We study the influence of higher curvature effects on stellar structure and conclude that the properties of stars are greatly impacted when such terms are dynamic. In particular the surface gravitational redshift which is connected to the equation of state and also the mass-radius ratio differs greatly from the corresponding values in general relativity as evidenced through our empirical comparisons. A model of a superdense star with strange star equation of state is constructed within the framework of the Einstein--Gauss--Bonnet theory. Under these assumptions large classes of solutions are admitted by the field equations. We isolate a particular class with the ansatz of the Vaidya--Tikekar superdense star spatial gravitational potential. The model is found to satisfy elementary requirements for physical applicability and stability. The parameter values chosen are consistent with observed star models. A significant effect of the higher curvature terms is to reduce the speed of sound and to drastically reduce the values of the surface gravitational redshift compared to the Einstein counterpart. These latter results have implications for interpretations of observations in relativistic astrophysics which are often made against the background of the standard general theory of relativity.
Disavowed by one its fathers, ill defined, never empirically tested, the Wheeler-DeWitt equation has nevertheless had a powerful influence on fundamental physics. A well deserved one.
Maximum mass limit of strange stars in quadratic curvature-matter coupled gravity
IUCAA Centre for Astronomy Research and Development (ICARD), Department of Physics, Cooch Behar Panchanan Barma University（印度天体物理中心天文研究与发展中心（ICARD），库奇比哈尔潘查南·巴尔马大学物理系）；Faculty of Symbiotic Systems Science, Fukushima University（福岛大学共生系统科学学院）
In this paper, we study the physical characteristics of anisotropic spherically symmetric quark star candidates for $R+2σT$ gravity model, where $R$, $σ$ and $T$ depict scalar curvature, coupling parameter, and the trace of the energy-momentum tensor, respectively. In order to analyze the structure formation of quark stars, we consider the Heintzmann solution and assume that strange quark matter is characterized by MIT bag model equation of state. We evaluate the unknown parameters through matching conditions and obtain the values of radii of strange quark stars using modified Tolman-Oppenheim…
In this paper, we study the existence of strange star in the background of f (T) modified gravity where T is a scalar torsion. In KB metric space, we derive the equations of motion using anisotropic property within the spherically strange star with modified chaplygin gas in the framework of modified f (T) gravity. Then we obtain many physical quantities to describe the physical status such as anisotropic behavior, energy conditions and stability. By the matching condition, we calculate the unknown parameters to evaluate the numerical values of mass, surface redshift etc. from our model to make…
In this work, we have first obtained the hydrostatic equilibrium equation in dilaton gravity. Then we have examined some of the structural characteristics of strange quark star in dilaton gravity with a background of Einstein gravity. We have shown that the variations of dilaton parameter do not affect the maximum mass of quark star, while the variations of the cosmological constant lead to change in the structural characteristics of the quark star. We have investigated the stability of strange quark stars that studied by MIT Bag model, in dilaton gravity. We have also provided limiting values…
In this work, we study the existence of strange star in the background of $f(\mathbb{T},\mathcal{T})$ gravity in the Einstein spacetime geometry, where $\mathbb{T}$ is the torsion tensor and $\mathcal{T}$ is the trace of the energy-momentum tensor. The equations of motion are derived for anisotropic pressure within the spherically symmetric strange star. We explore the physical features like energy conditions, mass-radius relations, modified TOV equations, principal of causality, adiabatic index, redshift and stability analysis of our model. These features are realistic and appealing to furthe…
The compact stars are promising candidates associated with the generation of gravitational waves (GWs). In this work, we study a special type of compact stars known as strange stars in the $f(\mathcal{R})$ gravity Palatini formalism. Here we consider three promising $f(\mathcal{R})$ gravity models viz., Starobinsky, Hu-Sawicki and Gogoi-Goswami models in the domain of MIT Bag model and linear equations of state (EoSs). We compute the stellar structures numerically and constrained the $f(\mathcal{R})$ model parameters with a set of probable strange star candidates. The study shows that the cons…
We study the electromagnetic spectrum of accretion disks around neutron and strange stars in $\mathcal{R}^2$ gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50\% for the electromagnetic flux and about 20\% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Kelperian velocity and with equal masses.
We studied plane symmetric cosmological model in the presence of quark and strange quark matter with the help of f(R,T) theory. To decipher solutions of Plane symmetric space-time, we used power law relation between scale factor and deceleration parameter. We considered the special law of variation of Hubble's parameter proposed by Berman (Nuovo Cimento B 74, 182, 1983) which yields constant deceleration parameter. We also discussed the physical behavior of the solutions by using some physical parameters.
This work is devoted on the recently introduced Einstein-Gauss-Bonnet gravity in four dimensions. The theory can bypass the Lovelock's theorem and avoids Ostrogradsky instability. The integrated part of this theory is the GB term gives rise to a non-trivial contribution to the gravitational dynamics in the limit $D \to 4$. Our main interest is to explore a class of static and spherically symmetric compact objects made of strange matter in the color flavor locked (CFL) phase, in which all three quark flavors participate to pairing symmetrically. It is therefore natural to ask for compact object…
In the present work we study strange stars in $f(R)$ theories of gravity in the Palatini formalism. We consider two concrete well-known cases, namely the $R+R^2/(6 M^2)$ model as well as the $R-μ^4/R$ model for two different values of the mass parameter $M$ or $μ$. We integrate the modified Tolman-Oppenheimer-Volkoff equations numerically, and we show the mass-radius diagram for each model separately. The standard case corresponding to the General Relativity is also shown in the same figure for comparison. Our numerical results show that the interior solution can be vastly different depending …
In the present paper we investigate self-consistently slowly rotating neutron and strange stars in R-squared gravity. For this purpose we first derive the equations describing the structure of the slowly rotating compact stars in $f(R)$-gravity and then simultaneously solve the exterior and the interior problem. The structure of the slowly rotating neutron stars is studied for two different hadronic equations of state and a strange matter equation of state. The moment of inertia and its dependence on the stellar mass and the $R$-squared gravity parameter $a$ is also examined in details. We fin…
The existence of strange matter in compact stars may pose striking sequels of the various physical phenomena. As an alternative to neutron stars, a new class of compact stars called strange stars should exist if the strange matter hypothesis is true. In the present article, we investigate the possible construction of the strange stars in quark matter phases based on the MIT bag model. We consider scenarios in which strange stars have no crusts. Then we apply two types of equations of state to quantify the mass-radius diagram for static strange star models performing the numerical calculation t…
We study the oscillations of neutron and strange stars in $R^2$ gravity. More precisely the nonradial $f$-modes are examined and the differences with pure general relativity are investigated. Using these results we build several gravitational wave asteroseismology relations. Our goal is to determine up to what extend these relations are equation of state independent and whether they deviate enough from general relativity in order to produce an observable effect. The results show that the differences coming from $R^2$ gravity are up to 10\% and that will be difficult to be observed in the near …
This paper investigates the existence and properties of anisotropic strange quark stars in the context of massive Brans-Dicke theory. The field equations are constructed in Jordan frame by assuming a suitable potential function with MIT bag model. We employ the embedding class-one approach as well as junction conditions to determine the unknown metric functions. Radius of the strange star candidate, LMC X-4, is predicted through its observed mass for different values of the bag constant. We analyze the effects of coupling parameter as well as mass of scalar field on state determinants and exec…
When gravity is quantum, the point structure of space-time should be replaced by a non-commutative geometry. This is true even for quantum gravity in the infrared. Using the octonions as space-time coordinates, we construct a pre-spacetime, pre-quantum Lagrangian dynamics. We show that the symmetries of this non-commutative space unify the standard model of particle physics with $SU(2)_R$ chiral gravity. The algebra of the octonionic space yields spinor states which can be identified with three generations of quarks and leptons. The geometry of the space implies quantisation of electric charge…
For the accurate understanding of compact objects such as neutron stars and strange stars, the Tolmann-Openheimer-Volkof (TOV) equation has proved to be of great use. Hence, in this work, we obtain the TOV equation for the energy-momentum-conserved $f(R,T)$ theory of gravity to study strange quark stars. The $f(R,T)$ theory is important, especially in cosmology, because it solves certain incompleteness of the standard model. In general, there is no intrinsic conservation of the energy-momentum tensor in the $f(R,T)$ gravity. Since this conservation is important in the astrophysical context, we…
In the current article, we study anisotropic spherically symmetric strange star under the background of $f(R,T)$ gravity using the metric potentials of Tolman-Kuchowicz type~\cite{Tolman1939,Kuchowicz1968} as $λ(r)=\ln(1+ar^2+br^4)$ and $ν(r)=Br^2+2\ln C$ which are free from singularity, satisfy stability criteria and also well behaved. We calculate the value of constants $a$, $b$, $B$ and $C$ using matching conditions and the observed values of the masses and radii of known samples. To describe the strange quark matter (SQM) distribution, here we have used the phenomenological MIT bag model e…
We study relativistic non-rotating stars in the framework of Lovelock gravity. In particular, we consider the Gauss-Bonnet term in a five-dimensional spacetime, and we investigate the impact of the Gauss-Bonnet parameter on properties of the stars, both isotropic and anisotropic. For matter inside the star, we assume a relativistic gas of de-confined massless quarks. We integrate the modified Tolman-Oppenheimer-Volkoff equations numerically, and we obtain the mass-to-radius profile, the compactness of the star as well as the gravitational red-shift for several values of the Gauss-Bonnet parame…
According to various models, the orbital and the epicyclic frequencies of particles moving on a circular orbit around compact objects are related to the quasi-periodic oscillations observed in the X-ray flux of some pulsars or black hole candidates. It is expected that they originate from the inner edge of the accretion discs, deep into the gravitational field of the compact objects. Considering the planned new generation X-ray timing observatories with large collective areas, the quasi-periodic oscillations might be an excellent tool for testing gravity in strong field regime and respectively…
In the present work, we study about highly dense compact stars which are made of quarks, specially strange quarks, adopting the Krori-Barua (KB)~\cite{Krori1975} metric in the framework of $f(R,T)$ gravity. The equation of state (EOS) of a strange star can be represented by the MIT bag model as $p_r(r)=\frac{1}{3}[ρ(r)-4B_g]$ where $B_g$ is the bag constant, arises due to the quark pressure. Main motive behind our study is to find out singularity free and physically acceptable solutions for different features of strange stars. Here we also investigate the effect of alternative gravity in the f…
Gravitational wave echoes as probes of the maximum mass of strange stars in quadratic curvature-matter coupled gravity
IUCAA Centre for Astronomy Research and Development (ICARD), Department of Physics, Cooch Behar Panchanan Barma University（印度大学天体物理联盟天文研究与发展中心（ICARD），库奇比哈尔潘查南巴玛大学物理系）；Faculty of Symbiotic Systems Science, Fukushima University（福岛大学共生系统科学学院）
In this paper, we have studied homogeneous and anisotropic, locally rotationally symmetric (LRS) Bianchi type-I universe model with magnetized strange quark matter (MSQM) distribution and cosmological constant $Λ$ in $f(R,T)$ gravity. The exact solutions of the field equations are obtained under bilinear and special form of time varying deceleration parameter (DP). Firstly we have considered two specific form of bilinear DP with a single parameter of the form: $q=\frac{α(1-t)}{1+t}$ and $q=-\frac{αt}{1+t}$, which leads to a function with constant or linear nature with respect to the choice of …
The properties of dark matter admixed strange quark stars are investigated in the Starobinsky model of modified gravity. For quark matter we assume the MIT bag model, while self-interacting dark matter inside the star is modelled as a Bose-Einstein condensate with a polytropic equation of state. We numerically integrate the structure equations in the Einstein frame adopting the two-fluid formalism treating the curvature correction term non-perturbatively. Our findings show that strange quark stars (in agreement with current observational constraints) with the highest masses are equally affecte…
We study a specific model of anisotropic strange stars in the modified $f\left(R,\mathcal{T}\right)$-type gravity by deriving solutions to the modified Einstein field equations representing a spherically symmetric anisotropic stellar object. We take a standard assumption that $f(R,\mathcal{T})=R+2χ\mathcal{T}$, where $R$ is Ricci scalar, $\mathcal{T}$ is the trace of the energy-momentum tensor of matter, and $χ$ is a coupling constant. To obtain our solution to the modified Einstein equations, we successfully apply the `embedding class 1' techniques. We also consider the case when the strange …
In present paper a new compact star model in f(R,T) gravity is obtained where R and T denote the Ricci scalar and the trace of energy-momentum tensor $T_{μν}$ respectively. To develop the model we consider the spherically symmetric space-time along with anisotropic fluid distribution in presence of electric field with $f(R,T) = R + 2γT$ where $γ$ is a small positive constant. We have used the Chaplygin equation of state to explore the stellar model. The field equations for $f(R,T)$ gravity have been solved by employing the Krori-Barua ansatz already reported in literature [J. Phys. A, Math. Ge…
In this article we try to present spherically symmetric isotropic strange star model under the framework of $f(R,\mathcal{T})$ theory of gravity. To this end, we consider that the Lagrangian density is an arbitrary linear function of the Ricci scalar $R$ and the trace of the energy momentum tensor~$\mathcal{T}$ given as $f\left(R,\mathcal{T}\right)=R+2χT$. We also assume that the quark matter distribution is governed by the simplest form of the MIT bag model equation of state (EOS) as $p=\frac{1}{3}\left(ρ-4B\right)$, where $B$ is the bag constant. We have obtained an exact solution of the mod…
We obtain a class of new anisotropic relativistic solution in Einstein Gauss-Bonnet (EGB) gravity with Finch-Skea metric in hydrostatic equilibrium. The relativistic solutions are employed to construct anisotropic stellar models for strange star with MIT Bag equation of state $ p_{r}= \frac{1}{3} \left( ρ- 4 B_{g}\right)$, where $B_{g}$ is the Bag constants. Considering the mass and radius of a known star PSR J0348+0432 we construct stellar models in the framework of higher dimensions. We also predict the mass and radius of stars for different model parameters. The Gauss-Bonnet coupling term (…
We discuss a recently proposed limiting curvature theory of gravity and its application to the problem of singularities inside black holes. In this theory the growth of the curvature is suppressed by specially chosen inequality constraints included in the gravity action. We consider a case of a spherically symmetric four dimensional black hole and demonstrated that imposed curvature constraints modify a solution in the black hole interior. Instead of forming the curvature singularity the modified metric describes a space which is exponentially expanding in one direction and oscillating the oth…
Our present study involves the strange stars model in the framework of $f(R,T)$ theory of gravitation. We have taken a linear function of the Ricci scalar $R$ and the trace $T$ of the stress-energy tensor $T_{μν}$ for the expression of $f(R,T)$, i.e., $f(R,T)=R+ 2 γT $ to obtain the proposed model, where $γ$ is a coupling constant. Moreover, to solve the hydrostatic equilibrium equations, we consider a linear equation of state between the radial pressure $p_r$ and matter density $ρ$ as $p_r=αρ-β$, where $α$ and $β$ are some positive constants, Both $α,\,β$ depend on coupling constant $γ$ which…
We compute the entropies for general curvature squared gravities in arbitrary dimensions using the conserved charge and Virasoro algebra from surface term. We introduce an auxiliary tensor field in order to obtain the boundary action which establish a variational principle. Our results reproduce those in some specific models, that is, the Gauss-Bonnet, new massive gravity (NMG), and critical gravity.
We compute curvature-dependent graviton correlation functions and couplings as well as the full curvature potential $f(R)$ in asymptotically safe quantum gravity coupled to scalars. The setup is based on a systematic vertex expansion about metric backgrounds with constant curvatures initiated in arXiv:1711.09259 for positive curvatures. We extend these results to negative curvature and investigate the influence of minimally coupled scalars. The quantum equation of motion has two solutions for all accessible numbers of scalar fields. We observe that the solution at negative curvature is a minim…
The zeroth law of black hole mechanics is an assertion of constancy of the surface gravity on a stationary Killing horizon. The Hawking temperature of the black hole horizon is proportional to the surface gravity. Therefore, the constancy of the surface gravity is reminiscent of the zeroth law of ordinary thermodynamics. In this work, we provide a proof of the zeroth law in Lanczos-Lovelock theories of gravity, where the Einstein Hilbert action is supplemented by higher curvature terms.
The present work is focused on the investigation of the existence of compact structures describing anisotropic matter distributions within the framework of modified gravity theories, specifically f(R,$\mathcal{T}$) gravity theory. Additionally, we have taken f(R,$\mathcal{T}$) as a linear function of the Ricci scalar $R$ and the trace of the energy-momentum tensor $\mathcal{T}$ as $f(R,\mathcal{T})$=$R+2χ\mathcal{T}$,where $χ$ is a dimensionless coupling parameter, and the Lagrangian matter $\mathcal{L}_m=-\frac{1}{3}\left(2p_{t}+p_{r}\right)$, to describe the complete set of field equations f…
We construct not only an induced gravity model with the restricted diffeomorphisms, that is, transverse diffeomorphisms which preserves the curvature density, but also that with the full diffeomorphisms. By solving the equations of motion, it turns out that these models produce Einstein's equations with a certain Newton's constant in addition to the constraint for the curvature density. In the limit of the infinite Newton constant, the models give rise to induced gravity. Moreover, we discuss cosmological solutions on the basis of the gravitational models at hand.
This thesis explores two avenues into understanding the physics of black holes and horizons beyond general relativity, via analogue models and Lorentz violating theories. Analogue spacetimes have wildly different dynamics to general relativity; this allows the possibility of non-Killing horizons in stationary solutions. In the case of non-Killing horizons different definitions of surface gravity are truly different quantities. This also has application to modified theories of gravity, where there is no reason to expect all horizons to be Killing horizons. In Lorentz violating theories, the sit…
We extract exact charged black-hole solutions with flat transverse sections in the framework of D-dimensional Maxwell-f(T) gravity, and we analyze the singularities and horizons based on both torsion and curvature invariants. Interestingly enough, we find that in some particular solution subclasses there appear more singularities in the curvature scalars than in the torsion ones. This difference disappears in the uncharged case, or in the case where f(T) gravity becomes the usual linear-in-T teleparallel gravity, that is General Relativity. Curvature and torsion invariants behave very differen…
We derive the field equations for topologically massive gravity coupled with the most general quadratic curvature terms using the language of exterior differential forms and a first order constrained variational principle. We find variational field equations both in the presence and absence of torsion. We then show that spaces of constant negative curvature (i.e. the anti de-Sitter space $AdS_3$) and constant torsion provide exact solutions.
Recently, in the Einstein gravity, Majhi and Padmanabhan proposed a straightforward and transparent way in obtaining the Bekenstein-Hawking entropy by using the approach based on the Virasoro algebra and central charge. In this work, we generalize their approach to the modified gravity with higher curvature corrections and show that their approach can successfully lead to the corresponding Wald entropy in the higher curvature gravity. Our result shows that the approach is physically general.
A low-energy perturbation theory is developed from the nonperturbative framework of covariant Loop Quantum Gravity (LQG) by employing the background field method. The resulting perturbation theory is a 2-parameter expansion in the semiclassical and low-energy regime. The two expansion parameters are the large spin and small curvature. The leading order effective action coincides with the Einstein-Hilbert action. The subleading corrections organized by the two expansion parameters give the modifications of Einstein gravity in quantum and high-energy regime from LQG. The perturbation theory deve…
In this paper we describe a model of a four-dimensional spherically symmetric black hole in a limiting curvature theory of gravity. In this theory the Einstein-Hilbert action is modified by adding to the action terms providing inequality constraints on chosen curvature invariants. We demonstrated that in such a model, instead of formation of the curvature singularity, the spacetime remains regular in the black hole interior. For black holes with gravitational radius much larger than the radius $\ell$ of the critical curvature the obtained solutions describe a space exponentially expanding in o…
By generalizing different recent works to the context of higher curvature gravity, we provide a unifying framework for three related results: (i) If an asymptotically AdS spacetime computes the entanglement entropies of ball-shaped regions in a CFT using a generalized Ryu-Takayanagi formula up to second order in state deformations around the vacuum, then the spacetime satisfies the correct gravitational equations of motion up to second order around AdS; (ii) The holographic dual of entanglement entropy in higher curvature theories of gravity is given by Wald entropy plus a particular correctio…
We consider the effects of higher curvature terms on a holographic dual description of boundary conformal field theory. Specifically, we consider three-dimensional gravity with a specific combination of Ricci tensor square and curvature scalar square, so called, new massive gravity. We show that a boundary entropy and an entanglement entropy are given by similar expression with those of the Einstein gravity case when we introduce an {\it effective} Newton's constant and an {\it effective} cosmological constant. We also show that the holographic g-theorem still holds in this extension, and we g…
We provide new formulas for the energy of black hole solutions in the anti-de Sitter (AdS) sector of a generic gravity theory with curvature-squared terms. This is achieved by the addition of counterterms of the extrinsic type (\emph{Kounterterms}) which produces a finite variation of the total action. The procedure is compatible with the boundary conditions in asymptotically AdS spaces within the framework of Quadratic Curvature Gravity (QCG) theory. As a consequence, without resorting to background-substraction, or linearized methods, the conserved charges correctly reproduce the energy of s…
We investigate effects of the modified $f(R, \mathcal{T})$ gravity on the charged strange quark stars with the standard choice of $f(R, \mathcal{T})=R+2χ\mathcal{T}$. Those types of stars are supposed to be made of strange quark matter (SQM) whose distribution is governed by the phenomenological MIT bag EOS as $p=\frac{1}{3}(ρ-4B)$, where $B$ is the bag constant, while the form of charge distribution is chosen to be $q\left(r\right)=Q\left(r/R\right)^3=αr^3$ with $α$ as a constant. We derive the values of the unknown parameters by matching the interior spacetime to the exterior Reissner-Nordst…
In this note, we evaluate the Weyl-invariant quadratic curvature tensors for the particular Weyl's gauge field constructed in the $3+1$-dimensional noncompact Weyl-Einstein-Yang-Mills model. We subsequently extend the model to its higher curvature version. Here, we also compute Weyl-invariant extension of topological Gauss-Bonnet term for this specific choice of vector field.
We consider f(R) gravity theories in the presence of a scalar field minimally coupled to gravity with a self-interacting potential. When the scalar field backreacts to the metric we find at large distances scalarized Schwarzschild-AdS and Schwarzschild-AdS-like black hole solutions. At small distances due to strong curvature effects and the scalar dynamis we find a rich structure of scalarized black hole solutions. When the scalar field is conformally coupled to gravity we also find scalarized black hole solutions at small distances.
Building on the recently introduced notion of quantum Ricci curvature and motivated by considerations in nonperturbative quantum gravity, we advocate a new, global observable for curved metric spaces, the curvature profile. It is obtained by integrating the scale-dependent, quasi-local quantum Ricci curvature, and therefore also depends on a coarse-graining scale. To understand how the distribution of local, Gaussian curvature is reflected in the curvature profile, we compute it on a class of regular polygons with isolated conical singularities. We focus on the case of the tetrahedron, for whi…
