arXiv subjects

Shalabh Gautam

Publications and source records attributed to Shalabh Gautam.

6 recordsLinked to original sources

arXiv2026

A 3D Summation-by-Parts scheme on a Hyperboloidal Foliation of Minkowski

This paper summarises our previous work on a fully $3$D Summation-by-Parts scheme, derived for a class of linear wave equations on hyperboloidal slices on a fixed Minkowski background. The scheme is derived in spherical polar coordinates, and allows having grid points at the origin and on the $z$-axis, despite coordinate singularities, and at infinity, by introducing compactification followed by rescaling, and is proved to be stable. Reducing it to the standard Cauchy problem, or to finite spacelike slices with an outer boundary, will follow a similarly. Second-order accurate finite-difference methods are used to implement this scheme numerically, but higher-order finite-difference or spectral methods could also be used. Kreiss-Oliger dissipation operators are generalized to curvilinear coordinates and are defined everywhere in the domain, including at the boundary points, such that they satisfy the dissipative property in the energy norms. We also propose new norm convergence tests that include all the grid points at all resolutions and produce more accurate results. Promising results are obtained, giving hope for application to fully nonlinear systems, like the Einstein Field Equations, and extracting the resulting gravitational waves free of systematic errors or gauge ambiguities.

gr-qc↗

arXiv2026

3d Summation-by-Parts scheme for Linear Wave Equations on Hyperboloidal Slices

We derive a fully 3-dimensional Summation-By-Parts scheme for a class of linear wave equations on hyperboloidal slices that meet future null infinity on a Minkowski background. The scheme is derived in spherical polar coordinates, with a major strength being that it is provably stable and allows having grid points at the origin and on the $z$-axis, despite coordinate singularities, and at infinity, by introducing compactification followed by rescaling. Reducing it to the standard Cauchy problem, or on finite spacelike slices with an outer boundary, will follow a similar procedure. Interesting relations are obtained between the rescaling and compactification factors that simplify the equations, and the conditions on constraint addition terms are discovered to maintain symmetric hyperbolicity. Numerical implementation is achieved using finite-difference methods at second-order accuracy, which can be generalized to higher-order or spectral accuracies as well. Dissipation operators are given a more abstract treatment, which makes it possible to define them everywhere in the domain, including at the boundary points, in curvilinear coordinates, such that they satisfy the dissipative property (DP) in our energy norms. These generalizations reduce to the well-known Kreiss-Oliger dissipation operators whenever defined on a Cartesian grid in the bulk and satisfy the DP in the standard $L^2$-norms. We also propose new norm convergence tests that produce more accurate outputs. Promising results are obtained, giving hope for application to fully nonlinear systems, like the Einstein Field Equations, and extracting the resulting gravitational waves free of systematic errors or gauge ambiguities.

gr-qc↗

arXiv2024

Spherical Evolution of the Generalized Harmonic Gauge Formulation of General Relativity on Compactified Hyperboloidal Slices

We report on the successful numerical evolution of the compactified hyperboloidal initial value problem in general relativity using generalized harmonic gauge. We work in spherical symmetry, using a massless scalar field to drive dynamics. Our treatment is based on the dual-foliation approach, proceeding either by using a height function or by solving the eikonal equation to map between frames. Both are tested here with a naive implementation and with hyperboloidal layers. We present a broad suite of numerical evolutions, including pure gauge perturbations, constraint violating and satisfying data with and without scalar field matter. We present calculations of spacetimes with a regular center. For black hole spacetimes we use excision to remove part of the black hole interior. We demonstrate both pointwise and norm convergence at the expected rate of our discretization. We present evolutions in which the scalar field collapses to form a black hole. Evolving nonlinear scalar field perturbations of the Schwarzschild spacetime, we recover the expected quasinormal frequencies and tail decay rates from linear theory.

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arXiv2023

3D Evolution of a Good-Bad-Ugly-F Model on Compactified Hyperboloidal Slices

The Good-Bad-Ugly-F model is a system of semi-linear wave equations that mimics the asymptotic form of the Einstein field equations in generalized harmonic gauge with specific constraint damping and suitable gauge source functions. These constraint additions and gauge source functions eliminate logarithmic divergences appearing at the leading order in the asymptotic expansion of the metric components. In this work, as a step towards using compactified hyperboloidal slices in numerical relativity, we evolve this model numerically in spherical symmetry, axisymmetry and full 3d on such hyperboloidal slices. Promising numerical results are found in all cases. Our results show that nonlinear systems of wave equations with the asymptotics of the Einstein field equations in the above form can be reliably captured within hyperboloidal numerical evolution without assuming symmetry.

gr-qc↗

arXiv2021

Summation by Parts and Truncation Error Matching on Hyperboloidal Slices

We examine stability of summation by parts (SBP) numerical schemes that use hyperboloidal slices to include future null infinity in the computational domain. This inclusion serves to mitigate outer boundary effects and, in the future, will help reduce systematic errors in gravitational waveform extraction. We also study a setup with truncation error matching. Our SBP-Stable scheme guarantees energy-balance for a class of linear wave equations at the semidiscrete level. We develop also specialized dissipation operators. The whole construction is made at second order accuracy in spherical symmetry, but could be straightforwardly generalized to higher order or spectral accuracy without symmetry. In a practical implementation we evolve first a scalar field obeying the linear wave equation and observe, as expected, long term stability and norm convergence. We obtain similar results with a potential term. To examine the limitations of the approach we consider a massive field, whose equations of motion do not regularize, and whose dynamics near null infinity, which involve excited incoming pulses that can not be resolved by the code, is very different to that in the massless setting. We still observe excellent energy conservation, but convergence is not satisfactory. Overall our results suggest that compactified hyperboloidal slices are likely to be provably effective whenever the asymptotic solution space is close to that of the wave equation.

gr-qc↗

arXiv2019

The Hyperboloidal Numerical Evolution of a Good-Bad-Ugly Wave Equation

One method for the numerical treatment of future null-infinity is to decouple coordinates from the tensor basis and choose each in a careful manner. This dual-frame approach is hampered by logarithmically divergent terms that appear in a naive choice of evolved variables. Here we consider a system of wave equations that satisfy the weak-null condition and serve as a model system with similar nonlinearities to those present in the Einstein field equations in generalized harmonic gauge. We show that these equations can be explicitly regularized by a nonlinear change of variables. Working in spherical symmetry, a numerical implementation of this model using compactified hyperboloidal slices is then presented. Clean convergence is found for the regularized system. Although more complicated, it is expected that general relativity can be treated similarly.

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