arXiv subjects

Sergiu Klainerman

Publications and source records attributed to Sergiu Klainerman.

At least 19 recordsLinked to original sources

arXiv2026

Generalized Whiting transform and Flux Estimates for Wave Equations in Subextremal Kerr

All recent advances \cite{DRS}, \cite{S-Rita1}, \cite{S-Rita2}, \cite{Millet}, \cite{MaS}, \cite{MaS2} on the boundedness and decay for wave equations on subextremal Kerr spacetimes $\KK(a,m)$, with non-small angular rotation, rely in an essential way on Whiting's groundbreaking discovery of a powerful integro-differential transformation with the help of which he ruled out exponentially growing modes in \cite{W} (see also \cite{Yacov}, \cite{AMPW} and \cite{Rita} for further developments). We give a purely \textit{physical space}, generalized version, of the transformation which takes general solutions of {spin-$\sk$} wave equations in $\KK=\KK(a,m)$ to solutions of a secondary wave equation on a new metric manifold $\KKt=\KKt(a,m)$ with two stationary, asymptotically flat, ends. Moreover, for a certain subdomain $\DDt\subset\DDt_-$, which can be identified as the exterior of a black hole region, the new metric $\gt$ of $\KKt(a, m)$ is Lorentzian and the time translation $\T $ is timelike. Using the geometric properties of the two ends one can then deduce, by classical asymptotic Fourier analysis techniques, the desired bound for the flux of $\psi$ at the future event horizon. This estimate played a crucial role in our companion paper \cite{He-K1}.

math.AP↗

arXiv2026

Inevitable shock formation for 3-D compressible Euler flows

We prove that solutions arising from smooth, sufficiently small, compactly supported perturbations of non-vacuum constant states in three-dimensional irrotational compressible flow must blow up in finite time, without any symmetry assumptions or other restrictions on the initial data. Moreover, we prove that shock inevitably forms at the boundary of the maximal Cauchy development, and that its formation time agrees, in the small-data asymptotic regime, with the lifespan predicted by the radiation-field analysis.

math.AP↗

arXiv2026

Formation of Trapped Surfaces from Spacelike Initial Data

We make use of the free data formalism developed in \cite{CK25,CK26} to provide a direct construction of short-pulse type Cauchy data. The construction of the spacelike short-pulse follows from the local existence result established in \cite{CK26}. The forward integration construction in \cite{CK26} allows us to show that such data can be extended to a set of asymptotically flat Cauchy data. This greatly extends the result of Li--Yu \cite{LiYu}.

gr-qc↗

arXiv2026

A Physical space derivation of Morawetz-Energy estimates in Kerr spacetimes with large angular momentum

We revisit the derivation of Morawetz--energy estimates for scalar wave equations in the domain of outer communication of a Kerr spacetime $\KK(a,m)$. Our goal is to develop robust physical-space methods which are well suited for extension to realistic perturbations of Kerr. The proof rests on several ingredients. First, we derive conditional Morawetz estimates which extend the physical-space techniques initiated by Andersson and Blue \cite{AB}, and later adapted in \cite{GKS} to perturbations of slowly rotating Kerr, by exploiting a physical-space characterization of the full $r$-range of trapped null geodesics. Second, we use an idea introduced by Stogin \cite{St} in the axially symmetric case to handle the low-frequency difficulties in the Morawetz estimates. In the general case, the control of the lower-order terms also requires making full use of the principal trapping term in the Morawetz bulk norm, together with a new use of Hardy-type inequalities. A further new ingredient is the control of the boundary terms generated by the Morawetz estimates. This is based on two additional ideas: physical-space versions of Whiting's transform \cite{W}, developed in a forthcoming paper \cite{H-K2}, which yield a flux-independent energy estimate; and an adaptation of the Andersson--Blue invariant-operator method, which turns that estimate into a bound for the horizon flux. Finally, a continuity argument yields an unconditional global-in-time Morawetz estimate, while a new energy estimate is obtained from the construction of a causal vectorfield which is Killing on the trapping set. The results proved here are restricted to scalar wave equations, corresponding to spin $0$, in the range $|a|/m\leq 0.75$. We expect this restriction to be technical, and the methods developed in this paper to extend to the Teukolsky equation.

gr-qc↗

arXiv2026

Forward Construction of Vacuum Initial Data with Borderline Decay

We make use of the free data formalism developed in \cite{CK25} to construct solutions of the Einstein vacuum constraint equations by integrating in the forward direction. This, together with a new gauge condition based on effective uniformization, allows us to construct general solutions with limited decay at spacelike infinity. In particular, we construct solutions with minimal and even borderline decay, as considered in \cite{Shen23}, \cite{Shen24} in connection with the stability of the Minkowski space. In a forthcoming paper, we make use of the techniques we develop here to identify and construct a general class of short-pulse Cauchy data that lead to the formation of trapped surfaces, extending the well-known result of \cite{LiYu}.

gr-qc↗

arXiv2025

Solving the constraint equation for general free data

We revisit the problem of solving the Einstein constraint equations in vacuum by a new method, which allows us to prescribe four scalar quantities, representing the full dynamical degrees of freedom of the constraint system. We show that once appropriate gauge conditions have been chosen and four scalars freely specified (modulo $\ell\leq 1$ modes), we can rewrite the constraint equations as a well-posed system of coupled transport and elliptic equations on $2$-spheres, which we solve by an iteration procedure. Our method provides a large class of exterior solutions of the constraint equations that can be matched to given interior solutions, according to the existing gluing techniques. As such, it can be applied to provide a large class of initial Cauchy data sets evolving to black holes, generalizing the well-known result of the formation of trapped surfaces due to Li and Yu. Though in our main theorem, we only specify conditions consistent with $g-g_{Schw}=O(r^{-1-\delta})$, $k=O(r^{-2-\delta})$, the method is flexible enough to be applied in many other situations. It can, in particular, be easily adapted to construct arbitrarily fast decaying data. We expect, moreover, that our method can also be applied to construct data with slower decay, such as that used by Shen. In fact, an important motivation for developing our method is to show that the result of Shen is sharp, i.e., construct small, smooth initial data sets which violate Shen's decay conditions, and for which the stability of the Minkowski space result is wrong.

gr-qc↗

arXiv2025

Decay estimates of wave equations in un-isotropic media

We prove decay estimates for solutions to non-isotropic linear systems of wave equations. The defining feature of these estimates is that they depend only on the commutation properties of the system with the scaling vector field. As application we give two surprisingly simple proofs for small data global regularity results non-isotropic systems of wave equations in $\mathbb{R}^{1+3}$ with cubic semilinear nonlinearities. We hope that the techniques presented here are relevant for the more difficult and important case of biaxial refraction in crystal optics.

math.AP↗

arXiv2024

A canonical foliation on null infinity in perturbations of Kerr

Kerr stability for small angular momentum has been proved in the series of works by Klainerman-Szeftel, Giorgi-Klainerman-Szeftel and Shen. Some of the most basic conclusions of the result, concerning various physical quantities on the future null infinity are derived in the work of Klainerman-Szeftel. Further important conclusions were later derived in An-He-Shen and Chen-Klainerman. In this paper, based on the existence and uniqueness results for GCM spheres by Klainerman-Szeftel, we establish the existence of a canonical foliation on the future null infinity for which the null energy, linear momentum, center of mass and angular momentum are well defined and satisfy the expected physical laws of gravitational radiation. The rigid character of this foliation eliminates the usual ambiguities related to these quantities in the physics literature. We also show that under the initial assumption of Klainerman-Szeftel, the center of mass of the black hole has a large deformation (recoil) after the perturbation.

gr-qc↗

arXiv2024

Formation of Trapped Surfaces in Geodesic Foliation

We revisit the classical results of the formation of trapped surfaces for the Einstein vacuum equation relying on the geodesic foliation, rather than the double null foliation used in all previous results, starting with the seminal work of Christodoulou \cite{Chr1} and continued in \cite{KRodn}, \cite{An}, \cite{AnLuk}, \cite{KLR}, \cite{An1}. The main advantage of the method is that it only requires information on the incoming curvature along the incoming initial null hypersurface. The result is based on a version of the non-integrable PT frame introduced in \cite{KS:Kerr} and \cite{GKS}, associated to the geodesic foliation.

gr-qc↗

arXiv2024

Regularity of the Future Event Horizon in Perturbations of Kerr

The goal of the paper is to show that the event horizons of the spacetimes constructed in \cite{KS}, see also \cite{KS-Schw}, in the proof of the nonlinear stability of slowly rotating Kerr spacetimes $\mathcal{K}(a_0,m_0)$, are necessarily smooth null hypersurfaces. Moreover we show that the result remains true for the entire range of $|a_0|/m_0$ for which stability can be established.

gr-qc↗

arXiv2022

Brief introduction to the nonlinear stability of Kerr

This a brief introduction to the sequence of works \cite{KS:Kerr}, \cite{GKS-2022}, \cite{KS-GCM1}, \cite{KS-GCM2} and \cite{Shen} which establish the nonlinear stability of Kerr black holes with small angular momentum. We are delighted to dedicate this article to Demetrios Christodoulou for whom we both have great admiration. The first author would also like to thank Demetrios for the magic moments of friendship, discussions and collaboration he enjoyed together with him.

math.AP↗

arXiv2022

Wave equations estimates and the nonlinear stability of slowly rotating Kerr black holes

This is the last part of our proof of the nonlinear stability of the Kerr family for small angular momentum, i.e $|a|/m\ll 1$, in which we deal with the nonlinear wave type estimates needed to complete the project. More precisely we provide complete proofs for Theorems M1 and M2 as well the curvature estimates of Theorem M8, which were stated without proof in sections 3.7.1 and 9.4.7 of \cite{KS:Kerr}. Our procedure is based on a new general interest formalism (detailed in Part I of this work), which extends the one used in the stability of Minkowski space. Together with \cite{KS:Kerr} and the GCM papers \cite{KS-GCM1}, \cite{KS-GCM2}, \cite{Shen}, this work completes proof of the Main Theorem stated in Section 3.4 of \cite{KS:Kerr}.

math.AP↗

arXiv2021

Kerr stability for small angular momentum

This is our main paper in a series in which we prove the full, unconditional, nonlinear stability of the Kerr family $Kerr(a, m)$ for small angular momentum, i.e. $|a|/m\ll 1$, in the context of asymptotically flat solutions of the Einstein vacuum equations (EVE). Three papers in the series, \cite{KS-GCM1} and \cite{KS-GCM2} and \cite{GKS1} have already been released. We expect that the remaining ones \cite{GKS2}, \cite{KS:Kerr-B} and \cite{Shen} will appear shortly. Our work extends the strategy developed in \cite{KS}, in which only axial polarized perturbations of Schwarzschild were treated, by developing new geometric and analytic ideas on how to deal with with general perturbations of Kerr. We note that the restriction to small angular momentum appears only in connection to Morawetz type estimates in \cite{GKS2} and \cite{KS:Kerr-B}

math.AP↗

arXiv2020

A general formalism for the stability of Kerr

The goal of this paper is to provide a geometric framework for analyzing the uniform decay properties of solutions to the Teukolsky equation in the fully nonlinear setting of perturbations of Kerr. It contains the first nonlinear version of the Chandrasekhar transformation introduced in the linearized setting in \cite{D-H-R-Kerr} and \cite{Ma} with the intent to use it in our ongoing project to prove the full nonlinear stability of slowly rotating Kerr as solution to the Einstein vacuum equations.

math.AP↗

arXiv2019

Effective results on uniformization and intrinsic GCM spheres in perturbations of Kerr

This is a follow-up of our paper \cite{KS-Kerr1} on the construction of general covariant modulated (GCM) spheres in perturbations of Kerr, which we expect to play a central role in establishing their nonlinear stability. We reformulate the main results of that paper using a canonical definition of $\ell=1$ modes on a $2$-sphere embedded in a $1+3$ vacuum manifold. This is based on a new, effective, version of the classical uniformization theorem which allows us to define such modes and prove their stability for spheres with comparable metrics. The reformulation allows us to prove a second, intrinsic, existence theorem for GCM spheres, expressed purely in terms of geometric quantities defined on it. A natural definition of angular momentum for such GCM spheres is also introduced, which we expect to play a key role in determining the final angular momentum for general perturbations of Kerr.

math.AP↗

arXiv2019

Constructions of GCM spheres in perturbations of Kerr

This the first in a series of papers whose ultimate goal is to establish the full nonlinear stability of the Kerr family for $|a|\ll m$. The paper builds on the strategy laid out in \cite{KS} in the context of the nonlinear stability of Schwarzschild for axially symmetric polarized perturbations. In fact the central idea of \cite{KS} was the introduction and construction of generally covariant modulated (GCM) hypersurfaces on which specific geometric quantities take Schwarzschildian values. This was made possible by taking into account the full general covariance of the Einstein vacuum equations. The goal of this paper is to get rid of the symmetry restriction in the construction of GCM spheres and thus remove an essential obstruction in extending the result of \cite{KS} to a full stability proof of the Kerr family.

math.AP↗

arXiv2018

Global solution for massive Maxwell-Klein-Gordon equations

We derive the asymptotic properties of the mMKG system (Maxwell coupled with a massive Klein-Gordon scalar field), in the exterior of the domain of influence of a compact set. This complements the previous well known results, restricted to compactly supported initial conditions, based on the so called hyperboloidal method. That method takes advantage of the commutation properties of the Maxwell and Klein Gordon with the generators of the Poincar\'e group to resolve the difficulties caused by the fact that they have, separately, different asymptotic properties. Though the hyperboloidal method is very robust and applies well to other related systems it has the well known drawback that it requires compactly supported data. In this paper we remove this limitation based on a further extension of the vector-field method adapted to the exterior region. Our method applies, in particular, to nontrivial charges. The full problem could then be treated by patching together the new estimates in the exterior with the hyperboloidal ones in the interior. This purely physical space approach introduced here maintains the robust properties of the old method and can thus be applied to other situations such as the coupled Einstein Klein-Gordon equation.

math.AP↗

arXiv2017

Global Nonlinear Stability of Schwarzschild Spacetime under Polarized Perturbations

We prove the nonlinear stability of the Schwarzschild spacetime under axially symmetric polarized perturbations, i.e. solutions of the Einstein vacuum equations for asymptotically flat $1+3$ dimensional Lorentzian metrics which admit a hypersurface orthogonal spacelike Killing vectorfield with closed orbits. While building on the remarkable advances made in last 15 years on establishing quantitative linear stability, the paper introduces a series of new ideas among which we emphasize the general covariant modulation (GCM) procedure which allows us to construct, dynamically, the center of mass frame of the final state. The mass of the final state itself is tracked using the well known Hawking mass relative to a well adapted foliation itself connected to the center of mass frame. Our work here is the first to prove the nonlinear stability of Schwarzschild in a restricted class of nontrivial perturbations. To a large extent, the restriction to this class of perturbations is only needed to ensure that the final state of evolution is another Schwarzschild space. We are thus confident that our procedure may apply in a more general setting.

gr-qc↗