arXiv subjects

Sari Ghanem

Publications and source records attributed to Sari Ghanem.

10 recordsLinked to original sources

arXiv2026

Dispersive estimates for a system of tensorial quasilinear wave equations satisfying the weak-null condition

We establish both global existence and decay properties for solutions with small data for a general class of coupled system of tensorial quasilinear hyperbolic wave equations in three space dimensions, that covers the dynamical Einstein equations coupled to a class of non-linear matter sources that do not satisfy the null condition of Christodoulou and Klainerman, and have new different non-linearities than the one treated by Lindblad-Rodnianski, for which their celebrated seminal $L^\infty$-estimate does not work, to the best of our knowledge. Global existence of solutions for a general class of quasilinear wave equations satisfying the weak-null condition, with small initial data, is largely an open problem at present. There is no known theory to prove decay for the class of non-linear hyperbolic partial differential equations that we treat in this paper. We establish a technique based on novel decoupling of the higher order energy estimates, at the level of the $L^2$-norm of the Lie derivatives of the tangential components, without involving all the other components, up to some good factor. This generalizes our previous results to include new non-linearities that are not present in the Einstein-Yang-Mills system in the Lorenz gauge.

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arXiv2026

Decoupled energy estimates for tensorial non-linear wave equations and applications

We prove energy estimates for solutions to a tensorial system of coupled non-linear wave equations, in a way that is suitable to deal with the structure of the non-linearity that arises from the Einstein-Yang-Mills system in the Lorenz gauge as well as with other new different non-linearities. We establish suitable bounds on the $L^2$-norm of each component in a frame decomposition of the tensorial solutions, in way that does not involve all the other components of the tensor, which would allow us to decouple the higher order energy estimates for certain components from the other components. We achieve this partly by exploiting the tensorial structure of the coupled non-linear wave equations, where the background metric that is \`a priori unknown, is a perturbation of the Minkowski space-time in a certain fixed system of coordinates, and by exploiting the structure of the commutator term for the Lie derivatives of the solutions. These decoupled energy estimates for each component of the tensor in a frame, are new and motivated by a problem that we address in a subsequent paper to prove the exterior non-linear stability of the $(1+3)$-Minkowski space-time governed by a general class of perturbations, that includes the non-linearities that arise from the Einstein-Yang-Mills system in the Lorenz gauge as well as other new non-linearities, which have a different non-linear structure than the one treated by Lindblad-Rodnianski, for which their seminal $L^\infty$-estimate does not work to the best of our knowledge. The decoupled energy bounds on each component in a frame derived here allow us to replace the celebrated $L^\infty$-estimate of Lindblad-Rodnianski in a novel way that permits us to treat these new non-linear structures.

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arXiv2024

Overview of the proof of the exterior stability of the $(1+3)$-Minkowski space-time governed by the Einstein-Yang-Mills system in the Lorenz gauge

We study the Einstein-Yang-Mills system in both the Lorenz and harmonic gauges, where the Yang-Mills fields are valued in any arbitrary Lie algebra $\cal G$, associated to any compact Lie group $G$. This gives a system of hyperbolic partial partial differential that does not satisfy the null condition and that has new complications that are not present for the Einstein vacuum equations nor for the Einstein-Maxwell system. We prove the exterior stability of the Minkowski space-time, $\mathbb{R}^{1+3}$, governed by the fully coupled Einstein-Yang-Mills system in the Lorenz gauge, valued in any arbitrary Lie algebra $\cal G$, without any assumption of spherical symmetry. We start with an arbitrary sufficiently small initial data, defined in a suitable energy norm for the perturbations of the Yang-Mills potential and of the Minkowski space-time, and we show the well-posedness of the Cauchy development in the exterior, and we prove that this leads to solutions converging in the Lorenz gauge and in wave coordinates to the zero Yang-Mills fields and to the Minkowski space-time. This provides a first detailed proof of the exterior stability of Minkowski governed by the fully non-linear Einstein-Yang-Mills equations in the Lorenz gauge, by using a null frame decomposition that was first used by H. Lindblad and I. Rodnianski for the case of the Einstein vacuum equations. We note that in contrast to the much simpler case of the Einstein-Maxwell equations where one can omit the potential, in fact in the non-abelian case of the Einstein-Yang-Mills equations, the question of stability, or non-stability, is a purely gauge dependent statement and the partial differential equations depend on the gauge on the Yang-Mills potential that is needed to write up the equations.

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arXiv2023

The global stability of the Minkowski space-time solution to the Einstein-Yang-Mills equations in higher dimensions

This is a first in a series of papers in which we study the stability of the $(1+n)$-Minkowski space-time, for $n \geq 3$, solution to the Einstein-Yang-Mills equations, in both the Lorenz and harmonic gauges, associated to any arbitrary compact Lie group $G$, and for arbitrary small perturbations. In this first, we prove global stability of the Minkowski space-time, $\mathbb{R}^{1+n}$, in higher dimensions $n \geq 5$ (both in the interior and in the exterior); in the paper that follows, we prove exterior stability for $n=4$; and its sequel, we prove exterior stability for $n=3$, and in all these cases, stability is studied as a solution to the fully coupled Einstein-Yang-Mills system in the Lorenz and harmonic gauges. We show here that for $n \geq 5$, the $\mathbb{R}^{1+n}$ Minkowski space-time in wave coordinates is stable as solution to the Einstein-Yang-Mills system in the Lorenz gauge on the Yang-Mills potential, for sufficiently small perturbations of the Einstein-Yang-Mills potential and metric, and leads to a global Cauchy development. We also obtain dispersive estimates in wave coordinates on the gauge invariant norm of the Yang-Mills curvature, on the Yang-Mills potential in the Lorenz gauge, and on the perturbations of the metric. In this manuscript, we detail all the material of our proof so as to provide lecture notes for Ph.D. students wanting to learn the Cauchy problem for the Einstein-Yang-Mills system.

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arXiv2023

Exterior stability of the $(1+3)$-dimensional Minkowski space-time solution to the Einstein-Yang-Mills equations

We prove the exterior stability of the Minkowski space-time, $\mathbb{R}^{1+3}$, solution to the Einstein-Yang-Mills system in both the Lorenz and harmonic gauges, where the Yang-Mills fields are valued in any arbitrary Lie algebra $\cal{G}$, associated to any compact Lie group $G$. We start with an arbitrary sufficiently small initial data, defined in a suitable energy norm for the perturbations of the Yang-Mills potential and of the Minkowski space-time, and we show the well-posedness of the Cauchy development in the exterior of the fully coupled Einstein-Yang-Mills equations in the Lorenz gauge and in wave coordinates, and we prove that this leads to solutions converging to the zero Yang-Mills curvature and to the Minkowski space-time. Furthermore, we obtain dispersive estimates in wave coordinates on the Yang-Mills potential in the Lorenz gauge and on the metric, as well as on the gauge invariant norm of the Yang-Mills curvature. This provides a new proof to the exterior stability result by P. Mondal and S. T. Yau, based on an alternative approach, by using a null frame decomposition that was first used by H. Lindblad and I. Rodnianski for the case of the Einstein vacuum equations. In this third paper of a series, we detail all the new material concerning our proof so as to provide lecture notes for Ph.D. students wanting to learn non-linear hyperbolic differential equations and stability problems in mathematical General Relativity.

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arXiv2023

Energy estimates for the Einstein-Yang-Mills fields and applications

We prove exterior energy estimates for tensorial non-linear wave equations, where the background metric is a perturbation of the Minkowski space-time, and where the derivatives are the Minkowski covariant derivatives. We obtain bounds in the exterior region of the Minkowski space-time, for the weighted $L^2$ norm on each component, separately, of the covariant derivative of the tensorial solutions, and we also control a space-time integral in the exterior of the covariant tangential derivatives of the solutions. As a special application, we use here these energy estimates to prove the exterior stability of the Minkowski space-time, $\mathbb{R}^{1+4}$, as solution to the coupled Einstein-Yang-Mills system associated to any compact Lie group $G$, in the Lorenz gauge and in wave coordinates. The bounds in the exterior for the $L^2$ norm on the covariant derivatives of each component, separately, of the tensor solution, as well as the bound on the space-time integral of the covariant tangential derivatives, are motivated by a problem that we will address in a paper that follows to prove the exterior stability of the $(1+3)$-Minkowski space-time for perturbations governed by the Einstein-Yang-Mills equations.

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arXiv2016

The decay of the $SU(2)$ Yang-Mills fields on the Schwarzschild black hole for spherically symmetric small energy initial data

We prove uniform decay estimates in the entire exterior of the Schwarzschild black hole for gauge invariant norms on the Yang-Mills fields valued in the Lie algebra associated to the Lie group $SU(2)$. We assume that the initial data are spherically symmetric satisfying a certain Ansatz, and have small energy, which eliminates the stationary solutions which do not decay. In particular, there don't exist any Coulomb type solutions satisfying this Ansatz. We first prove a Morawetz type estimate for the Yang-Mills fields within this setting, using the Yang-Mills equations directly. We then adapt the proof constructed in previous work by the first author to show local energy decay and uniform decay of the $L^{\infty}$ norm of the middle components in the entire exterior of the Schwarzschild black hole, including the event horizon.

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arXiv2014

On uniform decay of the Maxwell fields on black hole space-times

This is the second in a series of papers in which we take a systematic study of gauge field theories such as the Maxwell equations and the Yang-Mills equations, on curved space-times. In this paper, we study the Maxwell equations in the domain of outer-communication of the Schwarzschild black hole. We show that if we assume that the middle components of the non-stationary solutions of the Maxwell equations verify a Morawetz type estimate supported around the trapped surface, then we can prove uniform decay properties for components of the Maxwell fields in the entire exterior of the Schwarzschild black hole, including the event horizon, by making only use of Sobolev inequalities combined with energy estimates using the Maxwell equations directly. This proof is entirely gauge independent, and does not pass through the scalar wave equation on the Schwarzschild black hole, and does not need to separate the middle components for the Maxwell fields. However, proving a Morawetz estimate directly using the Maxwell equations, without refering to the scalar wave equation, seems to be out of reach of the mathematical community as of today; which I was not able to solve yet in this work. If one is able to prove the Morawetz estimate directly using the Maxwell equations, this combined with the present work would give full conceptual proof of decay of the Maxwell fields on the Schwarzschild black hole, and would then be in particular useful for the non-abelian case of the Yang-Mills equations where the separation of the middle components cannot occur. The whole manuscript is written in an expository way where we detail all the calculations.

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arXiv2013

Global regularity for the critical 2-D dissipative quasi-geostrophic equation with force

This is a remark that by using an adaptation of the technique invented by A. Kiselev, F. Nazarov, and A. Voldberg, with a modified scaling argument, we can prove global regularity of the critical 2-D dissipative quasi-geostrophic equation with smooth periodic force, under the assumption that the initial data is smooth and periodic, and the force is $α$-Hölder continuous in space, $α> 0$.

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arXiv2013

The global existence of Yang-Mills fields on curved space-times

This is an introductory chapter in a series in which we take a systematic study of the Yang-Mills equations on curved space-times. In this first, we provide standard material that consists in writing the proof of the global existence of Yang-Mills fields on arbitrary curved space-times using the Klainerman-Rodnianski parametrix combined with suitable Grönwall type inequalities. While the Chruściel-Shatah argument requires a simultaneous control of the $L^{\infty}_{loc}$ and the $H^{2}_{loc}$ norms of the Yang-Mills curvature, we can get away by controlling only the $H^{1}_{loc}$ norm instead, and write a new gauge independent proof on arbitrary, fixed, sufficiently smooth, globally hyperbolic, curved 4-dimensional Lorentzian manifolds. This manuscript is written in an expository way in order to provide notes to Master's level students willing to learn mathematical General Relativity.

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