# Einstein field equations in perspectives from PDE and functional analysis

The Einstein field equations have been subject of research in theoretical physics, and differential geometry, apparently with methods from classical analysis and geometry. In particular, solutions in closed form have been of interest.

It seems that the classical programme of the PDE community, i.e., (i) existence (ii) uniqueness (iii) regularity, heavily employing concepts from functional analysis, has not found prominent application in general relativity.

Why is this the case? Is it simple due to the Sobolev theory on manifolds still being rather fresh, or do serious technical obstacles exist? Or have I overlooked something?

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What would qualify as a "prominent application"?The standard introductory text Wald's General Relativity covers this material in Chapter 10. Also several groups have this as their research focus. – Kelly Davis Jul 13 '13 at 19:32
Sobolev spaces on manifolds has been around for a long time and are used routinely for PDE's on manifolds. They are certainly used for theorems on the existence and regularity of solutions to Einstein's equations. – Deane Yang Jul 13 '13 at 23:10
Although it is by now rather straightforward to prove the existence, uniqueness, and regularity of the initial value problem to Einstein's equations for small time, there are indeed serious technical difficulties to proving anything ore than that But there is by now a lot of work in this direction. – Deane Yang Jul 13 '13 at 23:13
@DeaneYang: Was the study of Sobolev spaces on manifolds initiated specifically for the purpose of solving the Cauchy Problem in General Relativity? I have been unable to find any authoritative references for the topic except Choquet-Bruhat’s General Relativity and the Einstein Equations. – user36116 Nov 13 '13 at 6:30
You can try looking at the original papers and see if they give any references to prior work on Sobolev spaces on manifolds. I don't know the answer. – Deane Yang Nov 13 '13 at 12:20

The statement

It seems that the classical programme of the PDE community, i.e., (i) existence (ii) uniqueness (iii) regularity, heavily employing concepts from functional analysis, has not found prominent application in general relativity.

is just plain wrong. You are overlooking quite a lot of stuff. For a modern presentation of the mathematics I would second Ben Whale in recommending Hans Ringstrom's book The Cauchy Problem in General Relativity. Here let me give a bit of historical remarks.

The local wellposedness (after imposing gauge conditions, for reasons already described in Rafe Mazzeo and Peter Michor's answers) of Einstein's equations is a result more than 60 years old! The original proof was published by Yvonne Choquet-Bruhat in 1952 (before she took her current last name), and yes, more modern presentations usually use heavily the notion of Sobolev spaces on manifolds.

Insofar as the local Cauchy problem is concerned, the Einstein system is actually, in some aspects, easier than some of the elliptic and parabolic problems on manifolds. This is principally due to the fact that in appropriate gauge conditions the equations of motion are manifestly hyperbolic (in fact a quasilinear wave equation). Such equations automatically enjoy finite speed of propagation, and thus we can more easily localise the analysis onto individual coordinate patches. That is to say, for the local problem we don't need much of the machinery of global analysis where the geometry and topology of the underlying manifold in the large may come into play.

Perhaps the only conceptually tricky bit of the Cauchy problem for the Einstein system is that, from the very get-go, there is no preferred notion of time on the solution manifold. So whereas in classical non-geometric PDEs the notion of a local existence theorem is stated in the form "there exists $t> 0$ such that a solution exists on $(0,t) \times D$ where $D$ is some domain", the corresponding theorem in general relativity would more naturally look like "there exists a manifold $M$ into which the initial data embeds as a codimension 1 Riemannian manifold with the initial conditions satisfied." This lack of a preferred notion of time also makes it more difficult to interpret what is meant by "global in time solution" to the Cauchy problem.

In 1969 Choquet-Bruhat and Robert Geroch showed that, essentially due to the hyperbolic nature of the equations, one can define the maximal manifold $M$ into which all solutions embed. With this notion one can then formulate the question of "global Cauchy problem" as a question of studying the geometric properties of this maximal solution.

It is well known that this maximal solution can be, in general, quite bad. There are plenty of explicit solutions to Einstein's equation which illustrate this, none-the-least the classical families of black hole space-times. In particular, the classical Schwarzschild solutions are geodesically incomplete in its maximum extension, while the classical Minkowski space is geodesically complete. A natural question, for example, is to ask about the genericity of "geodesic completeness" as a property for maximal solutions of the Cauchy problem, in terms of the initial data. That geodesic incompleteness is generic (in the sense that there exists open sets of initial data that are close to Schwarzschild initial data that leads always to incomplete solutions) turns out to be something that can be established geometrically with minimum amounts of PDE theory. One can read the result off of the incompleteness theorem of Penrose, which establishes a sufficient condition for geodesic incompleteness (the existence of trapped surfaces) and that the existence of trapped surface is an "open condition" and hence is preserved for small perturbations by classical Cauchy stability of the initial value problem at finite times. (A more detailed account is given in Mihalis Dafermos' exposé "The formation of black holes in general relativity (after D. Christodoulou)" for the Seminaire Bourbaki; see Astérisque (2012) 64, Exp. No. 1051.)

The genericity of the case of completeness turns out to be much harder, in terms of the PDE portion. Whereas for the incompleteness case the geometric criterion of Penrose allows us to use "finite time Cauchy stability", which is automatically true in view of the local wellposedness theorems, for the completeness case we have no such criterion and in fact needs to prove global Cauchy stability. In terms of the classical theory of nonlinear PDEs, this corresponds to, roughly speaking, proving global in time existence, with suitable decays, for small data to an initial value problem. Even for simple nonlinear wave equations this is not understood until at least the 70s and 80s. The developments in those two decades led to the understanding that in the physically relevant case of 3 space and 1 time dimensions, the appropriate small data global existence for nonlinear wave equations is not true in general. For Einstein's field equations (which, as described above, can be cast as a quasilinear wave equation, at least locally), this means one needs to search for geometric structure which affords one extra cancellations. As alluded to by Peter Michor, this was done by Demetrius Christodoulou and Sergiu Klainerman in their 500-page opus. (An alternative proof was more recently obtained by Igor Rodnianski and Hans Lindblad; the comparative shorter length testifies to how much the technology for nonlinear wave equations have evolved in the past two decades.)

Along this line of investigation, a further development was produced by Christodoulou in the past few years. As discussed above by Penrose's theorem one automatically has that initial data close to that of Schwarzschild space-time will lead to geodesically incomplete space-times. One may ask whether one can construct data initially far from that of Schwarzschild and still get singularity formation. And the answer turns out to be yes.

The result of Christodoulou and Klainerman on the global stability of Minkowski space actually yielded much more information then just geodesic completeness. It gives precise asymptotic convergence of such a "small data" solution to Minkowski space, and gave a complete description of the "conformal infinity" for such solutions. The aforementioned "quick" proof of stability of geodesic incompleteness by way of Penrose's theorem does not yield nearly as much information. And an ongoing active program of research (involving too many people to be reasonably listed here) is to demonstrate that similar asymptotic stability result with convergence and so forth can be had also for the classical black hole space-times. This is the motivation for the recent bloom in the study of the linear wave equation on black hole space-times.

There are also many other aspects of the global existence problem in general relativity. For example, one can try to consider situations where the initial manifold has compact topology or situations where the initial manifold has symmetries. The former is carefully studied in cosmological settings (I refer again to Ringstrom's book); though oftentimes with high degrees of symmetry so that the system reduces to that of a nonlinear system of ODEs. A spectacular example of the latter is the analysis by Choquet-Bruhat and Vincent Moncrief of solutions with an $U(1)$ symmetry. Under this particular symmetry assumption, and with the so-called CMC gauge condition, the evolution equations of the Einstein vacuum problem reduces to a coupled system of (a) an ODE on the cotangent bundle of Teichmuller space (describing the geometry of the "constant time" slices) and (b) a wave(map) equation.

Coming back to the local well-posedness problem in 3+1 dimensions: in the regime of "classical" solutions, existence, uniqueness, and regularity follows from a very similar analysis to that of Hughes, Kato, Marsden. To close the argument one needs to work in the Sobolev space $H^{5/2+}$ (or classically $H^3$ if you don't want to deal with fractional numbers of derivatives). One may ask about the optimal regularity for the local well-posedness statements. The scaling regularity should be $H^{3/2}$. However, the equation is quasilinear, and it is generally the case that compared to semilinear problems we cannot expect to go all the way down to scaling critical (see for example these two papers of Hans Lindblad 1 2). This problem motivated a body of literature studying the below-classical-regularity local existence problem for quasilinear wave equations. Between the groups of Klainerman-Rodnianski and that of Hart Smith and Daniel Tataru (esp. this paper and this other one), for general quasilinear wave equations the sharp exponent $H^{2+}$ was obtained. For the Einstein vacuum equations specifically, though, one may do better: a recently posted series of pre-prints by Klainerman, Rodnianski, and Jeremie Szeftel obtains a local existence theorem at the regularity level $H^2$ (these are arXiv items: http://arxiv.org/abs/1204.1772 http://arxiv.org/abs/1204.1767 http://arxiv.org/abs/1204.1768 http://arxiv.org/abs/1204.1769 http://arxiv.org/abs/1204.1770 http://arxiv.org/abs/1204.1771 http://arxiv.org/abs/1301.0112).

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It has found application. The main problem is that Einstein's equations have no type due to the large symmetry group (the whole diffeomorphism group). So first one has to fix a gauge; this is done by fixing a space like hypersurface $\Sigma$.

Added in edit: As Deane Yang mentioned, after fixing a gauge, Einsteins equations become hyperbolic.

Then the initial conditions are a Riemannian metric on $\Sigma$ and a second funcdamental form along $\Sigma$. It is one of the great advances in nonlinear PDE-theory that then there exists a solution and it is unique and it is smooth, if all initial data are small and smooth. This was proved by Christodoulou and Klainerman. See, e.g.,

• MR1946854 (2004f:58036) Reviewed Klainerman, Sergiu(1-PRIN); Nicolò, Francesco(I-ROME2M) The evolution problem in general relativity. Progress in Mathematical Physics, 25. Birkhäuser Boston, Inc., Boston, MA, 2003. xiv+385 pp. ISBN: 0-8176-4254-4

for an account of this.

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To me, Einstein's equation do have a type; they're hyperbolic. You do have to deal with the degeneracy due to the invariance under the group of diffeomorphisms, but once you do that, the system is clearly hyperbolic. This is analogous to how the Yang-Mills equations are now viewed as elliptic PDE's and the Ricci flow is parabolic. – Deane Yang Jul 13 '13 at 22:56
Also, the existence, uniqueness, and regularity of a solution to the initial value problem for vacuum space-time and for small time was done much earlier, I believe by Choquet-Bruhat. Christodoulou and Klainerman did indeed prove a spectacular theorem (followed by others) back in the late 80's, that, given appropriate sufficiently small initial data, there exists a unique global solution for all time. – Deane Yang Jul 13 '13 at 23:07

For a through overview of the use of functional analysis in General Relativity read Alan Rendall's "Partial Differential Equations in General Relativity". It's basically an elongated literature review but you'll read about things like distributional solutions of Einstein's equations as well as applications to numerical cosmology. The book doesn't contain technical details, but does have lots of references and it does, pretty much, mention every active area of application of functional analysis to General Relativity.

I also suggest Ringstorm's "The Cauchy Problem in General Relativity". Pretty much the first third of the book discusses Sobolov spaces in the abstract. The rest of the book discusses their explicit application to existence, uniqueness and reguality of Einstein's equations. Ringstorm pays particular attention to the interplay of geometric details and functional analysis. I found it refreshing to see the two sides of GR brought so nicely together. Though he does only present one of the many proofs of existence. One of my favorite parts is the combination of functional analysis, differential topology and a neat use of Zorn's lemma to prove global existence.

It's worthwhile noting that not just hyperbolic equations are important in General Relativity. Depending on the way you gauge fix the constraint equations can be elliptic or parabolic or neither.

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I just want to weigh in to say that in fact quite a lot is now known about the evolutionary aspects of the Einstein equations. As comments above note, the equations have a diffeomorphism invariance which make it necessary to impose a gauge, but the first main well-posedness result goes back to the mid 1950's (Choquet-Bruhat), and the work of Klainerman, Rodnianski, Christadoulou, Lindblad, Dafermos and many others over the past two decades have utilized and in some cases motivated some of the key new advances in the theory of nonlinear hyperbolic equations. The use of modern techniques from nonlinear functional analysis pervade much of this work.

As for the stationary side of the theory, i.e. the study of the constraint equations, this is analytically less intricate (but is tied up with quite a lot of deep geometric ideas), but here too many ideas from nonlinear functional analysis (e.g. the Schauder fixed point theorem) are key techniques in the most recent existence theorems.

I think it is also incorrect to say that Sobolev theory on manifolds is rather `fresh' -- this goes back a half century or more and is the cornerstone of much of modern geometric analysis.

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