Bulk viscous matter and the cosmic acceleration of the universe in f(Q,T) gravity

2021 ◽  
Author(s):  
Simran Arora ◽  
S.K.J. Pacif ◽  
Abhishek Parida ◽  
P.K. Sahoo
Keyword(s):  
Cosmic Acceleration ◽  
Bulk Viscous ◽  
Acceleration Of The Universe ◽  
The Universe

Cosmic acceleration with tachyon field

2018 ◽  
Vol 33 (34) ◽  
pp. 1850199 ◽  
Author(s):  
A. I. Keskin
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
De Sitter ◽  
Tachyon Field ◽  
Minimal Coupling ◽  
Acceleration Of The Universe ◽  
The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

scholarly journals Accelerating cosmology in modified gravity: From convenient F(R) or string-inspired theory to bimetric F(R) gravity

2014 ◽  
Vol 11 (02) ◽  
pp. 1460006 ◽  
Author(s):  
Shin'ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Early Time ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Reconstruction Method ◽  
Free Theory ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Unified Description

We consider modified gravity which may describe the early-time inflation and/or late-time cosmic acceleration of the universe. In particular, we discuss the properties of F(R), F(G), string-inspired and scalar-Einstein–Gauss–Bonnet gravities, including their FRW equations and fluid or scalar-tensor description. Simplest accelerating cosmologies are investigated and possibility of unified description of the inflation with dark energy is described. The cosmological reconstruction program which permits to get the requested universe evolution from modified gravity is developed. As some extension, massive F(R) bigravity which is ghost-free theory is presented. Its scalar-tensor form turns out to be the easiest formulation. The cosmological reconstruction method for such bigravity is presented. The unified description of inflation with dark energy in F(R) bigravity turns out to be possible.

scholarly journals THE MINIMAL CURVATURE OF THE UNIVERSE IN MODIFIED GRAVITY AND CONFORMAL ANOMALY RESOLUTION OF THE INSTABILITIES

2004 ◽  
Vol 19 (08) ◽  
pp. 627-638 ◽  
Author(s):  
SHIN'ICHI NOJIRI ◽  
SERGEI D. ODINTSOV
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Cosmic Acceleration ◽  
Conformal Anomaly ◽  
Frw Universe ◽  
Conformal Fields ◽  
Acceleration Of The Universe ◽  
Minimal Curvature ◽  
The Universe ◽  
Universe Evolution

We discuss the modified gravity which may produce the current cosmic acceleration of the universe and eliminate the need for dark energy. It is shown that such models where the action quickly grows with the decrease of the curvature define the FRW universe with the minimal curvature. Infinite time is required to reach the minimal curvature during the universe evolution. It is demonstrated that quantum effects of conformal fields may strongly suppress the instabilities discovered in modified gravity. We also briefly speculate on the modification of gravity combined with the presence of the cosmological constant dark energy.

scholarly journals Bulk viscous quintessential inflation

2018 ◽  
Vol 27 (05) ◽  
pp. 1850052 ◽  
Author(s):  
Jaume Haro ◽  
Supriya Pan
Keyword(s):  
General Relativity ◽  
Particle Production ◽  
Cosmic Acceleration ◽  
Inflationary Universe ◽  
De Sitter ◽  
Thermodynamical Equilibrium ◽  
One Dimensional ◽  
Inflationary Phase ◽  
Bulk Viscous ◽  
The Universe

In a spatially-flat Friedmann–Lemaître–Robertson–Walker universe, the incorporation of bulk viscous process in general relativity leads to an appearance of a nonsingular background of the universe that both at early and late times depicts an accelerated universe. These early and late scenarios of the universe can be analytically calculated and mimicked, in the context of general relativity, by a single scalar field whose potential could also be obtained analytically where the early inflationary phase is described by a one-dimensional Higgs potential and the current acceleration is realized by an exponential potential. We show that the early inflationary universe leads to a power spectrum of the cosmological perturbations which match with current observational data, and after leaving the inflationary phase, the universe suffers a phase transition needed to explain the reheating of the universe via gravitational particle production. Furthermore, we find that at late times, the universe enters into the de Sitter phase that can explain the current cosmic acceleration. Finally, we also find that such bulk viscous-dominated universe attains the thermodynamical equilibrium, but in an asymptotic manner.

MODIFIED BRANEWORLD COSMOLOGIES IN THE PRESENCE OF STRINGY CORRECTIONS COUPLED TO A CANONICAL SCALAR FIELD

2009 ◽  
Vol 18 (05) ◽  
pp. 691-715 ◽  
Author(s):  
R. A. EL-NABULSI
Keyword(s):  
Scalar Field ◽  
Dimensional Space ◽  
High Energy ◽  
Cosmic Acceleration ◽  
Dimensional Model ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Matter Fields ◽  
Kaluza Klein ◽  
Canonical Scalar Field

The purpose of this paper is to study braneworld cosmologies in the presence of stringy corrections coupled to a canonical scalar field. Two independent models are explored which in their own right provide cosmologies exhibiting the present cosmic acceleration of the universe. The evolution of the cosmological scale factor is studied in four and ten-dimensional space–times. The four-dimensional model is preformed in the context of the nonminimal Maxwell–Gauss–Bonnet gravity while the ten-dimensional model is constructed from the implications of Kaluza–Klein cosmology together with the Gauss–Bonnet Lagrangian in the action with matter fields nonminimally coupled to gravity. Both the low energy and high energy limits are discussed for the two models and many interesting features are described in some detail.

scholarly journals An Analysis for Conservative and Non-conservative f(R, T) Gravity Models

2021 ◽  
pp. 1-11
Author(s):  
Diyadin Can ◽  
Ertan Güdekli
Keyword(s):  
Energy Condition ◽  
Hubble Constant ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Gravity Models ◽  
Energy Conditions ◽  
Theory Of Relativity ◽  
General Theory Of Relativity ◽  
Acceleration Of The Universe ◽  
The Universe

As it is known that General Theory of Relativity does not explain the current acceleration of the universe, so there are many attempts to generalize this theory in order to explain the cosmic acceleration without introducing some dark components such as the Dark Energy. Because of the crowd of models in literature, a need to check the models according to some criteria arises. In this study, we analyze two classes of models by means of energy condition restrictions and illustrate the analysis of those classes by graphical simulations. We consider the conservative and non-conservative cases of two classes of  models to perform the analysis. The results of the viability of the classes are discussed and it is found that the value of the Hubble constant has no effect on the viability of the models. Focusing on some general classes for the models, we restrict them by means of the so-called energy conditions the energy-momentum tensor on physical grounds. Besides, we find numerical values for coefficients of those classes of models.

COSMIC ACCELERATION AND MODIFIED GRAVITY

2007 ◽  
Vol 16 (12a) ◽  
pp. 2065-2074 ◽  
Author(s):  
MARK TRODDEN
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
Modified Gravity ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Late Time Acceleration ◽  
Acceleration Of The Universe ◽  
The Universe

I briefly discuss some attempts to construct a consistent modification to general relativity (GR) that might explain the observed late-time acceleration of the Universe and provide an alternative to dark energy. I describe the issues facing extensions to GR, illustrate these with a specific example, and discuss the resulting observational and theoretical obstacles.

scholarly journals A LATE TIME ACCELERATION OF THE UNIVERSE WITH TWO SCALAR FIELDS: MANY POSSIBILITIES

2006 ◽  
Vol 21 (35) ◽  
pp. 2663-2670 ◽  
Author(s):  
NARAYAN BANERJEE ◽  
SUDIPTA DAS
Keyword(s):  
Scalar Field ◽  
Scalar Fields ◽  
Cosmic Acceleration ◽  
The Other ◽  
Field Conditions ◽  
Late Time ◽  
Late Time Acceleration ◽  
Acceleration Of The Universe ◽  
The Universe

In the present work, an attempt has been made to explain the recent cosmic acceleration of the universe with two mutually interacting scalar fields, one being the Brans–Dicke scalar field and the other a quintessence scalar field. Conditions have been derived for which the quintessence scalar field has an early oscillation and it grows during a later time to govern the dynamics of the universe.

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