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Journal articles on the topic 'Friedmann-Robertson-Walker background equations'

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Published: 14 March 2023

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1

Shchigolev, Victor. "An approximate solution of the Yang - Mills equation on a spatially flat FRW cosmological background." International Journal of Physical Research 7, no. 2 (2019): 100. http://dx.doi.org/10.14419/ijpr.v7i2.29775.

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In this paper, an approximate solution for the Yang - Mills equation in a spatially flat Friedmann-Robertson-Walker universe is obtained. For this purpose, the well known method of solution of non-linear differential equations is used, viz. the homotopy perturbations method. This method has been developed as effective technique for solving different non-linear problems. Here, this method allowed us to obtain approximate solution for the essentially non-linear equation for the SO3 Yang-Mills fields on the curved space-time background of the spatially flat Friedmann-Robertson-Walker universe.
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2

Liu, Jianwen, Ruifang Wang, and Fabao Gao. "Dynamics of a Cosmological Model in f(R,T) Gravity: I. On Invariant Planes." Universe 8, no. 7 (2022): 365. http://dx.doi.org/10.3390/universe8070365.

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Under the background of perfect fluid and flat Friedmann–Lemaître–Robertson–Walker (FLRW) space-time, this paper mainly describes the dynamics of the cosmological model constructed in f(R,T) gravity on three invariant planes, by using the singularity theory and Poincaré compactification in differential equations.
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Paliathanasis, Andronikos. "Painlevé Analysis of the Cosmological Field Equations in Weyl Integrable Spacetime." Universe 8, no. 7 (2022): 345. http://dx.doi.org/10.3390/universe8070345.

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We apply a singularity analysis to investigate the integrability properties of the gravitational field equations in Weyl Integrable Spacetime for a spatially flat Friedmann–Lemaître–Robertson–Walker background spacetime induced by an ideal gas. We find that the field equations possess the Painlevé property in the presence of the cosmological constant, and the analytic solution is given by a left Laurent expansion.
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4

Coley, A. A., and B. O. J. Tupper. "Two-fluid Friedmann–Robertson–Walker cosmologies and their numerical predictions." Canadian Journal of Physics 64, no. 2 (1986): 204–9. http://dx.doi.org/10.1139/p86-036.

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Friedmann–Robertson–Walker models satisfying the Einstein field equations for a combination of two fluids, one of which is a comoving perfect fluid with the radiation equation of state [Formula: see text], representing the cosmic microwave background, are discussed. Existing models, in which the second fluid is a comoving perfect fluid, are reviewed and their numerical predictions calculated. These models are generalized by considering the case in which the second fluid is an imperfect fluid. This fluid is necessarily noncomoving, the tilt representing the motion of the local supercluster of g
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5

LAHIRI, JOYDEV, and GAUTAM BHATTACHARYA. "A COVARIANT APPROACH TO THE GENERALIZED MULTI-INFLATON COSMOLOGICAL PERTURBATION." International Journal of Modern Physics A 24, no. 20n21 (2009): 3893–916. http://dx.doi.org/10.1142/s0217751x09044231.

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Following the formalism developed in Astrophys. J.375, 443 (1991), differential equations for the gauge invariant scalar part of the metric perturbation in the Friedmann–Robertson–Walker background with multiple inflatons with arbitrary field metric are obtained without any specific choice of gauge. Subsequently, an algorithm for the solution of these equations in the slow-roll approximation is given without any prior choice of the basis system in the field manifold. Vector and tensor perturbations are also briefly reviewed.
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6

Amani, Ali R. "The bouncing cosmology with F(R) gravity and its reconstructing." International Journal of Modern Physics D 25, no. 06 (2016): 1650071. http://dx.doi.org/10.1142/s0218271816500711.

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In this paper, we study [Formula: see text] gravity by Hu–Sawicki model in Friedmann–Lemaître–Robertson–Walker (FLRW) background. The Friedmann equations are calculated by modified gravity action, and then the obtained Friedmann equations are written in terms of standard Friedmann equations. Next, the behavior of bouncing cosmology is investigated in the modified gravity model, i.e. this behavior can solve the problem of nonsingularity in standard big bang cosmology. We plot the cosmological parameters in terms of cosmic time and then the bouncing condition is investigated. In what follows, we
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7

Yılmaz, Nejat Tevfik. "Effective fluid FLRW cosmologies of minimal massive gravity." Modern Physics Letters A 30, no. 18 (2015): 1550087. http://dx.doi.org/10.1142/s021773231550087x.

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By using a solution ansatz we partially decouple the metric and the Stückelberg sectors of the minimal massive gravity (MMGR). In this scheme for a diagonal physical metric we find the general solutions for the scalars of the theory and the particular fiducial (background) metric which leads to these solutions. Then we adopt this general formalism to construct the derivation of new Friedmann–Lemaitre–Robertson–Walker (FLRW) cosmologies of the theory in the presence of a so-called effective ideal fluid which arises from our solution ansatz as a modifying, non-physical source for the Einstein an
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8

Pal, Subhajyoti, Sudip Mishra, and Subenoy Chakraborty. "Dynamical system analysis of a nonminimally coupled scalar field." International Journal of Modern Physics D 28, no. 15 (2019): 1950173. http://dx.doi.org/10.1142/s0218271819501736.

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This paper deals with a nonminimally coupled scalar field in the background of homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker (FLRW) flat spacetime. As Einstein field equations are coupled second-order nonlinear differential equations, it is very hard to find exact solutions. By suitable choice of variables, we transform Einstein field equations to an autonomous system and critical points are determined. We use center manifold theory to characterize nonhyperbolic critical points and are found to be saddle in nature. We discuss possible bifurcation scenarios, which indicate the e
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9

Paliathanasis, Andronikos, and Genly Leon. "Cosmological solutions in Hořava-Lifshitz scalar field theory." Zeitschrift für Naturforschung A 75, no. 6 (2020): 523–32. http://dx.doi.org/10.1515/zna-2020-0003.

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AbstractWe perform a detailed study of the integrability of the Hořava-Lifshitz scalar field cosmology in a Friedmann-Lemaître-Robertson-Walker background space-time. The approach we follow to determine the integrability is that of singularity analysis. More specifically, we test whether the gravitational field equations possess the Painlevé property. For the exponential potential of the scalar field, we are able to perform an analytic explicit integration of the field equations and write the solution in terms of a Laurent expansion and more specifically write the solution in terms of right Pa
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10

Dutta, Malay Krishna, and B. Modak. "Can Noether symmetry in modified f(G) gravity always yield cosmic evolution?" Modern Physics Letters A 32, no. 11 (2017): 1750046. http://dx.doi.org/10.1142/s0217732317500468.

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We discuss Noether symmetry approach in the modified theory of gravity with Gauss–Bonnet (GB) interaction-f(G) including an ideal fluid in Friedmann–Lemaître–Robertson–Walker (FLRW) background. It yields functional form of f(G) from the symmetry. The existence of Noether symmetry gives the scale factor in two cases, but these are not satisfied by field equations in general. In another case, the solution of field equations shows late-time transition to an accelerating expansion when matter is dust, however the solution including dust and radiation is always in accelerating era.
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11

Elizalde, E., V. V. Obukhov, and A. V. Timoshkin. "Inhomogeneous viscous dark fluid coupled with dark matter in the FRW universe." Modern Physics Letters A 29, no. 25 (2014): 1450132. http://dx.doi.org/10.1142/s0217732314501326.

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A cosmological model with an inhomogeneous viscous dark fluid coupled with dark matter in a flat Friedmann–Robertson–Walker (FRW) universe is investigated. The influence of dark matter on the behavior of an inhomogeneous viscous fluid of this kind, responsible for cosmic acceleration and for the appearance of different types of singularities, is analyzed in detail. In particular, the critical points corresponding to the solutions of the background equations in a useful approximation are obtained explicitly.
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12

Zubair, M., M. Zeeshan та Saira Waheed. "Cosmic evolution in the background of R (1 + αQ) gravity". Modern Physics Letters A 34, № 31 (2019): 1950253. http://dx.doi.org/10.1142/s0217732319502535.

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In this paper, we discuss the cosmic evolution in a modified theory involving non-minimal interaction of geometry and matter, labeled as [Formula: see text] gravity, where [Formula: see text] is the non-minimal interaction term. First, we develop the dynamical [Formula: see text] field equations for Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetime and then by using divergence of these equations, we explore its interesting outcome of non-conserved energy–momentum tensor (EMT). The presence of geometry matter coupling in such theories results in non-geodesic test particles motion and hence c
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13

Krishnan, Chethan, Ranjini Mondol, and M. M. Sheikh Jabbari. "Copernican Paradigm beyond FLRW." Symmetry 15, no. 2 (2023): 428. http://dx.doi.org/10.3390/sym15020428.

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We present the dipole cosmological principle, i.e., the notion that the Universe is a Copernican cosmology that agrees with the cosmic flow. It suits the most symmetric paradigm that generalizes the Friedmann–Lemaître–Robertson–Walker ansatz in the context of numerous suggestions that have appeared in the literature for non-kinematic components in the cosmic microwave background dipole. Field equations in our “dipole cosmology” are still ODEs, but we now have four instead of two Friedmann equations. The two extra functions can be regarded as additional scale factors that break the isotropy gro
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14

Dutta, Sourav, and Santu Mondal. "Lie and Noether symmetry analysis in Brans–Dicke cosmology." Modern Physics Letters A 33, no. 34 (2018): 1850198. http://dx.doi.org/10.1142/s0217732318501985.

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This paper is aimed to study the group invariant solutions of the evolution equations in Brans–Dicke cosmology. In this context, we have considered the flat homogeneous Brans–Dicke (BD) scalar field in the background of flat homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model and have used Lie and Noether symmetry on the augmented system. From Lie symmetry we have determined the unknown potential for two different values of the equation of state parameter w. Then assuming that the Lagrangian admits a Noether symmetry, an analytic solution of the system is ob
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15

CZINNER, VIKTOR G., and MÁTYÁS VASÚTH. "REVISITING ROTATIONAL PERTURBATIONS AND THE MICROWAVE BACKGROUND." International Journal of Modern Physics D 16, no. 11 (2007): 1715–23. http://dx.doi.org/10.1142/s0218271807011176.

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We consider general-relativistic rotational perturbations in homogeneous and isotropic Friedmann–Robertson–Walker (FRW) cosmologies. Taking linear perturbations of FRW models, the general solution to the field equations contains tensorial, vectorial and scalar functions. The vectorial terms are in connection with rotations in the given model and due to the Sachs–Wolfe effect they produce contributions to the temperature fluctuations of the cosmic microwave background radiation (CMBR). In this paper we obtain the analytic time dependence of these contributions in a spatially flat FRW model with
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16

Dutta, Sourav, Madan Mohan Panja, and Subenoy Chakraborty. "A study of dynamical equations for non-minimally coupled scalar field using Noether symmetric approach." Modern Physics Letters A 31, no. 19 (2016): 1650116. http://dx.doi.org/10.1142/s0217732316501169.

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Non-minimally coupled scalar field cosmology has been studied in this work within the framework of Einstein gravity. In the background of homogeneous and isotropic Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetime non-minimally coupled scalar field having self-interacting potential is taken as the source of the matter content. The constraint of imposing Noether symmetry on the Lagrangian of the system not only determines the infinitesimal generator (the symmetry vector) but also the coupling function and the self-interacting potential for the scalar field. By choosing appropriately a point
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17

Myrzakulov, Nurgissa, Ratbay Myrzakulov, and Lucrezia Ravera. "Metric-Affine Myrzakulov Gravity Theories." Symmetry 13, no. 10 (2021): 1855. http://dx.doi.org/10.3390/sym13101855.

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In this paper, we review the so-called Myrzakulov Gravity models (MG-N, with N = I, II, …, VIII) and derive their respective metric-affine generalizations (MAMG-N), discussing also their particular sub-cases. The field equations of the theories are obtained by regarding the metric tensor and the general affine connection as independent variables. We then focus on the case in which the function characterizing the aforementioned metric-affine models is linear and consider a Friedmann-Lemaître–Robertson–Walker background to study cosmological aspects and applications. Historical motivation for th
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18

Biswas, Sujay Kr, and Subenoy Chakraborty. "Interacting dark energy in f(T) cosmology: A dynamical system analysis." International Journal of Modern Physics D 24, no. 07 (2015): 1550046. http://dx.doi.org/10.1142/s0218271815500467.

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This paper deals with an interacting dark energy (DE) model in the framework of f(T) cosmology. A cosmologically viable form of f(T) is chosen (T is the torsion scalar in teleparallelism) in the background of flat homogeneous and isotropic Friedmann–Robertson–Walker (FRW) spacetime model of the universe. The matter content of the universe is chosen as dust interacting with minimally coupled scalar field. The evolution equations are reduced to an autonomous system of ordinary differential equations by suitable transformation of variables. The nature of critical points is analyzed by evaluating
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19

Haghani, Zahra, and Tiberiu Harko. "Effects of Quantum Metric Fluctuations on the Cosmological Evolution in Friedmann-Lemaitre-Robertson-Walker Geometries." Physics 3, no. 3 (2021): 689–714. http://dx.doi.org/10.3390/physics3030042.

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In this paper, the effects of the quantum metric fluctuations on the background cosmological dynamics of the universe are considered. To describe the quantum effects, the metric is assumed to be given by the sum of a classical component and a fluctuating component of quantum origin . At the classical level, the Einstein gravitational field equations are equivalent to a modified gravity theory, containing a non-minimal coupling between matter and geometry. The gravitational dynamics is determined by the expectation value of the fluctuating quantum correction term, which can be expressed in term
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20

Abebe, Amare, Peter K. S. Dunsby, and Deon Solomons. "Integrability conditions of quasi-Newtonian cosmologies in modified gravity." International Journal of Modern Physics D 26, no. 06 (2016): 1750054. http://dx.doi.org/10.1142/s0218271817500547.

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We investigate the integrability conditions of a class of shear-free perfect-fluid cosmological models within the framework of anisotropic fluid sources, applying our results to [Formula: see text] dark energy models. Generalizing earlier general relativistic results for timelike geodesics, we extend the potential and acceleration terms of the quasi-Newtonian formulation of integrable dust cosmological models about a linearized Friedmann–Lemaître–Robertson–Walker background and derive the equations that describe their dynamical evolutions. We show that in general, models with an anisotropic fl
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21

OGUSHI, SACHIKO. "HOLOGRAPHIC ENTROPY ON THE BRANE IN de SITTER SCHWARZSCHILD SPACE." Modern Physics Letters A 17, no. 01 (2002): 51–58. http://dx.doi.org/10.1142/s0217732302006084.

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The relationship between the entropy of de Sitter (dS) Schwarzschild space and that of the CFT, which lives on the brane, is discussed by using Friedmann–Robertson–Walker (FRW) equations and Cardy–Verlinde formula. The cosmological constant appears on the brane with time-like metric in dS Schwarzschild background. On the other hand, in case of the brane with space-like metric in dS Schwarzschild background, the cosmological constant of the brane does not appear because we can choose brane tension to cancel it. We show that when the brane crosses the horizon of dS Schwarzschild black hole, both
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22

CHEVALLIER, MICHEL, and DAVID POLARSKI. "ACCELERATING UNIVERSES WITH SCALING DARK MATTER." International Journal of Modern Physics D 10, no. 02 (2001): 213–23. http://dx.doi.org/10.1142/s0218271801000822.

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Friedmann–Robertson–Walker universes with a presently large fraction of the energy density stored in an X-component with wX<-1/3, are considered. We find all the critical points of the system for constant equations of state in that range. We consider further several background quantities that can distinguish the models with different wXvalues. Using a simple toy model with a varying equation of state, we show that even a large variation of wXat small redshifts is very difficult to observe with dL(z) measurements up to z~1. Therefore, it will require accurate measurements in the range 1<z
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23

Ketov, Sergei V., Ekaterina O. Pozdeeva, and Sergey Yu Vernov. "On the superstring-inspired quantum correction to the Starobinsky model of inflation." Journal of Cosmology and Astroparticle Physics 2022, no. 12 (2022): 032. http://dx.doi.org/10.1088/1475-7516/2022/12/032.

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Abstract Superstring/M-theory is the theory of quantum gravity that can provide the UV-completion to viable inflation models. We modify the Starobinsky inflation model by adding the Bel-Robinson tensor Tμνλρ squared term proposed as the leading quantum correction inspired by superstring theory. The (R + 1/6m 2 R 2 - β/8m 6 T 2) model under consideration has two parameters: the inflaton mass m and the string-inspired positive parameter β. We derive the equations of motion in the Friedmann-Lemaitre-Robertson-Walker universe and investigate its solutions. We find the physical bounds on the value
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24

Hu, Lingyi, Alan Heavens, and David Bacon. "Light bending by the cosmological constant." Journal of Cosmology and Astroparticle Physics 2022, no. 02 (2022): 009. http://dx.doi.org/10.1088/1475-7516/2022/02/009.

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Abstract We revisit the question of whether the cosmological constant Λ affects the cosmological gravitational bending of light, by numerical integration of the geodesic equations for a Swiss cheese model consisting of a point mass and a compensated vacuole, in a Friedmann-Robertson-Walker background. We find that there is virtually no dependence of the light bending on the cosmological constant that is not already accounted for in the angular diameter distances of the standard lensing equations, plus small modifications that arise because the bending is restricted to a finite region covered b
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25

Sofuoğlu, Değer, Rishi Kumar Tiwari, Amare Abebe, Alnadhief H. A. Alfedeel, and Eltegani I. Hassan. "The Cosmology of a Non-Minimally Coupled f(R,T) Gravitation." Physics 4, no. 4 (2022): 1348–58. http://dx.doi.org/10.3390/physics4040086.

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A non-minimally coupled cosmological scenario is considered in the context of f(R,T)=f1(R)+f2(R)f3(T) gravity (with R being the Ricci scalar and T the trace of the energy-momentum tensor) in the background of the flat Friedmann–Robertson–Walker (FRW) model. The field equations of this modified theory are solved using a time-dependent deceleration parameter for a dust. The behavior of the model is analyzed taking into account constraints from recent observed values the deceleration parameter. It is shown that the analyzed models can explain the transition from the decelerating phase to the acce
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26

Biswas, Sujay Kr, and Subenoy Chakraborty. "Interacting dark energy model in the brane scenario: A dynamical system analysis." International Journal of Geometric Methods in Modern Physics 16, no. 08 (2019): 1950115. http://dx.doi.org/10.1142/s0219887819501159.

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The present work is a second in the series of investigations of the background dynamics in brane cosmology when dark energy is coupled to dark matter by a suitable interaction. Here, dark matter is chosen in the form of perfect fluid with barotropic equation of state, while a real scalar field with self-interacting potential is chosen as dark energy. The scalar field potential is chosen as exponential or hyperbolic in nature and three different choices for the interactions between the dark species are considered. In the background of spatially flat, homogeneous and isotropic Friedmann–Robertso
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27

Semrén, Philip. "Electromagnetic, Gravitational, and Plasma-Related Perturbations of Locally Rotationally Symmetric Class II Spacetimes." Universe 8, no. 8 (2022): 406. http://dx.doi.org/10.3390/universe8080406.

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We investigate electromagnetic, gravitational, and plasma-related perturbations to the first order on homogeneous and hypersurface orthogonal locally rotationally symmetric (LRS) class II spacetimes. Due to the anisotropic nature of the studied backgrounds, we are able to include a non-zero magnetic field to the zeroth order. As a result of this inclusion, we find interesting interactions between the electromagnetic and gravitational variables already of the first order in the perturbations. The equations governing these perturbations are found by using the Ricci identities, the Bianchi identi
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28

Ntahompagaze, Joseph, Amare Abebe, and Manasse Mbonye. "A study of perturbations in scalar–tensor theory using 1 + 3 covariant approach." International Journal of Modern Physics D 27, no. 03 (2018): 1850033. http://dx.doi.org/10.1142/s0218271818500335.

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This work discusses scalar–tensor theories of gravity, with a focus on the Brans–Dicke sub-class, and one that also takes note of the latter’s equivalence with [Formula: see text] gravitation theories. A [Formula: see text] covariant formalism is used in this case to discuss covariant perturbations on a background Friedmann–Laimaître–Robertson–Walker (FLRW) spacetime. Linear perturbation equations are developed based on gauge-invariant gradient variables. Both scalar and harmonic decompositions are applied to obtain second-order equations. These equations can then be used for further analysis
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29

Tawfik, A. "Thermodynamics in the viscous early universe." Canadian Journal of Physics 88, no. 11 (2010): 825–31. http://dx.doi.org/10.1139/p10-058.

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Assuming that the matter in the background geometry is a free gas and that no phase transitions were occurring in the early Universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, the scaling factor a, and the curvature parameter k, can be derived from this simple model, which on one hand fulfills and entirely obeys the laws of thermodynamics, and on the other hand, its results are compatible with the Friedmann–Robertson–Walker model and the Einstein field equations. Including a fi
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30

Pacif, S. K. J., Md Salahuddin Khan, L. K. Paikroy, and Shalini Singh. "An accelerating cosmological model from a parametrization of Hubble parameter." Modern Physics Letters A 35, no. 05 (2019): 2050011. http://dx.doi.org/10.1142/s021773232050011x.

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In view of late-time cosmic acceleration, a dark energy cosmological model is revisited wherein Einstein’s cosmological constant is considered as a candidate of dark energy. Exact solution of Einstein field equations (EFEs) is derived in a homogeneous isotropic background in classical general relativity. The solution procedure is adopted in a model-independent way (or the cosmological parametrization). A simple parametrization of the Hubble parameter (H) as a function of cosmic time t is considered which yields an exponential type of evolution of the scale factor (a) and also shows a negative
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31

Amani, Ali R., and S. L. Dehneshin. "Interacting F(R, T) gravity with modified Chaplygin gas." Canadian Journal of Physics 93, no. 12 (2015): 1453–59. http://dx.doi.org/10.1139/cjp-2015-0024.

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In this paper, we have studied F(R, T) gravity as an arbitrary function of curvature and torsion scalars in Friedmann–Lemaître–Robertson–Walker background. Then, we have considered interacting model between F(R, T) gravity and modified Chaplygin gas. The novelty of this model is that the Universe includes both curvature and torsion cases, and one dominated by a Chaplygin gas. To calculate cosmological solutions, we obtained the Friedmann equations and also equation of state parameter of dark energy. By employing the interacting model we considered the total energy density and the total pressur
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32

Tawfik, A., and H. Magdy. "Thermodynamics of viscous matter and radiation in the early universe." Canadian Journal of Physics 90, no. 5 (2012): 433–40. http://dx.doi.org/10.1139/p2012-037.

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Assuming that the background geometry is filled with a free gas consisting of matter and radiation and that no phase transitions are occurring in the early universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, scale factor a, and curvature parameter k, can be derived from this simple model. On one hand, it obeys the laws of thermodynamics entirely. On the other hand, the results are compatible with the Friedmann–Lemaitre–Robertson–Walker model and the Einstein field equations. Th
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33

Espriu, Domènec, and Marc Rodoreda. "Effect of the cosmological parameters on gravitational waves: general analysis." Classical and Quantum Gravity 39, no. 1 (2021): 015012. http://dx.doi.org/10.1088/1361-6382/ac33bc.

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Abstract Some time ago it was pointed out that the presence of cosmological components could affect the propagation of gravitational waves (GW) beyond the usual cosmological redshift and that such effects might be observable in pulsar timing arrays (PTA). These analyses were done at leading order in the Hubble constant H 0, which is proportional to Λ 1 2 and ρ i 1 2 (ρ i being the various cosmological fluid densities). In this work, we study in detail the propagation of metric perturbations on a Schwarzschild–de Sitter (SdS) background, close to the place where GW are produced, and obtain solu
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34

Tsyba, P. Yu, O. V. Razina, and N. T. Suikimbayeva. "Reconstruction of cosmological models are inspired by generalization of the Chaplygin gas." Bulletin of the Karaganda University. "Physics" Series 105, no. 1 (2022): 27–34. http://dx.doi.org/10.31489/2022ph1/27-34.

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This paper considers models arising from the composition of the modified Gauss–Bonnet gravity (the Gauss– Bonnet invariant) and the general relativity (the Ricci scalar) against the background of a flat, homogeneous, and isotropic space-time described by the Friedmann–Robertson–Walker metric. Advantages arising from applying a theory containing higher-order invariants (Gauss–Bonnet invariant) consist in the presence of additional degrees of freedom, which makes it possible to study the influence of small-order effects on the dynamics of the system under study, which are in search and confirmed
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35

Ainamon, C., M. G. Ganiou, H. F. Abadji, and M. J. S. Houndjo. "Bianchi-I cosmology within f(T): Reconstruction method and dynamical study." International Journal of Geometric Methods in Modern Physics 18, no. 01 (2020): 2150012. http://dx.doi.org/10.1142/s0219887821500122.

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This paper is fundamentally devoted to the cosmological reconstruction and dynamic studying in homogeneous BIANCHI-I space-time under the [Formula: see text] background. Its content is supported by the fact that in the General Relativity description of the standard cosmological paradigm, the evolution from an anisotropic universe into an Friedmann–Lemaitre–Robertson–Walker (FLRW) one can be achieved by a period of inflationary expansion. Nowadays, modified gravity theories like [Formula: see text] are widely accepted to provide a real description of some universe evolution phases like inflatio
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36

Paliathanasis, Andronikos, and Genly Leon. "Integrability and cosmological solutions in Einstein-æther-Weyl theory." European Physical Journal C 81, no. 3 (2021). http://dx.doi.org/10.1140/epjc/s10052-021-09031-w.

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AbstractWe consider a Lorentz violating scalar field cosmological model given by the modified Einstein-æther theory defined in Weyl integrable geometry. The existence of exact and analytic solutions is investigated for the case of a spatially flat Friedmann–Lemaître–Robertson–Walker background space. We show that the theory admits cosmological solutions of special interests. In addition, we prove that the cosmological field equations admit the Lewis invariant as a second conservation law, which indicates the integrability of the field equations.
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37

Iosifidis, Damianos, and Lucrezia Ravera. "The cosmology of quadratic torsionful gravity." European Physical Journal C 81, no. 8 (2021). http://dx.doi.org/10.1140/epjc/s10052-021-09532-8.

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AbstractWe study the cosmology of a quadratic metric-compatible torsionful gravity theory in the presence of a perfect hyperfluid. The gravitational action is an extension of the Einstein–Cartan theory given by the usual Einstein–Hilbert contribution plus all the admitted quadratic parity even torsion scalars and the matter action also exhibits a dependence on the connection. The equations of motion are obtained by regarding the metric and the metric-compatible torsionful connection as independent variables. We then consider a Friedmann–Lemaître–Robertson–Walker background, analyze the conserv
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38

Paliathanasis, Andronikos. "Complex scalar fields in scalar-tensor and scalar-torsion theories." Modern Physics Letters A, October 27, 2022. http://dx.doi.org/10.1142/s0217732322501681.

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In this paper, we investigate the cosmological dynamics in a spatially flat Friedmann–Lemaître–Robertson–Walker geometry in scalar-tensor and scalar-torsion theories where the nonminimally coupled scalar field is a complex field. We derive the cosmological field equations and we make use of dimensionless variables in order to determine the stationary points and determine their stability properties. The physical properties of the stationary points are discussed while we find that the two-different theories, scalar-tensor and scalar-torsion theories, share many common features in terms of the ev
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39

Murorunkwere, Beatrice, Joseph Ntahompagaze, and Edward Jurua. "1 + 3 covariant perturbations in power-law f(R) gravity." European Physical Journal C 81, no. 4 (2021). http://dx.doi.org/10.1140/epjc/s10052-021-09134-4.

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AbstractWe applied the 1+3 covariant approach around the Friedmann–Lemaître–Robertson–Walker (FLRW) background, together with the equivalence between f(R) gravity and scalar-tensor theory to study cosmological perturbations. We defined the gradient variables in the 1 + 3 covariant approach which we used to derive a set of evolution equations. Harmonic decomposition was applied to partial differential equations to obtain ordinary differential equations used to analyse the behavior of the perturbation quantities. We focused on dust dominated area and the perturbation equations were applied to ba
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40

Ntahompagaze, Joseph, Shambel Sahlu, Amare Abebe, and Manasse R. Mbonye. "On multifluid perturbations in scalar–tensor cosmology." International Journal of Modern Physics D, November 30, 2020, 2050120. http://dx.doi.org/10.1142/s0218271820501205.

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In this paper, the scalar–tensor theory is applied to the study of perturbations in a multifluid universe, using the [Formula: see text] covariant approach. Both scalar and harmonic decompositions are instituted on the perturbation equations. In particular, as an application, we study perturbations on a background Friedmann-Robertson-Walker (FRW) cosmology consisting of both radiation and dust in the presence of a scalar field. We consider both radiation-dominated and dust-dominated epochs, respectively, and study the results. During the analysis, quasi-static approximation is instituted. It i
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41

Paliathanasis, Andronikos. "New exact and analytic solutions in Weyl Integrable cosmology from Noether symmetry analysis." Physica Scripta, August 4, 2022. http://dx.doi.org/10.1088/1402-4896/ac8702.

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Abstract We consider a cosmological model in a Friedmann-Lemaître-Robertson-Walker background space with an ideal gas defined in Weyl Integrable gravity. In the Weyl-Einstein a scalar field is introduced in a geometric way. Furthermore, the scalar field and the ideal gas interact in the gravitational Action Integral. Furthermore, we introduce a potential term for the scalar field potential and we show that the field equations admit a minisuperspace description. Noether's theorem is applied for the constraint of the potential function and the corresponding conservation laws are constructed. Fin
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42

Tiwari, R. K., D. Sofuoglu, R. Isik, B. K. Shukla, and E. Baysazan. "Non-minimally coupled transit cosmology in f(R,T) gravity." International Journal of Geometric Methods in Modern Physics, April 18, 2022. http://dx.doi.org/10.1142/s0219887822501183.

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A non-minimally coupled cosmological model is studied in [Formula: see text] gravity for a particular choice of the function in the background of flat Friedmann–Robertson–Walker universe. The modified field equations are solved with the help of a varying deceleration parameter. The time evolution of the model is analyzed for both the dynamic and kinematic quantities. For testing the viability of the results, energy conditions and the statefinder diagnostic are used. It has been shown that this model, which we discussed to examine the phase transition in the expansion of the universe, is compat
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43

Hama, Rattanasak, Tiberiu Harko, and Sorin V. Sabau. "Dark energy and accelerating cosmological evolution from osculating Barthel–Kropina geometry." European Physical Journal C 82, no. 4 (2022). http://dx.doi.org/10.1140/epjc/s10052-022-10318-9.

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AbstractFinsler geometry is an important extension of Riemann geometry, in which each point of the spacetime manifold is associated with an arbitrary internal variable. Two interesting Finsler geometries with many physical applications are the Randers and Kropina type geometries. A subclass of Finsler geometries is represented by the osculating Finsler spaces, in which the internal variable is a function of the base manifold coordinates only. In an osculating Finsler geometry, we introduce the Barthel connection, with the remarkable property that it is the Levi–Civita connection of a Riemannia
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44

Ildes, Medine, and Metin Arik. "Analytic solutions of scalar field cosmology, mathematical structures for early inflation and late time accelerated expansion." European Physical Journal C 83, no. 2 (2023). http://dx.doi.org/10.1140/epjc/s10052-023-11273-9.

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AbstractWe study the most general cosmological model with real scalar field which is minimally coupled to gravity. Our calculations are based on Friedmann–Lemaitre–Robertson–Walker (FLRW) background metric. Field equations consist of three differential equations. We switch independent variable from time to scale factor by change of variable $${\dot{a}}/a=H(a)$$ a ˙ / a = H ( a ) . Thus a new set of differential equations are analytically solvable with known methods. We formulate Hubble function, the scalar field, potential and energy density when one of them is given in the most general form.
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45

Hama, Rattanasak, Tiberiu Harko, Sorin V. Sabau, and Shahab Shahidi. "Cosmological evolution and dark energy in osculating Barthel–Randers geometry." European Physical Journal C 81, no. 8 (2021). http://dx.doi.org/10.1140/epjc/s10052-021-09517-7.

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AbstractWe consider the cosmological evolution in an osculating point Barthel–Randers type geometry, in which to each point of the space-time manifold an arbitrary point vector field is associated. This Finsler type geometry is assumed to describe the physical properties of the gravitational field, as well as the cosmological dynamics. For the Barthel–Randers geometry the connection is given by the Levi-Civita connection of the associated Riemann metric. The generalized Friedmann equations in the Barthel–Randers geometry are obtained by considering that the background Riemannian metric in the
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46

Paliathanasis, Andronikos. "f(T) cosmology with nonzero curvature." Modern Physics Letters A 36, no. 38 (2021). http://dx.doi.org/10.1142/s0217732321502618.

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We investigate exact and analytic solutions in [Formula: see text] gravity within the context of a Friedmann–Lemaître–Robertson–Walker background space with nonzero spatial curvature. For the power-law theory [Formula: see text] we find that the field equations admit an exact solution with a linear scalar factor for negative and positive spatial curvature. That Milne-like solution is asymptotic behavior for the scale factor near the initial singularity for the model [Formula: see text]. The analytic solution for that specific theory is presented in terms of Painlevé series for [Formula: see te
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47

Manna, Goutam, Arijit Panda, Aninda Karmakar, Saibal Ray, and Md Rabiul Islam. "$f(\bar{R}, L(X))$-gravity in the context of dark energy with power law expansion and energy conditions." Chinese Physics C, November 3, 2022. http://dx.doi.org/10.1088/1674-1137/ac9fbe.

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Abstract The motto of this work is to generate a general formalism of $f(\bar{R}, L(X))-$gravity in the context of dark energy under the framework of the {\bf K-}essence emergent geometry with the Dirac-Born-Infeld (DBI) variety of action, where $\bar{R}$ is the familiar Ricci scalar, $L(X)$ is the DBI type non-canonical Lagrangian with $X={1\over 2}g^{\mu\nu}\nabla_{\mu}\phi\nabla_{\nu}\phi$ and $\phi$ is the {\bf K-}essence scalar field. The emergent gravity metric $\G_{\mu\nu}$ and the well known gravitational metric $g_{\mu\nu}$ are not conformally equivalent. We have constructed a modifie
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48

Asghari, Mahnaz, and Ahmad Sheykhi. "Observational constraints of the modified cosmology through Barrow entropy." European Physical Journal C 82, no. 5 (2022). http://dx.doi.org/10.1140/epjc/s10052-022-10262-8.

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AbstractTaking into account a fractal structure for the black hole horizon, Barrow argued that the area law of entropy is modified due to quantum-gravitational effects (Barrow in Phys Lett B 808:135643, 10.1016/j.physletb.2020.135643, 2020). Accordingly, the corrected entropy takes the form $$S \sim A^{1+{\Delta }/2}$$ S ∼ A 1 + Δ / 2 , where $$0\le {\Delta }\le 1$$ 0 ≤ Δ ≤ 1 indicates the amount of the quantum-gravitational deformation effects. In this paper, based on Barrow entropy, we first derive the modified gravitational field equations through the Clausius relation. We then consider the
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49

Boumaza, Hamza. "Cosmology of cubic galileon in modified teleparallel gravity." European Physical Journal C 81, no. 2 (2021). http://dx.doi.org/10.1140/epjc/s10052-021-08916-0.

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AbstractIn this present paper, we study the cosmological evolution of the cubic galileon along with modified teleparallel gravity at perturbed and non-perturbed levels. We show the dynamical equations of motion and investigate the evolution of different cosmological parameters by using the dynamical variables analysis. In addition, a detailed analysis of different cosmological evolution in the matter, radiation and de Sitter eras is presented by solving the dynamical equations numerically. In our analysis, we find that the equations of motion in the Friedmann–Robertson–Walker (FRW) background
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50

Bouali, Amine, Himanshu Chaudhary, Rattanasak Hama, Tiberiu Harko, Sorin V. Sabau, and Marco San Martín. "Cosmological tests of the osculating Barthel–Kropina dark energy model." European Physical Journal C 83, no. 2 (2023). http://dx.doi.org/10.1140/epjc/s10052-023-11265-9.

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AbstractWe further investigate the dark energy model based on the Finsler geometry inspired osculating Barthel–Kropina cosmology. The Barthel–Kropina cosmological approach is based on the introduction of a Barthel connection in an osculating Finsler geometry, with the connection having the property that it is the Levi-Civita connection of a Riemannian metric. From the generalized Friedmann equations of the Barthel–Kropina model, obtained by assuming that the background Riemannian metric is of the Friedmann–Lemaitre–Robertson–Walker type, an effective geometric dark energy component can be gene
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