time dynamics of a phantom Universe with general potential, and the recent proposal of evading the Big Rip through wormhole tunneling.PACS Nos.: 98.80.–k, 04.50.+h, 04.20.–q

scholarly journals Gravitational waves in the presence of viscosity

2019 ◽  
Vol 28 (10) ◽  
pp. 1950133 ◽  
Author(s):  
Iver Brevik ◽  
Shin’ichi Nojiri
Keyword(s):  
Experimental Evidence ◽  
Gravitational Waves ◽  
Early Universe ◽  
Bulk Viscosity ◽  
Shear Viscosity ◽  
Hubble Parameter ◽  
Decay Constant ◽  
Fluid Density ◽  
Big Rip ◽  
Good Agreement

We analyze gravitational waves propagating in an isotropic cosmic fluid endowed with a bulk viscosity [Formula: see text] and a shear viscosity [Formula: see text], assuming these coefficients to vary with fluid density [Formula: see text] as [Formula: see text], with [Formula: see text] favored by experimental evidence. We give the general governing equation for the gravitational waves and focus thereafter on two examples. The first concerns waves in the very late universe, close to the Big Rip, where the fate of the cosmic fluid is dependent highly on the values of the parameters. Our second example considers the very early universe, the lepton era, the motivation for this choice being that the microscopical bulk viscosity as calculated from statistical mechanics is then at maximum. We find that the gravitational waves on such an underlying medium are damped, having a decay constant equal to the inverse of the conformal Hubble parameter. Our results turn out to be in good agreement with other viscosity-based approaches.

Observational constraints of emergent universe in f(R,T) gravity with bulk viscosity

2020 ◽  
Vol 17 (07) ◽  
pp. 2050102
Author(s):  
Partha Sarathi Debnath ◽  
Bikash Chandra Paul
Keyword(s):  
Bulk Viscosity ◽  
Modified Gravity ◽  
Equilibrium Points ◽  
Momentum Tensor ◽  
General Function ◽  
Emergent Universe ◽  
Data Set ◽  
Cosmological Solutions ◽  
Coupling Parameters ◽  
The Eu

Emergent universe (EU) cosmological models with viscosity in a modified gravity which contains a general function [Formula: see text], where [Formula: see text] and [Formula: see text] denote the curvature scalar and the trace of the energy–momentum tensor, respectively, are studied in a flat Friedmann–Robertson–Walker metric. Cosmological solutions are obtained in [Formula: see text] theory of gravity, which represented as [Formula: see text] with bulk viscosity that described by Eckart theory, truncated Israel Stewart theory and full Israel Stewart Theory. The physical and geometrical features of the EU models in [Formula: see text] gravity, where [Formula: see text] and [Formula: see text] are coupling parameters, with bulk viscosity are studied in details. Constraints of the EU models parameters in [Formula: see text] gravity with bulk viscosity are estimated from observational data set. The stability analysis of the equilibrium points admitting cosmological solutions of the dynamical system associated with the evolution in the modified gravity is studied in Eckart theory, truncated Israel Stewart theory and full Israel Stewart theory.

A study of the bulk viscous pressure in scalar fields and holographic Ricci dark energy considered in the modified gravity framework

2020 ◽  
Vol 98 (7) ◽  
pp. 664-674 ◽  
Author(s):  
Sthiti Chakrabarti ◽  
Surajit Chattopadhyay ◽  
Irina Radinschi
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Bulk Viscosity ◽  
Modified Gravity ◽  
Scalar Fields ◽  
Effective Equation ◽  
Ricci Dark Energy ◽  
Scalar Field Models ◽  
Viscous Pressure ◽  
Bulk Viscous Pressure

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.

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