arXiv subjects

Yashmitha Kumaran

Publications and source records attributed to Yashmitha Kumaran.

9 recordsLinked to original sources

arXiv2026

Screened Scalar Hair and the Weak-Lensing Separation of Black Holes from Neutron Stars in Quadratic $f(R)$ Gravity

Quadratic $f(R)$ gravity carries a massive scalar degree of freedom, the scalaron, whose finite range $\lambda$ screens its influence on the geometry outside a compact object. We show that this screening severs the exterior of a black hole from that of a neutron star of the same mass. The correction to the Schwarzschild metric is Yukawa-suppressed, falling as $e^{-r/\lambda}$ instead of polynomially in the coupling, and is thus invisible to any expansion in powers of that coupling; also, the exterior is intrinsically isotropic, so that inverting the temporal potential alone, as in single-potential solutions, fails to solve ]field equations. Applying the Gauss-Bonnet theorem to this geometry, we find the leading deflection angle to be exactly the general-relativistic $4GM/b$, the scalaron contributions cancelling identically in the combination that bends light. The first correction carries the unfamiliar signature $\lambda^{-1/2}b^{-3/2}e^{-b/\lambda}$, screened beyond the scalaron range and confirmed against direct quadrature to around 20% at $b=4\lambda$, improving to 7% at $b=12\lambda$. What survives is not a difference of degree but of kind. The three ingredients that deliver it, non-analyticity in the coupling, the intrinsically isotropic gauge and the pressure-weighted scalar charge, are here obtained within a single, self-consistent derivation for the first time. A static black hole carries no scalar hair, and lenses precisely as GR requires; a neutron star acquires a scalar charge weighted by the pressure supporting it against collapse, and does not. Weak lensing hence closes as a discriminant of the theory, whilst the horizon, as against a material surface, remains one in principle. The observational advantage lies not in bending angles at large $b$ but in the strong-field imaging of the photon sphere, and in the stellar interior, where the scalar charge is fixed by EoS.

gr-qc↗

arXiv2026

Weak gravitational lensing by a dark-matter-admixed neutron star: a self-consistent two-fluid halo and the Gauss--Bonnet deflection angle

We compute the weak gravitational deflection of light by a neutron star that carries a self-consistent dark-matter component, in the regime in which the dark matter forms an "extended halo" reaching beyond the baryonic surface. Two observations motivate this configuration. First, a dark-matter core confined within the baryonic radius leaves no distinctive lensing signature, since by Birkhoff's theorem the exterior is Schwarzschild with the total mass, so a ray passing outside the star feels only that mass and the compactness it implies. Second, the signature we seek resides in the complementary case, in which the ray genuinely traverses the halo, so that the surrounding density enters the optical geometry directly. We model the star by integrating the coupled two-fluid Tolman--Oppenheimer--Volkoff equations, with the baryonic and dark components interacting solely through gravity, and we obtain the deflection angle from the resulting external profile through the Gibbons--Werner construction of the Gauss--Bonnet theorem. The defining feature of the approach is that the deflection is tied to a "self-consistent" two-fluid profile rather than to a medium inserted by hand. We show that, for impact parameters smaller than the halo radius, the deflection departs measurably from the point-mass prediction, the deficit being governed by the dark-matter fraction and the halo extent, and we argue that the "shape" of this deficit furnishes a geometric, lensing-based handle on the degeneracy between dark matter and the nuclear equation of state. The framework thereby unites the structural modelling of compact stars with their gravitational-lensing phenomenology.

astro-ph.HE↗

arXiv2026

Production of Gravitational Waves in the Early Universe From turbulence triggered by first-order phase transitions

This project is aimed at studying the first-order phase transitions, that is presumed to have ensued in the early universe, and its consequences on the primordial gravitational waves. The effects of bubble nucleation, growth, and coalescence are reviewed. The resulting first-order phase transition is taken as the source of the gravitational waves that were produced, in order to determine the energy density, amplitude, and frequency spectra of the relic gravitational wave background. This is accomplished by modelling the first-order phase transition as a turbulent fluid and employing relativistic hydrodynamic equations to estimate the required physical quantities. Two models are majorly studied for all the analysis done in this project. Both models compute the necessary gravitational wave spectra using the exponential Kraichnan function as the temporal decorrelation function. Also, both models contemplate the turbulence in the flow of plasma to be stationary, obeying the conditions dictated by the Kolmogorov turbulence. However, the first model uses a de-coherence function that depends on the wavenumber and time, while, the second model uses the top hat correlation, to compute the anisotropic stress. The new model introduced here adheres to the freely decaying turbulence model, but employs the time dependent de-coherence function in its computations.

gr-qc↗

arXiv2025

Implications of σ-cut potential on Antikaon condensates in neutron stars

We investigate the properties of neutron stars with antikaon condensation in the framework of the Relativistic Mean-Field (RMF) model with a $σ$-cut potential. The well-known RMF models, TM1 and TM1e, are used to analyze the structure and composition of neutron stars. The antikaon condensation part of the equation of state (EoS) is constrained from the experimental data of K$^{-}$ atomic and kaon-nucleon scattering. The $σ$-cut potential, which is known to make the EoS stiffer at high densities, is modulated by a free parameter $f_{s}$. Our present analysis suggests that one can obtain neutron star configurations heavier than 2$M_{\odot}$ with antikaon condensates in most cases for $f_{s}$ = 0.6. The antikaon phase transition is a second-order for $f_{s}$ = 0.6 for both TM1 and TM1e parameter sets. The calculated global properties of neutron stars with antikaon condensates i.e., mass and radius seem to be in resonable agreement with other theoretical and observational data.

astro-ph.HE↗

arXiv2023

Shadow and deflection angle of asymptotic, magnetically-charged, non-singular black hole

In this paper, we present a detailed analysis of an asymptotic, magnetically-charged, non-singular (AMCNS) black hole. By utilizing the Gauss-bonnet theorem, we aim to unravel the intricate astrophysics associated with this unique black hole. The study explored various aspects including the black hole's gravitational field, intrinsic properties, light bending, the shadow and greybody bounding of the black hole. Through rigorous calculations and simulations, we derive the weak deflection angle of the optical metric of AMCNS black hole. Additionally, we investigate the impact of the dark matter medium on the deflection angle, examined the distinctive features of the black hole's shadow, and bound its greybody factors. Our findings not only deepen our understanding of gravitational lensing but also pave the way for future improvements in black hole theories by minimizing restrictive assumptions and incorporating a more realistic representation of these cosmic phenomena.

gr-qc↗

arXiv2022

Deflection angle and shadow of the Reissner-Nordström black hole with higher-order magnetic correction in Einstein-nonlinear-Maxwell fields

Nonlinear electrodynamics is known as the generalizations of Maxwell electrodynamics at strong fields and presents interesting features such as curing the classical divergences present in the linear theory when coupled to general relativity. In this paper, we consider the asymptotically flat Reissner-Nordström black hole solution with higher-order magnetic correction in Einstein-nonlinear-Maxwell fields. We study the effect of the magnetic charge parameters on the black hole, viz. weak deflection angle of photons and massive particles using Gauss-bonnet theorem. Moreover, we apply the Keeton-Petters formalism to confirm our results of the weak deflection angle. Apart from vacuum, their influence in the presence of different media such as plasma and dark matter are probed as well. Finally, we examine the black hole shadow cast using the null-geodesics method and investigate its spherically in-falling thin accretion disk. Our inferences show how the magnetic charge parameter $p$ affects the other physical quantities; so, we impose some constraints on this parameter using the observations from the Event Horizon Telescope.

gr-qc↗

arXiv2021

Deriving Weak Deflection Angle by Black Holes or Wormholes using Gauss-Bonnet Theorem

In this review, various researches on finding the bending angle of light deflected by a massive gravitating object which regard the Gauss-Bonnet theorem as the premise have been revised. Primarily, the Gibbons and Werner method is studied apropos of the gravitational lensing phenomenon in the weak field limits. Some exclusive instances are deliberated while calculating the deflection angle, beginning with the finite-distance corrections on non-asymptotically flat spacetimes. Effects of plasma medium is then inspected to observe its contribution to the deflection angle. Finally, the Jacobi metric is explored as an alternative method, only to arrive at similar results. All of the cases are probed in three constructs, one as a generic statement of explanation, one for black holes, and one for wormholes, so as to gain a perspective on every kind of influence.

gr-qc↗

arXiv2021

Weak deflection angle by asymptotically flat black holes in Horndeski theory using Gauss-Bonnet theorem

The principal objective of this project is to investigate the gravitational lensing by asymptotically flat black holes in the framework of Horndeski theory in weak field limits. To achieve this objective, we utilize the Gauss-Bonnet theorem to the optical geometry of asymptotically flat black holes and apply the Gibbons-Werner technique to achieve the deflection angle of photons in weak field limits. Subsequently, we manifest the influence of plasma medium on deflection of photons by asymptotically flat black holes in the context of Horndeski theory. We also examine the graphical impact of deflection angle on asymptotically flat black holes in the background of Horndeski theory in plasma medium as well as non-plasma medium.

gr-qc↗

arXiv2019

Weak Deflection Angle of Extended Uncertainty Principle Black Holes

In this paper, we have discussed the effects of quantum fluctuations spewed by a black hole on its deflection angle. The Gauss-Bonnet theorem (GBT) is exploited with quantum corrections through the Extended Uncertainty Principle (EUP) and the corresponding deflection angle is obtained. Moreover, we have attempted to broaden the scope of our work by subsuming the effects of plasma medium on the deflection angle as well. To demonstrate the degree of difference, the acquired results are compared with the prevailing findings.

gr-qc↗