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In this study we explore the astrophysical implications of pressure anisotropy on the physical characteristics of millisecond pulsars within the framework of $f(Q)$ gravity. Starting off with the field equations for anisotropic matter distributions, we adopt the physically salient Durgapal-Fuloria ansatz together with a well-motivated anisotropic factor for the interior matter distribution. This leads to a nonlinear second order differential equation which is integrated to give the complete gravitational and thermodynamical properties of the stellar object. The resulting model is subjected to rigorous tests to ensure that it qualifies as a physically viable compact object within the $f(Q)$-gravity framework. We study in detail two factors, one being non-metricity (gravitational interaction) and the other the pressure anisotropy (matter variation) and their contributions to the mass, radius and stability of the star. Our analyses indicate that our models are well-behaved, singularity-free and can account for the existence of a wide range of observed pulsars with masses ranging from 2.08 to 2.67 $M_{\odot}$, with the upper value being in the so-called {\em mass gap} regime observed in gravitational events such as GW190814. A novel finding of this work is two-fold: an increase in the non-metricity factor (fixed anisotropy) leads to a spectrum of pulsars with radii ranging from a minimum of 10 km whereas a variation in anisotropy (fixed non-metricity) restricts the radii to a narrow window between ($12 - 13.2$) km for a 450 Hz pulsar. In addition, contributions from anisotropy outweigh the impact of nonmetricity in predicting the masses of pulsars with the ratio, $\left(\frac{M_\Delta}{M_Q}\right)_{max} \approx 2.43 $ for low mass pulsars., Comment: 6 figures and 2 tables (Submssion for JCAP)

This current analysis offers novel wormhole solutions in the background of newly developed extended $f(\mathcal{R},\mathcal{L}_m)$ gravity. We use an anisotropic matter source and a particular type of energy density demonstrating cold dark matter halo and quantum wave dark matter halo to calculate two different wormhole solutions. The properties of the exotic matter within the wormhole geometry and the matter contents via energy conditions are studied in detail, both analytically and graphically, by showing valid and invalid regions. The calculated shape functions of wormhole geometry satisfy the required conditions in both cases. Further, we investigate the stability of the shell around wormhole structures by considering black hole solutions in the framework of cold dark matter halo and quantum wave dark matter by assuming matter contents located at the shell follow the phantom-like equations of state. Then, for quintessence and phantom energy type equations of state, the choice of cold dark matter halo has maximum stability at lower equilibrium shell radii and declines as the radius rises. More petite wormhole throats have an unstable configuration for the dark energy matter content, while higher wormhole throat radius has the lowest stability. For the choice of quantum wave dark matter, the shell around the wormhole structure is unstable for both quintessence and phantom energy, while dark energy shows a stable configuration for smaller values of wormhole throat.

Inspired by the conundrum of the gravitational event, GW190814 which brings to light the coalescence of a 23 $ M_{\odot}$ black hole with a yet to be determined secondary component, we look to modelling compact objects within the framework of $f(\mathcal{Q})$ gravity by employing the method of gravitational decoupling. We impose a quadratic equation of state (EOS) for the interior matter distribution which in the appropriate limit reduces to the MIT bag model. The governing field equations arising from gravitational decoupling bifurcates into the $\rho=\theta^0_0$ and $p_r=\theta^1_1$ sectors leading to two distinct classes of solutions. Both families of solutions are subjected to rigorous tests qualifying them to describe a plethora of compact objects including neutron stars, strange stars and the possible progenitor of the secondary component of GW190814. Using observational data of mass-radius relations for compact objects LMC X-4, Cen X-3, PSR J1614-2230 and PSR J0740+6620 we show that it is possible to generate stellar masses and radii beyond 2.0 $ M_{\odot}$ for neutron stars. Our findings reveal that the most { suitable and versatile model in this framework is the quadratic EOS}, which accounts for a range of low mass stars as well as typical stellar candidates describing the secondary component of GW190814., Comment: Accepted in The Astrophysical Journal Supplement Series

In this work we take our cue from the observations of gravitational waves of the GW190814 event which suggests that source of the signals can be ascribed to a compact binary coalescence of a 22.2 to 24.3$ M_{\odot}$ black hole and a compact object endowed with a mass of 2.50 to 2.67$M_{\odot}$. In the current exposition, we are concerned with modeling of the lower mass component of the coalescence pair. We utilise the $f(\mathcal{Q})$ gravity together with Minimum Geometric Deformation (MGD) technique to obtain compact stellar objects with masses aligned with the GW190814 event. Starting off with the Tolman IV ansatz for one of the metric functions, together with a MIT Bag model equation of state we are able to reduce the problem of fully describing the gravitational behaviour of the seed solution to a quadrature. Through the MGD technique, we introduce anisotropy by deforming the radial part of the gravitational potential. This enables us to obtain two new classes of solutions which depend on the metricity parameter, $\cal Q$ and the deformation constant, $\beta$. We show that these two parameters play a crucial role in determining the thermodynamical behaviour and stability of our models. In particular, we show that the interplay between the metricity parameter and the deformation constant leads to predicted mass of the progenitor articulating as the secondary component of GW190814., Comment: 11 Figures and 3 Tables

In this paper, we discuss the Bianchi type-I cosmological model in the framework of symmetric teleparallel gravity say $f(Q)$ gravity in which the non-metricity term $Q$ is responsible for the gravitational interaction. We consider a special form of the $f\left( Q\right) $ function which can be cast as $f\left( Q\right) =\lambda Q^{n}$, where $\lambda $ and $n$ both are the dynamical model parameters. Such a choice can be viewed as a hybrid scale factor that leads to a relation between cosmic time and redshift as $% t=\left( \frac{\alpha t_{0}}{\beta }\right) W\left[ \frac{\beta }{\alpha }e^{% \frac{\beta -\ln \left( 1+z\right) }{\alpha }}\right] $ which describes a $% \Lambda $CDM model of the Universe with the expansion evolving from decelerating to an acceleration phase. The best values for the model parameters i.e. $\alpha $ and $\beta $ that would accord with the most current observational datasets are then estimated. We make use of 57 points from the Hubble dataset, 1048 points from the supernovae of type Ia dataset and 6 points from the BAO dataset. We use the Markov Chain Monte Carlo (MCMC) technique in conjunction with Bayesian analysis and the likelihood function. Further, we study the validity of our model with the investigation of the thermodynamical quantities, energy conditions along with some physical variables such as the EoS, and jerk parameters. Next, our results are discussed in light of current observational data and trends., Comment: Journal of High Energy Astrophysics accepted version

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 $\theta$ 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., Comment: 16 pages, and 12 figures

In this exposition, we seek solutions of the Einstein-Maxwell field equations in the presence of a massive scalar field cast in the Brans-Dicke (BD) formalism which describes charged anisotropic strange stars. The interior spacetime is described by a spherically symmetric static metric of embedding class I. This reduces the problem to a single-generating function of the metric potential which is chosen by appealing to physics based on regularity at each interior point of the stellar interior. The resulting model is subjected to rigorous physical checks based on stability, causality and regularity. We show that our solutions describe compact objects such as PSR J1903+327; Cen X-3; EXO 1785-248 \& LMC X-4 to an excellent approximation. Novel results of our investigation reveal that the scalar field leads to higher surface charge densities which in turn affects the compactness and upper and lower values imposed by the modified Buchdahl limit for charged stars. Our results also show that the electric and scalar fields which originate from entirely different sources couple to alter physical characteristics such as mass-radius relation and surface redshift of compact objects. This superposition of the electric and scalar fields is enhanced by an increase in the BD coupling constant, $\omega_{BD}$., Comment: 9 figures and 6 tables

We model a radiating star undergoing dissipative gravitational collapse in the form of radial heat flux. The exterior of the collapsing star is described by the generalised Vaidya solution representing a mixture of null radiation and strings. Our model generalises previously known results of constant string density atmosphere to include inhomogeneities in the exterior spacetime. By utilising a causal heat transport equation of the Maxwell-Cattaneo form we show that relaxational effects are enhanced in the presence of inhomogeneities due to the string density., Comment: 12 pages, 4 figures

Utilizing an ansatz developed by Maurya and co-workers we present a class of exact solutions of the Einstein-Maxwell field equations describing a spherically symmetric compact object. A detailed physical analysis of these solutions in terms of stability, compactness and regularity indicate that these solutions may be used to model strange star candidates. In particular, we model the strange star candidate Her X-1 and show that our solution conforms to observational data to an excellent degree of accuracy. An interesting and novel phenomenon which arises in this model is the fact that the relative difference between the electromagnetic force and the force due to pressure anisotropy changes sign within the stellar interior. This may be a an additional mechanism required for stability against cracking of the stellar object.

In this work we present an exact solution of the Einstein-Maxwell field equations describing compact, charged objects within the framework of classical general relativity. Our model is constructed by embedding a four-dimensional spherically symmetric static metric into a five dimensional flat metric. The source term for the matter field is composed of a perfect fluid distribution with charge. We show that our model obeys all the physical requirements and stability conditions necessary for a realistic stellar model. Our theoretical model approximates observations of neutron stars and pulsars to a very good degree of accuracy., Comment: 14 figures

A new class of solution describing an anisotropic stellar configuration satisfying Karmarkar's condition i.e. spherically symmetric metric of embedding class 1, is reported. It has been shown that the compact star model is physically well-behaved and meet all the physical requirements for a stable configuration in hydrostatic equilibrium. Our model describes compact stars like Vela X-1 and 4U1608-52 to a very good approximation.

We obtain a new solution of the TOV-equation for an anisotropic fluid distribution by imposing the Karmarkar condition. In order to close the system of equations we postulate an interesting form for the grr gravitational potential which allows us to solve for gtt metric component via the Karmarkar condition. We demonstrate that the new interior solution has well-behaved physical attributes and can be utilized to model relativistic static fluid spheres. By using observational data sets for the radii and mass-to-radius relations for compact stars such as 4U 1538-52, LMC X-4 and PSR J1614- 2230 we show that our solution describes these objects to a very good degree of accuracy. The physical plausibility of the solution depends on a parameter $c$ for a particular star. For 4U 1538-52 the solution behaves well for $0.1574 \le c \le 0.46$ which corresponds to $1 \ge v^2_{r0} \ge 0:13$, $0.91 \ge v^2_{t0}\ge 0.04$ and $17.8 \ge \Gamma_0 \ge 3.8$. For LMC X-4 the solution behaves well for $0.1235 \le c \le 0.35$ which corresponds to $0.99 \ge v^2_{r0} \ge 0.15$, $0.91 \ge v^2_{t0}\ge 0.06$ and $15.35 \ge \Gamma_0 \ge 3.35$. Our model approximates the pulsar PSR J1614-2230 for $0.05 \le c \le 0.13$ which corresponds to $1 \ge v^2_{r0}\ge 0.21$, $0.94 \ge v^2_{t0} \ge 0.13$ and $8.34 \ge \Gamma_0 \ge 2.34$. Our observations of the behavior of the thermodynamical and physical variables of these compact objects lead us to conclude that the parameter c plays an important role in determining the equation of state of the stellar material. It is apparent that smaller values of c lead to stiffer equation of states and vice-versa., Comment: 9 pages, 15 Figures Accepted in EPJC

In this paper we investigate the role of causal heat transport in a spatially homogeneous, locally-rotationally symmetric Bianchi type-V cosmological model. In particular, the causal temperature profile of the cosmological fluid is obtained within the framework of extended irreversible thermodynamics. We demonstrate that relaxational effects can alter the temperature profile when the cosmological fluid is out of hydrostatic equilibrium., Comment: 9 pages, 2 figures

In this work we present an exact model of a spherically symmetric star undergoing dissipative collapse in the form of a radial heat flux. The interior of the star is matched smoothly to the generalised Vaidya line element representing a two-fluid atmosphere comprising null radiation and a string fluid. The influence of the string density on the thermal behaviour of the model is investigated by employing a causal heat transport equation of Maxwell-Cattaneo form., Comment: 12 pages, 2 figures

In this paper we investigate the physics of a radiating star undergoing dissipative collapse in the form of a radial heat flux. Our treatment clearly demonstrates how the presence of shear affects the collapse process; we are in a position to contrast the physical features of the collapsing sphere in the presence of shear with the shear-free case. By employing a causal heat transport equation of the Maxwell-Cattaneo form we show that the shear leads to an enhancement of the core temperature thus emphasizing that relaxational effects cannot be ignored when the star leaves hydrostatic equilibrium., Comment: 15 pages, To appear in Int. J. Mod. Phys. D

We model the gravitational behaviour of a radiating star when the exterior geometry is the generalised Vaidya spacetime. The interior matter distribution is shear-free and undergoing radial heat flow. The exterior energy momentum tensor is a superposition of a null fluid and a string fluid. An analysis of the junction conditions at the stellar surface shows that the pressure at the boundary depends on the interior heat flux and the exterior string density. The results for a relativistic radiating star undergoing nonadiabatic collapse are obtained as a special case. For a particular model we demonstrate that the radiating fluid sphere collapses without the appearance of the horizon at the boundary., Comment: 10 pages, To appear in Gen. Relativ. Gravit

A recent study of dissipative collapse considered a contracting sphere in which the areal and proper radii are equal throughout its evolution. The interior spacetime was matched to the exterior Vaidya spacetime which generated a temporal evolution equation at the boundary of the collapsing sphere. We present a solution of the boundary condition which allows the study of the gravitational and thermodynamical behaviour of this particular radiating model., Comment: 10 pages, 3 figures

We investigate the causal temperature profiles in a recent model of a radiating star undergoing dissipative gravitational collapse without the formation of an horizon. It is shown that this simple exact model provides a physically reasonable behaviour for the temperature profile within the framework of extended irreversible thermodynamics., Comment: 8 pages, 2 figures, submitted to Mod. Phys. Lett. A

We study the effects of pressure anisotropy and heat dissipation in a spherically symmetric radiating star undergoing gravitational collapse. An exact solution of the Einstein field equations is presented in which the model has a Friedmann-like limit when the heat flux vanishes. The behaviour of the temperature profile of the evolving star is investigated within the framework of causal thermodynamics. In particular, we show that there are significant differences between the relaxation time for the heat flux and the relaxation time for the shear stress., Comment: 14 pages, 3 figures

In a recent approach in modelling a radiating relativistic star undergoing gravitational collapse the role of the Weyl stresses was emphasised. It is possible to generate a model which is physically reasonable by approximately solving the junction conditions at the boundary of the star. In this paper we demonstrate that it is possible to solve the Einstein field equations and the junction conditions exactly. This exact solution contains the Friedmann dust solution as a limiting case. We briefly consider the radiative transfer within the framework of extended irreversible thermodynamics and show that relaxational effects significantly alter the temperature profiles., Comment: 10 pages, submitted to IJMP-D

We consider the end state of collapsing null radiation with a string fluid. It is shown that, if diffusive transport is assumed for the string, that a naked singularity can form (at least locally). The model has the advantage of not being asymptotically flat. We also analyse the case of a radiation-string two-fluid and show that a locally naked singularity can result in the collapse of such matter. We contrast this model with that of strange quark matter., Comment: RevTeX 4.0 (8 pages - no figures). submitted to Phys Rev D. Some changes to abstract, introduction and conclusion - references updated

We present here a simple model of radiative gravitational collapse with radial heat flux which describes qualitatively the stages close to the formation of a superdense cold star. Starting with a static general solution for a cold star, the model can generate solutions for the earlier evolutionary stages. The temporal evolution of the model is specified by solving the junction conditions appropriate for radiating gravitational collapse., Comment: 13 pages, including 3 figures, submitted to IJMP-D

We study the analogue of the Oppenheimer-Snyder model of a collapsing sphere of homogeneous dust on the Randall-Sundrum type brane. We show that the collapsing sphere has the Vaidya radiation envelope which is followed by the brane analogue of the Schwarzschild solution described by the Reissner-Nordstrom metric. The collapsing solution is matched to the brane generalized Vaidya solution and which in turn is matched to the Reissner-Nordstrom metric. The mediation by the Vaidya radiation zone is the new feature introduced by the brane. Since the radiating mediation is essential, we are led to the remarkable conclusion that a collapsing sphere on the brane does indeed, in contrast to general relativity, radiate null radiation., Comment: Minor changes, main results remain unchanged, to appear in Phys. Lett. B