Exotica or the failure of the strong cosmic censorship in four dimensions

In this letter a generic counterexample to the strong cosmic censor conjecture is exhibited. More precisely---taking into account that the conjecture lacks any precise formulation yet---first we make sense of what one would mean by a "generic counterexample" by introducing the mathematically unambigous and logically stronger concept of a "robust counterexample". Then making use of Penrose' nonlinear graviton construction (i.e., twistor theory) and a Wick rotation trick we construct a smooth Ricci-flat but not flat Lorentzian metric on the largest member of the Gompf--Taubes uncountable radial family of large exotic ${\mathbb R}^4$'s. We observe that this solution of the Lorentzian vacuum Einstein's equations with vanishing cosmological constant provides us with a sort of counterexample which is weaker than a "robust counterexample" but still reasonable to consider as a "generic counterexample". It is interesting that this kind of counterexample exists only in four dimensions.Comment: LaTeX, 11 pages, 1 figure, the final published versio

Note on a reformulation of the strong cosmic censor conjceture based on computability

In this letter we provide a reformulation of the strong cosmic censor conjecture taking into account recent results on Malament--Hogarth space-times. We claim that the strong version of the cosmic censor conjecture can be formulated by postulating that a physically relevant space-time is either globally hyperbolic or possesses the Malament--Hogarth property. But it is known that a Malament--Hogarth space-time in principle is capable for performing non-Turing computations such as checking consistency of ZFC set theory. In this way we get an intimate conjectured link between the cosmic censorship scenario and computability theory.Comment: LaTeX, 9 pages, 1 eps-figure; minor typos corrected and journal reference adde

S-duality in Abelian gauge theory revisited

Definition of the partition function of U(1) gauge theory is extended to a class of four-manifolds containing all compact spaces and certain asymptotically locally flat (ALF) ones including the multi-Taub--NUT spaces. The partition function is calculated via zeta-function regularization with special attention to its modular properties. In the compact case, compared with the purely topological result of Witten, we find a non-trivial curvature correction to the modular weights of the partition function. But S-duality can be restored by adding gravitational counter terms to the Lagrangian in the usual way. In the ALF case however we encounter non-trivial difficulties stemming from original non-compact ALF phenomena. Fortunately our careful definition of the partition function makes it possible to circumnavigate them and conclude that the partition function has the same modular properties as in the compact case.Comment: LaTeX; 22 pages, no figure

The topology of asymptotically locally flat gravitational instantons

In this letter we demonstrate that the intersection form of the Hausel--Hunsicker--Mazzeo compactification of a four dimensional ALF gravitational instanton is definite and diagonalizable over the integers if one of the Kahler forms of the hyper-Kahler gravitational instanton metric is exact. This leads to the topological classification of these spaces. The proof exploits the relationship between L^2 cohomology and U(1) anti-instantons over gravitational instantons recognized by Hitchin. We then interprete these as reducible points in a singular SU(2) anti-instanton moduli space over the compactification leading to the identification of its intersection form. This observation on the intersection form might be a useful tool in the full geometric classification of various asymptotically locally flat gravitational instantons.Comment: 9 pages, LaTeX, no figures; Some typos corrected, slightly differs from the published versio

Gravity as a four dimensional algebraic quantum field theory

Based on a family of indefinite unitary representations of the diffeomorphism group of an oriented smooth 4-manifold, a manifestly covariant 4 dimensional and non-perturbative algebraic quantum field theory formulation of gravity is exhibited. More precisely among the bounded linear operators acting on these representation spaces we identify algebraic curvature tensors hence a net of local quantum observables can be constructed from C*-algebras generated by local curvature tensors and vector fields. This algebraic quantum field theory is extracted from structures provided by an oriented smooth 4-manifold only hence possesses a diffeomorphism symmetry. In this way classical general relativity exactly in 4 dimensions naturally embeds into a quantum framework. Several Hilbert space representations of the theory are found. First a “tautological representation” of the limiting global C*-algebra is constructed allowing to associate to any oriented smooth 4-manifold a von Neumann algebra in a canonical fashion. Secondly, influenced by the Dougan–Mason approach to gravitational quasilocal energy-momentum, we construct certain representations what we call “positive mass representations” with unbroken diffeomorphism symmetry. Thirdly, we also obtain “classical representaions” with spontaneously broken diffeomorphism symmetry corresponding to the classical limit of the theory which turns out to be general relativity. Finally we observe that the whole family of “positive mass representations” comprise a 2 dimensional conformal field theory in the sense of G. Segal

Gravitational interpretation of the Hitchin equations

By referring to theorems of Donaldson and Hitchin, we exhibit a rigorous AdS/CFT-type correspondence between classical 2+1 dimensional vacuum general relativity theory on S x R and SO(3) Hitchin theory (regarded as a classical conformal field theory) on the spacelike past boundary S, a compact, oriented Riemann surface of genus greater than one. Within this framework we can interpret the 2+1 dimensional vacuum Einstein equation as a decoupled ``dual'' version of the 2 dimensional SO(3) Hitchin equations. More precisely, we prove that if over S with a fixed conformal class a real solution of the SO(3) Hitchin equations with induced flat SO(2,1) connection is given, then there exists a certain cohomology class of non-isometric, singular, flat Lorentzian metrics on S x R whose Levi--Civita connections are precisely the lifts of this induced flat connection and the conformal class induced by this cohomology class on S agrees with the fixed one. Conversely, given a singular, flat Lorentzian metric on S x R the restriction of its Levi--Civita connection gives rise to a real solution of the SO(3) Hitchin equations on S with respect to the conformal class induced by the corresponding cohomology class of the Lorentzian metric.Comment: LaTeX, 14 pages, no figures; compared with the previous version, Proposition 2.4 and its proof presented in a more clear for

Spin(7)-manifolds and symmetric Yang--Mills instantons

In this Letter we establish a relationship between symmetric SU(2) Yang--Mills instantons and metrics with Spin(7)-holonomy. Our method is based on a slight extension of that of Bryant and Salamon developed to construct explicit manifolds with special holonomies in 1989. More precisely, we prove that making use of symmetric SU(2) Yang--Mills instantons on Riemannian spin-manifolds, we can construct metrics on the chiral spinor bundle whose holonomies are within Spin(7). Moreover if the resulting space is connected, simply connected and complete, the holonomy coincides with Spin(7). The basic example is the metric constructed on the chiral spinor bundle of the round four-sphere by using a generic SU(2)-instanton of unit action; hence it is a five-parameter deformation of the Bryant--Salamon example, also found by Gibbons, Page and Pope.Comment: 10 pages, no figures, LaTeX. More references have been added; but this version differs from the published on

Szimmetria és Csoporthatások az Algebrai Topológiában = Symmetry and Group Actions in Algebraic Topology

Pályázatunk három, lazán összefüggő problémakört érint. Több kiemelkedő eredményt értünk el a Thom polinomok, és általánosabban, az ekvivariáns obstrukciók elméletében. A Thom sorok bevezetése és a Morin szingularitások Thom polinomjainak kiszámítása a terület legfontosabb eredményei az utóbbi években. A geometriai oldalon, komoly előrehaladást értünk el a hiperkahler modulusterek geometriájának leírásában, és sikerült bebizonyítanunk a Batyrev-Materov tükör reziduum sejtést tórikus orbifoldokra. Végezetül, projektünk algebrai eredményei között megemlítjük új 2-karakterisztikai jelenségek felfedezését az ortogonális csoport reprezentációelméletében, és a Zamolodcsikov periodicitási sejtés bizonyítását Y-rendszerekre. Ezenkívül, új algebrai egyenlőtlenségeket találtunk szemidefinit mátrixokra, és ezeket felhasználva megjavítottuk a legjobb ismert alsó becslést valós lineáris funkcionálok szorzatára. | The project deals with three loosely interconnected areas of mathematics. We obtained a number of outstanding results in the theory of Thom polynomials, and more generally, in equivariant obstruction theory. In particular, the introduction of Thom series, and the calculation of the Thom polynomials of Morin singularities are the most important advances in the subject in the last few years. On the more geometric side, we made serious progress in the description of the geometry of hyperkahler moduli spaces, and proved the Batyrev-Materov mirror residue conjecture for toric orbifolds. Finally, the more algebraic results of our project include discovering new characteristic-2 phenomena in the representation theory of the orthogonal group, and proving the Zamolodchikov periodicity conjecture for Y-systems. We also found new algebraic inequalities for semidefinite matrices, and using these, improved the best known lower bound on products of real linear functionals

A rigidity theorem for nonvacuum initial data

In this note we prove a theorem on non-vacuum initial data for general relativity. The result presents a ``rigidity phenomenon'' for the extrinsic curvature, caused by the non-positive scalar curvature. More precisely, we state that in the case of asymptotically flat non-vacuum initial data if the metric has everywhere non-positive scalar curvature then the extrinsic curvature cannot be compactly supported.Comment: This is an extended and published version: LaTex, 10 pages, no figure

Geometric construction of new Yang-Mills instantons over Taub-NUT space

In this paper we exhibit a one-parameter family of new Taub-NUT instantons parameterized by a half-line. The endpoint of the half-line will be the reducible Yang-Mills instanton corresponding to the Eguchi-Hanson-Gibbons L^2 harmonic 2-form, while at an inner point we recover the Pope-Yuille instanton constructed as a projection of the Levi-Civita connection onto the positive su(2) subalgebra of the Lie algebra so(4). Our method imitates the Jackiw-Nohl-Rebbi construction originally designed for flat R^4. That is we find a one-parameter family of harmonic functions on the Taub-NUT space with a point singularity, rescale the metric and project the obtained Levi-Civita connection onto the other negative su(2) subalgebra of so(4). Our solutions will possess the full U(2) symmetry, and thus provide more solutions to the recently proposed U(2) symmetric ansatz of Kim and Yoon.Comment: 13 pages, LaTex, no figure