Matter-geometry Coupling Research Articles

Existence of non-singular stellar solutions within the context of electromagnetic field: a comparison between minimal and non-minimal gravity models

In this paper, we explore the existence of various non-singular compact stellar solutions influenced by the Maxwell field within the matter-geometry coupling based modified gravity. We start this analysis by considering a static spherically symmetric spacetime which is associated with the isotropic matter distribution. We then determine the field equations corresponding to two specific functions of this modified theory. Along with these models, we also adopt different forms of the matter Lagrangian. We observe several unknowns in these equations such as the metric potentials, charge and fluid parameters. Thus, the embedding class-one condition and a particular realistic equation of state is used to construct their corresponding solutions. The former condition provides the metric components possessing three constants, and we calculate them through junction conditions. Further, four developed models are graphically analyzed under different parametric values. Finally, we find all our developed solutions well-agreeing with the physical requirements, offering valuable insights for future explorations of the stellar compositions in this theory.

Stability of charged anisotropic thin shell gravastars admitting conformal motion in [formula omitted] gravity

This paper investigates the interesting features of gravastar configuration under charged anisotropic fluid distribution considering geometry-matter coupling theory. In the context of f(R,T) gravity, we derive exact solutions of the Einstein field equations using the linear form of the function f(R,T)=R+2γT, where R is the Ricci scalar and T is the trace of the energy–momentum tensor with conformal motion. We consider three different regions of gravastar, the interior region having a definite radius r, the intermediate layer of matter with the thickness ϵ, and the outer region with the radius r+ϵ. For the viability of our developed solution, we examine the metric potentials, energy density, and pressure components inside the thin shell for different values of theory parameter γ, and charge Q. Using the well-known matching conditions, we interpret the dynamical properties of gravastar such as surface charge density, surface pressure, proper length, equation of state parameter, entropy, and evolution of shell energy under the allowed range of parameters. Moreover, we analyze the presence of realistic matter inside the thin shell through energy conditions. The dynamical stability of the gravastar is checked through the equilibrium profile and speed of sound parameter, which shows the stable configuration of the gravastar in our theoretical studies.

The first variation of the matter energy–momentum tensor with respect to the metric, and its implications on modified gravity theories

The first order variation of the matter energy–momentum tensor Tμν with respect to the metric tensor gαβ plays an important role in modified gravity theories with geometry-matter coupling, and in particular in the f(R,T) modified gravity theory. We obtain the expression of the variation δTμν/δgαβ for the baryonic matter described by an equation given in a parametric form, with the basic thermodynamic variables represented by the particle number density, and by the specific entropy, respectively. The first variation of the matter energy–momentum tensor turns out to be independent on the matter Lagrangian, and can be expressed in terms of the pressure, the energy–momentum tensor itself, and the matter fluid four-velocity. We apply the obtained results for the case of the f(R,T) gravity theory, where R is the Ricci scalar, and T is the trace of the matter energy–momentum tensor, which thus becomes a unique theory, also independent on the choice of the matter Lagrangian. A simple cosmological model, in which the Hilbert–Einstein Lagrangian is generalized through the addition of a term proportional to Tn is considered in detail, and it is shown that it gives a very good description of the observational values of the Hubble parameter up to a redshift of z≈2.5.

Invariant Bianchi type I cosmological models and conservation laws in f(R,T)=f1(R)+f2(R)f3(T) gravity

f(R,T) theory of gravity can explain the dark energy and the observed late time acceleration of the universe even without cosmological constant. It passes the solar system test and appears to be an alternative of dark matter. We obtain the accelerating solutions of Einstein’s field equations, corresponding to Bianchi I spacetime with perfect fluid, in f(R,T) gravity theory without shear. The non-static non-degenerating indefinite smooth solutions of field equations are derived by considering f(R,T)=f1(R)+f2(R)f3(T) with non-minimal matter–geometry coupling. The classical Lie symmetry method is used for symmetry reduction of field equations and for obtaining the exact solutions in terms of arbitrary functions. The graphical representations of solutions are shown by considering the possibilities of arbitrary functions. The conservation laws of Einstein’s equations in f(R,T) gravity are also obtained.

Evolution of gravitational waves in non-minimal coupling between geometry and matter theories of gravity

We consider some specific models of non-minimal matter–geometry coupling theories and investigate the propagation of the gravitational waves in them. Extracting the temporal evolution of the gravitational wave equation within the framework of a flat FRW universe with a perfect fluid distribution, we analyze the waveforms traveling during the time. We find that while both the amplitude and frequency of the GWs decay with time in all considered models, the rate of reduction is highly sensitive to the values of the equation of state parameter and input parameters of the considered models.

The Effect of f(R, T) Modified Gravity on the Mass and Radius of Pulsar HerX1

Millisecond pulsars are the perfect testable to examine potential matter-geometry coupling and its physical consequences in the context of the recent Neutron Star Interior Composition Explorer discoveries. We apply the field equations of modified gravity, f(R, T) = R + α T, to a spherically symmetric spacetime, where R is the Ricci scalar, α is a dimensional parameter, and T is the matter of the geometry. Five unknown functions are present in the output system of differential equations, which consists of three equations. To close the system, we make explicit assumptions about the anisotropy and the radial metric potential, g rr . We then solve the output differential equations and derive the explicit forms of the components of the energy-momentum tensor, i.e., density, radial, and tangential pressures. We look into the possibility that all of the physical parameters in the star can be reexpressed in terms of α and the compactness parameters, C = 2 GM Rc−2. We show that, for a given mass, the size permitted by Einstein’s general relativity is less due to the matter-geometry coupling in f(R, T). The validity of the hypothesis was validated by observations from an extra 21 pulsars. To achieve a surface density that is compatible with a neutron core at nuclear saturation density, the mass–radius curve enables masses up to 3.35M ⊙. We emphasize that although there is no assumption of an equation of state, the model fits well with a linear behavior. When comparing the surface densities of these 20 pulsars, we divided them into three groups. We show that these three groups are compatible with neutron cores.

Some aspects of inflationary scenario in the modified f(𝕋,𝒯 ) gravity

Inflationary cosmology was the subject of an investigation in the [Formula: see text] gravity context, for which [Formula: see text] stands for the torsion scalar while [Formula: see text] is the trace of the energy–momentum tensor using three different class of inflation potentials well known in the literature. In order to find the range of geometry-matter coupling parameter to describe cosmological inflation scenario, we determined the slow-roll parameters and predict the scalar spectral index [Formula: see text], the tensor to scalar ratio [Formula: see text] and tensor spectral index [Formula: see text] in function in inflation potential parameters. The results show that the range of geometry-matter coupling parameter found is in agreement with the PLANCK 2018 data and WMAP data.

Impact of minimal matter-geometry coupling on anisotropic quark stars

This paper inspects the impact of minimal matter-geometry coupling present in [Formula: see text] model of [Formula: see text] theory on the physical attributes of anisotropic quark stars. The geometry of considered stellar candidates is modeled via spherically symmetric static spacetime whose metric functions are influenced by Heintzmann solutions. The inner matter distribution of the stellar system is assumed as anisotropic with the phenomenological MIT bag model equation of state. The expressions of unknown parameters that appear in Heintzmann solution are evaluated in terms of mass and radius by the continuity of interior and exterior geometries. Further, insertion of masses and radii of some particular observed stellar models will yield their numerical values. In order to discuss the physical acceptability as well as stability of the quarks stars based on the considered solutions, we have checked the physical behavior of matter variables, mass and related quantities, energy conditions, equilibrium of forces, adiabatic index and Herrera’s cracking concept. The energy conditions are fulfilled ensuring the compatibility of assumed matter and geometry of quark stars. It is also found that all compact star candidates exhibit stable structures corresponding to the proposed values of the model parameters. Hence, the considered [Formula: see text] model shows consistency with all the physical conditions and presents a viable study to the nature of anisotropic massive stellar system.

Quark stars with 2.6 M_odot in a non-minimal geometry-matter coupling theory of gravity

This work analyses the hydrostatic equilibrium configurations of strange stars in a non-minimal geometry-matter coupling (GMC) theory of gravity. Those stars are made of strange quark matter, whose distribution is governed by the MIT equation of state. The non-minimal GMC theory is described by the following gravitational action: f(R,L)=R/2+L+sigma RL, where R represents the curvature scalar, L is the matter Lagrangian density, and sigma is the coupling parameter. When considering this theory, the strange stars become larger and more massive. In particular, when sigma =50 km^2, the theory can achieve the 2.6 M_odot , which is suitable for describing the pulsars PSR J2215+5135 and PSR J1614-2230, and the mass of the secondary object in the GW190814 event. The 2.6 M_odot is a value hardly achievable in General Relativity, even considering fast rotation effects, and is also compatible with the mass of PSR J0952-0607 (M = 2.35 pm 0.17 ~M_odot ), the heaviest and fastest pulsar in the disk of the Milky Way, recently measured, supporting the possible existence of strange quark matter in its composition. The non-minimal GMC theory can also give feasible results to describe the macroscopical features of strange star candidates.

Palatini formulation of the conformally invariant fleft( R,L_mright) gravity theory

We investigate the field equations of the conformally invariant models of gravity with curvature-matter coupling, constructed in Weyl geometry, using the Palatini formalism. We consider the case in which the Lagrangian is given by the sum of the square of the Weyl scalar, the strength of the field associated to the Weyl vector, and a conformally invariant geometry-matter coupling term, constructed from the matter Lagrangian and the Weyl scalar. After substituting the Weyl scalar in terms of its Riemannian counterpart, the quadratic action is defined in Riemann geometry and involves a nonminimal coupling between the Ricci scalar and the matter Lagrangian. For the sake of generality, a more general Lagrangian, in which the Weyl vector is nonminimally coupled with an arbitrary function of the Ricci scalar, is also considered. By varying the action independently with respect to the metric and the connection, the independent connection can be expressed as the Levi-Civita connection of an auxiliary Ricci scalar- and Weyl vector-dependent metric, which is related to the physical metric by means of a conformal transformation. The field equations are obtained in both the metric and the Palatini formulations. The cosmological implications of the Palatini field equations are investigated for three distinct models corresponding to different forms of the coupling functions. A comparison with the standard Lambda CDM model is also performed, and we find that the Palatini type cosmological models can give an acceptable description of the observations.

Impact of Rastall Gravity on Mass, Radius, and Sound Speed of the Pulsar PSR J0740+6620

Millisecond pulsars are perfect laboratories to test possible matter–geometry coupling and its physical implications in light of recent Neutron Star Interior Composition Explorer (NICER) observations. We apply Rastall field equations of gravity, where matter and geometry are nonminimally coupled, to Krori–Barua interior spacetime whereas the matter source is assumed to be anisotropic fluid. We show that all physical quantities inside the star can be expressed in terms of Rastall, ϵ, and compactness, C = 2GM/Rc 2, parameters. Using NICER and X-ray Multi-Mirror Newton X-ray-observational constraints on the mass and radius of the pulsar PSR J0740+6620, we determine the Rastall parameter to be at most ϵ = 0.041 in the positive range. The obtained solution provides a stable compact object; in addition the squared sound speed does not violate the conjectured sound speed unlike the general relativistic treatment. We note that no equations of state are assumed; the model however fits well with linear patterns with bag constants. In general, for ϵ > 0, the theory predicts a slightly larger-size star in comparison to general relativity for the same mass. This has been explained as an additional force, due to matter–geometry coupling, in the hydrodynamic equilibrium equation, which contributes to partially diminishing the gravitational force effect. Consequently, we calculate the maximal compactness as allowed by the strong energy condition to be C = 0.735, which is ∼2% higher than general relativity prediction. Moreover, for the surface density at saturation nuclear density ρ nuc = 2.7 × 1014 g cm−3, we estimate the maximum mass M = 4M ⊙ at radius R = 16 km.

Charged quark stars in f(R,T) gravity* *JMZP acknowledges financial support from the PCI program of the Brazilian agency "Conselho Nacional de Desenvolvimento Científico e Tecnológico"–CNPq. T. Tangphati was supported by King Mongkut's University of Technology Thonburi's Post-doctoral Fellowship. A. Pradhan thanks IUCCA, Pune, India for providing facilities under associateship programmes

Recent advances in nuclear theory and new astrophysical observations have led to the need for specific theoretical models applicable to dense-matter physics phenomena. Quantum chromodynamics (QCD) predicts the existence of non-nucleonic degrees of freedom at high densities in neutron-star matter, such as quark matter. Within a confining quark matter model, which consists of homogeneous, neutral 3-flavor interacting quark matter with corrections, we examine the structure of compact stars composed of a charged perfect fluid in the context of gravity. The system of differential equations describing the structure of charged compact stars has been derived and numerically solved for a gravity model with . For simplicity, we assumed that the charge density is proportional to the energy density, namely, . It is demonstrated that the matter-geometry coupling constant β and charge parameter α affect the total gravitational mass and the radius of the star.

Non-trivial class of anisotropic compact stellar model in Rastall gravity

We investigated Rastall gravity, for an anisotropic star with a static spherical symmetry, whereas the matter-geometry coupling as assumed in Rastall Theory (RT) is expected to play a crucial role differentiating RT from General Relativity (GR). Indeed, all the obtained results confirm that RT is not equivalent to GR, however, it produces same amount of anisotropy as GR for static spherically symmetric stellar models. We used the observational constraints on the mass and the radius of the pulsar Her X-1 to determine the model parameters confirming the physical viability of the model. We found that the matter-geometry coupling in RT allows slightly less size than GR for a given mass. We confirmed the model viability via other twenty pulsars’ observations. Utilizing the strong energy condition we determined an upper bound on compactness U_text {max}sim 0.603, in agreement with Buchdahl limit, whereas Rastall parameter epsilon =-0.1. For a surface density compatible with a neutron core at nuclear saturation density the mass-radius curve allows masses up to 3.53 M_odot . We note that there is no equation of state is assumed, however the model fits well with linear behaviour. We split the twenty pulsars into four groups according to the boundary densities. Three groups are compatible with neutron cores while one group fits perfectly with higher boundary density 8times 10^{14}hbox {g/cm}^3 which suggests that those pulsars may have quark-gluon cores.

Effect of arbitrary matter-geometry coupling on thermodynamics in f(R) theories of gravity

In this paper, the thermodynamics of the Friedmann–Lemaître–Robertson–Walker universe have been explored in f(R) theories of gravity with arbitrary matter-geometry coupling. The equivalence between the modified Friedmann equations with any spatial curvature and the first law of thermodynamics is confirmed, where the assumption of the entropy plays a crucial role. Then laws of thermodynamics in our considering case are obtained. They can reduce to the ones given in Einstein’s general theory of relativity under certain conditions. Moreover, a particular model is investigated through the obtained generalized second law of thermodynamics with observational results of cosmographic parameters.

Effects of non-minimal matter-geometry coupling on embedding class-one anisotropic solutions

This paper investigates some particular anisotropic star models in gravity, where . We adopt a standard model , where ϖ indicates a coupling constant. We take spherically symmetric spacetime and develop solutions to the modified field equations corresponding to different choices of the matter Lagrangian by applying ‘embedding class-one’ scheme. For this purpose, we utilize bag model equation of state and investigate some physical aspects of compact models such as RXJ 1856-37, 4U 1820-30, Cen X-3, SAX J 1808.4-3658 and Her X-I. We use masses and radii of these stars and employ the vanishing radial pressure condition at the boundary to calculate the value of their respective bag constant . Further, we fix ϖ = ± 4 to analyze the behavior of resulting state variables, anisotropy, mass, compactness, surface redshift as well as energy bounds through graphical interpretation for each star model. Two different physical tests are performed to check the stability of the developed solutions. We conclude that ϖ = −4 is more suitable choice for the considered modified model to obtain stable structures of the compact bodies.

Anisotropic stars of class one space–time in [formula omitted] gravity under the simplest linear functional of the matter–geometry coupling

In the present paper, we have explored the existence of compact structures describing anisotropic matter distributions within the framework of f(R,T) theory, where R is Ricci scalar and T is a trace of energy–momentum tensor. We have chosen f(R,T) as a linear function of R and T, i.e., f(R,T)=R+2χT with χ as matter–geometry coupling constant. We employed the embedding class one technique in order to obtain a full space–time description inside the stellar configuration. After specifying space–time geometry, we determined the complete solution of modified field equations by using the MIT-Bag model equation of state, i.e., pr=13(ρ−4Bg) that describes the strange quark matter distribution inside the stellar system, where Bg represents a bag constant. We checked the physical viability of the solution through different physical tests. Moreover, using observed mass values for the various strange star candidates, we have predicted the exact radii by taking distinct values for χ and Bg. Accordingly, we thoroughly show that with decreasing the value of χ, the stellar systems under examinations become progressively massive and larger in size, transforming them into less dense compact objects. It additionally uncovers that for χ<0 the fR,T gravity arises as an adequate theory for explaining the magnetars, super-Chandrasekhar stars and massive pulsars, which are assumed to be the reason behind the overluminous SNeIa e.g. SN 2003fg, SN 2006gz, SN 2007if, SN 2009dc and remain mostly unexplained in the background of general relativity.

Testing an inflationary era in curvature–matter coupling gravity

In this paper, we investigate the cosmological inflation in the context of a minimal matter–geometry coupling, which is based on the [Formula: see text] gravity theory, where [Formula: see text] is a generic function of the curvature scalar [Formula: see text] and the trace [Formula: see text] of the energy–momentum tensor. Assuming that the slow-roll inflation conditions hold true in [Formula: see text] gravity, we obtain the various inflation-related observables such as the tensor-to-scalar ratio [Formula: see text], the scalar spectral index [Formula: see text], the running [Formula: see text] of the spectral index and the tensor spectral [Formula: see text] for two specific [Formula: see text] models from the Hubble slow-roll parameters. To observe the viability of these models, a numerical analysis of such parameters has been done. The results showed that, by using the various values of free parameters, it is possible to obtain a viable compatible with the observational data.

Bouncing universe models in an extended gravity theory

Some bouncing models are investigated in the framework of an extended theory of gravity. The extended gravity model is a simple extension of the General Relativity where an additional matter geometry coupling is introduced to account for the late time cosmic speed up phenomena. The dynamics of the models are discussed in the background of a flat FRW universe. Some viable models are reconstructed for specifically assumed bouncing scale factors. The behavior of the models are found to be decided mostly by the parameters of the respective models. The extended gravity based minimal matter-geometry coupling parameter has a role to remove the omega singularity occurring at the bouncing epoch. It is noted that the constructed models violate the energy conditions, however, in some cases this violation leads to the evolution of the models in phantom phase. The stability of the models are analyzed under linear homogeneous perturbations and it is found that, near the bounce, the models show instability but the perturbations decay out smoothly to provide stable models at late times.

Geodesic deviation, Raychaudhuri equation, Newtonian limit, and tidal forces in Weyl-type f(Q,\xa0T) gravity

We consider the geodesic deviation equation, describing the relative accelerations of nearby particles, and the Raychaudhuri equation, giving the evolution of the kinematical quantities associated with deformations (expansion, shear and rotation) in the Weyl-type f(Q, T) gravity, in which the non-metricity Q is represented in the standard Weyl form, fully determined by the Weyl vector, while T represents the trace of the matter energy–momentum tensor. The effects of the Weyl geometry and of the extra force induced by the non-metricity–matter coupling are explicitly taken into account. The Newtonian limit of the theory is investigated, and the generalized Poisson equation, containing correction terms coming from the Weyl geometry, and from the geometry matter coupling, is derived. As a physical application of the geodesic deviation equation the modifications of the tidal forces, due to the non-metricity–matter coupling, are obtained in the weak-field approximation. The tidal motion of test particles is directly influenced by the gradients of the extra force, and of the Weyl vector. As a concrete astrophysical example we obtain the expression of the Roche limit (the orbital distance at which a satellite begins to be tidally torn apart by the body it orbits) in the Weyl-type f(Q, T) gravity.

Bouncing scenario of general relativistic hydrodynamics in extended gravity

In this paper, we have framed bouncing cosmological model of the Universe in the presence of general relativistic hydrodynamics in an extended theory of gravity. The metric assumed here is the flat Friedmann–Robertson–Walker space–time and the stress energy tensor is of perfect fluid. Since general relativity (GR) has certain issues with late time cosmic speed up phenomena, here we have introduced an additional matter geometry coupling that described the extended gravity to GR. The dynamical parameters are derived and analyzed. The dynamical behavior of the equation of state parameter has been analyzed. We have observed that the bouncing behavior is mostly controlled by the coupling parameter.