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Sudan Hansraj

Publications and source records attributed to Sudan Hansraj.

At least 19 recordsLinked to original sources

Conformal flatness selects a universal isothermal attractor in pure Lovelock gravity

We determine the complete solution set of the conformally flat isotropic pure Lovelock field equations for every order $N\ge2$ and every admissible dimension $d\ge2N+1$. Conformal flatness and pressure isotropy combine into a single identity that factorises exactly, splitting the solutions into two branches. The first is the constant-density Schwarzschild interior, which persists at every order with both metric potentials keeping their Einstein form. The second has no Einstein counterpart and is governed by the single dimension--order parameter $k=(d-2N)/[4(1-N)]$; we obtain its physically admissible orbit in explicit closed parametric form, both potentials included. A phase-space analysis on the projective line shows that no solution of this branch has a pressure-free boundary at finite radius, so only the Schwarzschild branch can describe a bounded star. The second instead loses all memory of its central data and relaxes onto a pure Lovelock isothermal sphere with $ρ\propto r^{-2N}$, a higher-curvature analogue of the singular isothermal halo, which we show to be the attractor of the whole family, approached at the closed-form rate $λ_*=-(d-2)/(d-2N)$ and with limiting equation of state fixed by $(d,N)$ alone. We prove that $p/ρ$ increases monotonically outward there, and that of the two regular-centre orientations only one is admissible. Since $k$ is rational the spatial potential is algebraic of degree $u+v$, where $2k=-u/v$; radical inversion is guaranteed for degree at most four, while four representative cases have full symmetric Galois group.

gr-qc

Static anisotropic stars in Lovelock gravity: Universal closed-form equations

A characteristic-polynomial formulation is already known for static perfect fluids in Lovelock gravity [Phys. Rev. D 113, 084008 (2026)]. We extend it to the complete anisotropic matter sector. For arbitrary dimension and independently coupled curvature orders, we derive closed, summation-free expressions for the density, radial pressure, tangential pressure and anisotropy in terms of a single polynomial \(W\), its derivatives and two derived polynomials \(V\) and \(U\). The dimension-dependent combinatorial coefficients are obtained directly from the generalized-delta antisymmetrisation and independently confirmed through an inductive argument. Imposing pressure isotropy recovers the known total-derivative equation, which we resolve into an explicit operator affine in Lovelock order and linear in dimension. This structure proves that, apart from the branch of vanishing angular sectional curvature, the interior Schwarzschild geometry is the unique isotropic interior common to arbitrary Lovelock couplings. It also identifies the fixed-sign obstruction to bounded pure Lovelock spheres in \(d=2N+1\) with the polynomial identity \(V\equiv0\), and shows how a lower-order term or cosmological term removes this obstruction. Finally, we integrate the resulting arbitrary-order stellar equations with all admissible orders active in \(d=10\) and \(d=11\). For the positive single-parameter hierarchy of Lovelock couplings considered, the general-relativistic sequence topology persists while higher orders increase the maximum mass cumulatively.

gr-qc

Effective Constrained Scalar--Gauss--Bonnet Inflation Motivated by $f(R,\mathcal{G})$ Gravity

We develop an effective framework for inflation in a constrained scalar--Gauss--Bonnet theory motivated by a restricted sector of $f(R,\mathcal{G})$ gravity. Using unified parametrizations of the Hubble expansion rate and the Gauss--Bonnet coupling function within a generalized slow-roll formalism, we derive analytical expressions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$, and study their dependence on the model parameters. We show that the Hubble parametrization mainly controls the scalar sector through the slow-roll parameter $ε_1$, while the Gauss--Bonnet-induced contribution $ε_4$ can significantly affect the scalar tilt and strongly suppress primordial tensor modes, naturally leading to very small values of $r$. A representative benchmark solution yields $n_s \simeq 0.958$ and $r \simeq 2.7 \times 10^{-4}$, marginally compatible with current Planck, ACT, and BICEP/Keck constraints. We further investigate the scalar perturbation structure of the exactly constrained theory, where the Lagrange-multiplier constraint forces the lapse perturbation to vanish and, together with the gravitational momentum constraint, implies $\dot{\mathcal{R}}=0$, eliminating the propagating scalar degree of freedom at linear order. This exact result clarifies that the generalized slow-roll treatment should be interpreted as an effective softly constrained description. We also discuss perturbative stability conditions, including the positivity of the relevant kinetic coefficients and propagation speeds. Our results demonstrate that the effective constrained scalar--Gauss--Bonnet framework provides a flexible and observationally viable description of inflation while clarifying the distinction between the exact constrained limit and its effective slow-roll realization.

physics.gen-ph

Dynamical systems approach to stellar modelling in $f(G, B)$ gravity

The novel proposal to invoke the split of the Ricci scalar into bulk and boundary terms in the gravitational action, opens up a new avenue of investigation into stellar dynamics. The Lagrangian contains functional forms of the bulk term while the boundary term do not contribute to the dynamics. The advantage of the proposition is that the stellar structure equations are up to order two, thus the theory is not haunted by ghosts. We obtain explicitly the defining equations for the thermodynamical variables and the geometry for the pure quadratic case, since the linear case amounts to general relativity. In trying to establish the vacuum geometry associated with the theory it turns out that two possible metrics emerge through the vanishing of the energy-momentum tensor. Next, we analyse the isotropy equation and make the observation that it is autonomous. It is rare that this happens in astrophysical modelling. This behaviour prompted the use of dynamical systems to understand the stability properties of fixed points or invariant submanifolds. It was necessary to choose a gauge in order to split the autonomous equation into a system from which we could plot a phase portrait and deduce the stability of solution trajectories. We find that the invariant submanifolds were generally stable with nearby paths approaching them.

gr-qc

Is dark energy necessary for the sustainability of traversable wormholes?

In the standard approach to studying wormhole geometry, the presence of dark energy is unavoidable to ensure traversability. The dark energy provides the negative gravity effect to keep the throat open. The question we analyse is whether the same can be achieved without dark energy. It turns out that if we couple the trace of energy-momentum with the standard Einstein-Hilbert Lagrangian and utilise a sppecific equation of state then dark energy may be obviated. The Casimir stress energy is known to result in the violation of the null energy condition (NEC) on the energy momentum tensor. This phenomenon makes such an equation of state (EoS) an ideal candidate to generate traversable wormhole (WH) geometries. The laboratory proven phenomenon provides a mechanism to sustain an open WH throat without having to appeal to dark energy. We generate two classes of WH solutions with this in $f(R,T)$ gravity theory, where $R$ represents the Ricci scalar and $T$ is the trace of the stress-energy tensor. For the background geometry we choose a static and spherically symmetric metric, and derive the field equations for exact WH solutions. For the specific choice of the Casimir EoS relating the energy-momentum tensor components [ Kar and Sahdev: Phys. Rev D {\bf 52} 2030 (1995)] and different choices for redshift functions, we determine the WH geometry completely. The obtained WH solutions violate the NECs and all qualitative constraints demanded of physically realisable WHs are satisfied. This is demonstrated via graphical plots for a suitably chosen range of values of the $f(R,T)$ coupling parameter. Furthermore, our study involved an investigation into the repulsive effect of gravity, which revealed that its presence leads to a negative deflection angle for photons traveling along null geodesics.

gr-qc

Generating exact polytropes in non-conservative unimodular geometries

The trace-free Einstein equations contain one equation less than the complete field equations. In a static and spherically symmetric spacetime, the number of field equations is thus reduced to two. The equation of pressure isotropy of general relativity, however, is preserved thus showing that any known perfect fluid spacetime is a suitable candidate for the trace-free scenario. The extra freedom in imposing two constraints may now be exploited to include polytopes, something that is difficult in general relativity. The point here is that using any known exact solution one can find a polytropic star for various values of the polytropic index. One arrives at Tolman-Oppenheimer-Volkoff type equations and can study their solutions explicitly. Two examples of well-known stellar distributions that generate polytropes with physically reasonable behaviour are discussed. These models are regular, exhibit a sound speed that is never superluminal and are adiabatically stable in the sense of Chandrasekhar. We investigate a compactness measure confirming that our results are consistent with some observational data.

gr-qc

The criteria of the anisotropic quark star models in Rastall gravity

Quark stars are terrestrial laboratories to study fundamental physics at ultrahigh densities and temperatures. In this work, we investigate the internal structure and the physical properties of quark stars (QSs) in the Rastall gravity. Rastall gravity is considered a non-conservative theory of gravity, which is an effective gravity theory at high energy density, e.g., relevant to the early universe and dense, compact objects. We derive the hydrostatic equilibrium structure for QSs with the inclusion of anisotropic pressure. More specifically, we find the QS mass-radius relations for the MIT bag model. We focus on the model depending on the Rastall free parameter $η$ and examine the deviations from the General Relativity (GR) counterparts.

gr-qc

Physical Implications of Pure Lovelock Geometry on Stellar Structure

We construct an exact anisotropic star model with a linear barotropic equation of state and with Finch-Skea potential within the framework of pure Lovelock gravity. A comparison with the corresponding Einstein model in a suitable limit is easily deduced. Evidently higher curvature effects induced by the Lovelock contributions generate lower densities, pressures, surface tensions and anisotropy factors when compared to its Einstein counterpart. The maximum moment of inertia is attained for the Einstein case and hence it may be inferred that Lovelock effects soften the equation of state. The model satisfies various stability tests.

gr-qc

Gravitationally decoupled strange star model beyond standard maximum mass limit in Einstein-Gauss-Bonnet gravity

The recent theoretical advance known as the Minimal Geometric Deformation (MGD) method has initiated a renewed interest in investigating higher curvature gravitational effects in relativistic astrophysics. In this work, we model a strange star within the context of Einstein-Gauss-Bonnet gravity with the help of the MGD technique. Starting off with the Tolman metric ansatz together with the MIT Bag model equation of state applicable to hadronic matter, anisotropy is introduced via the superposition of the seed source and the decoupled energy-momentum tensor. The solution of the governing systems of equations bifurcates into two distinct models, namely the mimicking of the $θ$ sector to the seed radial pressure and energy density and a regular fluid model. Each of these models can be interpreted as self-gravitating static, compact objects with the exterior described by the vacuum Boulware-Deser solution. Utilizing observational data for three stellar candidates, viz., PSR J1614-2230, PSR J1903+317, and LMC X-4 we subject our solutions to rigorous viability tests based on regularity and stability. We find that the Einstein-Gauss-Bonnet parameter and the decoupling constant compete against each other for ensuring physically realizable stellar structures. The novel feature of work is the demonstration of stable compact objects with stellar masses in excess of $M= 2 M_{\odot}$ without appealing to exotic matter. The analysis contributes new insights and physical consequences concerning the development of ultra-compact astrophysical entities.

gr-qc

Strange stars in the framework of higher curvature gravity

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.

gr-qc

All Conformally Flat Einstein--Gauss--Bonnet static Metrics

It is known that the standard Schwarzschild interior metric is conformally flat and generates a constant density sphere in any spacetime dimension in Einstein and Einstein--Gauss--Bonnet gravity. This motivates the questions: In EGB does the conformal flatness criterion yield the Schwarzschild metric? Does the assumption of constant density generate the Schwarzschild interior spacetime? The answer to both questions turn out in the negative in general. In the case of the constant density sphere, a generalised Schwarzschild metric emerges. When we invoke the conformal flatness condition the Schwarschild interior solution is obtained as one solution and another metric which does not yield a constant density hypersphere in EGB theory is found. For the latter solution one of the gravitational metrics is obtained explicitly while the other is determined up to quadratures in 5 and 6 dimensions. The physical properties of these new solutions are studied with the use of numerical methods and a parameter space is located for which both models display pleasing physical behaviour.

gr-qc

Isotropic compact stars in 4D Einstein-Gauss-Bonnet gravity

Recently it has been proposed that the Gauss-Bonnet coupling parameter of Lovelock gravity may suitably be rescaled in order to admit physically viable models of celestial phenomena such that higher curvature effects are active in standard four dimensions as opposed to the usual higher dimensions. We investigate the consequences of this modification in the context of stellar modelling. The evolution of perfect fluid distributions is governed by the pressure isotropy condition and through stipulation of one of the metric potentials complete models emerge from solutions of the master differential equation. New classes of exact solution with this approach have been reported. One particular model is analysed in detail and shown to comport with elementary physical requirements demanded of realistic compact stars suggesting that the modified theory is not inconsistent with observations.

gr-qc

Shadow images of Kerr-like wormholes

Investigations of shadows of astrophysical entities constitute a major source of insight into the evolution of compact objects. Such effects depend on the nature of the compact object and arise on account of the strong gravitational lensing that casts a shadow on the bright background. We consider the Kerr-like wormhole spacetime (Phys.\ Rev.\ D 97:024040, 2018), which is a modification of the Kerr black hole that degenerates into wormholes for nonzero values of the deviation parameter $λ^2$. The results suggest that the Kerr spacetime can reproduce far away from the throat of the wormhole. We obtain the shapes of the shadow for the Kerr-like wormholes and discuss the effect of the spin $a$, the inclination angle $θ_0$, and the deviation parameter $λ^2$ on the size and nature of the shadow. As a consequence, it is discovered that the shadow is distorted due to the spin as well as the deviation parameter and the radius of the shadow decreases with $λ^2$ if the ADM mass of the Kerr-like wormholes is considered.

gr-qc

Impact of the Rastall parameter on perfect fluid spheres

We examine the effects of the Rastall parameter on the behaviour of spherically symmetric static distributions of perfect fluid matter. It was claimed by Visser [Physics Letters B, 782, 83, (2018)] that the Rastall proposition is completely equivalent to the Einstein theory. While many authors have raised contrary arguments, our intention is to analyze the properties of Rastall gravity through variation of the Rastall parameter in the context of perfect fluids spheres that may be used to model neutron stars or cold fluid planets. This analysis also serves to counter the claim that Rastall gravity is equivalent to the standard Einstein theory. It turns out that the condition of pressure isotropy is exactly the same as for Einstein gravity and hence that any known solution of the Einstein equations may be used to study the effects of the Rastall dynamical quantities. Moreover, by choosing the well studied Tolman metrics, we discover that in the majority of cases there is substantial deviation from the Einstein case when the Rastall parameter vanishes and in cases where the Einstein model displays defective behaviour, certain Rastall models obey the well known elementary requirements for physical plausibility. These empirical findings do not support the idea that Rastall theory is equivalent to Einstein theory as several deviations in physical behavior are displayed as counter-examples.

gr-qc

Equilibrium stellar configurations in Rastall theory and linear equation of state

Amongst a number of modified theories of gravity, the Rastall theory has been propounded to address some shortcomings of the standard theory of general relativity. Our purpose is to investigate this framework's capacity to analyse stellar structure in the context of elementary requirements for physical plausibility such as positive definite functions for the energy density and pressure, conformity to the causality criterion and the existence of an equation of state. We consider the analogue of the Saslaw \textit{et al} \cite{saslaw} isothermal model of general relativity and show that the Rastall version satisfies the basic requirements unlike its counterpart. Then we examine in turn the consequences of suppressing one of the inverse square law fall off of the energy density or the linear equation of state. In addition, the case of a constant spatial gravitational potential is studied on account of this prescription being a necessary and sufficient condition for isothermal behaviour in Einstein theory and its most general tensor extension Lovelock gravity.

gr-qc

Qualitative analysis of the Tolman metrics within the unimodular framework

We investigate the behaviour of the Tolman metrics within the formalism of the trace-free (or unimodular) gravity. While this approach is similar to the standard Einstein field equations, some subtlety arises. The effective number of independent field equations is reduced by one on account of the density and pressure appearing as an inseparable entity the inertial mass density. Further energy is not conserved within the trace-free theory but the conservation law may be supplemented to the field equations. This presentation of the field equations offers a different avenue to determine the density and pressure explicitly. It turns out that an extra integration constant is always in evidence. While this constant has little impact on the dynamics and energy conditions, it makes a significant impact on the gravitational mass and equation of state. Graphical plots are generated to analyse the behaviour of physical quantities qualitatively.

gr-qc

Spherically symmetric isothermal fluids in $f(R,T)$ gravity

We analyze the isothermal property in static fluid spheres within the framework of the modified $f(R, T)$ theory of gravitation. The equation of pressure isotropy of the standard Einstein theory is preserved however, the energy density and pressure are expressed in terms of both gravitational potentials. Invoking the isothermal prescription requires that the isotropy condition assumes the role of a consistency condition and an exact model generalizing that of general relativity is found. Moreover it is found that the Einstein model is unstable and acausal while the $f(R, T)$ counterpart is well behaved on account of the freedom available through an additional coupling constant. The case of a constant spatial gravitational potential is considered and the complete model is determined. This model is markedly different from its Einstein counterpart which is known to be isothermal. Dropping the restriction on the density and imposing a linear barotropic equation of state generates an exact solution and consequently a stellar distribution as the vanishing of the pressure is possible and a boundary hypersurface exists. Finally we comment on the case of relaxing the equation of state but demanding an inverse square fall-off of the density - this case proves intractable.

gr-qc

Deflection of light by black holes and massless wormholes in massive gravity

Weak gravitational lensing by black holes and wormholes in the context of massive gravity (Bebronne and Tinyakov 2009) theory is studied. The particular solution examined is characterized by two integration constants, the mass $M$ and an extra parameter $S$ namely `scalar charge'. These black hole reduce to the standard Schwarzschild black hole solutions when the scalar charge is zero and the mass is positive. In addition, a parameter $λ$ in the metric characterizes so-called 'hair'. The geodesic equations are used to examine the behavior of the deflection angle in four relevant cases of the parameter $λ$. Then, by introducing a simple coordinate transformation $r^λ=S+v^2$ into the black hole metric, we were able to find a massless wormhole solution of Einstein-Rosen (ER) \cite{Einstein} type with scalar charge $S$. The programme is then repeated in terms of the Gauss--Bonnet theorem in the weak field limit after a method is established to deal with the angle of deflection using different domains of integration depending on the parameter $λ$. In particular, we have found new analytical results corresponding to four special cases which generalize the well known deflection angles reported in the literature. Finally, we have established the time delay problem in the spacetime of black holes and wormholes, respectively.

gr-qc