Generic cosmic-censorship violation in anti-de Sitter space

We consider (four-dimensional) gravity coupled to a scalar field with potential V(phi). The potential satisfies the positive energy theorem for solutions that asymptotically tend to a negative local minimum. We show that for a large class of such potentials, there is an open set of smooth initial data that evolve to naked singularities. Hence cosmic censorship does not hold for certain reasonable matter theories in asymptotically anti-de Sitter spacetimes. The asymptotically flat case is more subtle. We suspect that potentials with a local Minkowski minimum may similarly lead to violations of cosmic censorship in asymptotically flat spacetimes, but we do not have definite results.     

Kalb–Ramond Dipole Solution in Low-Energy Bosonic String Theory

We construct a new solution subspace for the bosonic string theory toroidally compactified to 3 dimensions. This subspace corresponds to the complex harmonic scalar field coupled to the effective 3–dimensional gravity. We calculate a class of the asymptotically flat and free of the Dirac string peculiarity solutions which describes a Kalb–Ramond dipole source with the generally nontrivial dilaton characteristics.     

Asymptotic structure near event horizon and Cardy-Verlinde formula for general asymptotically flat stationary black hole

The asymptotically anti-de Sitter structure near event horizon of general asymptotically flat stationary black hole is found, and the Cardy-Verlinde formula is generalized to the asymptotically flat black holes in the Einstein-Maxwell theory and low-energy effective field theory describing string. The result that the entropy of conformal field theory (CFT) agrees precisely with black-hole entropy provides a CFT interpretation of the Bekenstein-Hawking entropy of the asymptotically flat stationary black holes.     

Spinor fields at spacelike infinity

The concept of a spinor structure at spacelike infinity is introduced for space-times which are asymptotically flat. It is shown how zero-rest-mass fields on space-time acquire smooth limits on this structure and that these limits satisfy certain differential equations characterized by the helicity and regularity of the field. The geometry of the limits of twistor fields is also discussed, and it seems possible that one can define the momentum and angular momentum of an asymptotically flat space-time in terms of a twistor space at spacelike infinity.     

Nonrotating cosmic strings interacting with gravitational waves in Einstein-Maxwell-dilaton gravity

We present methods for the construction of exact diagonal cylindrically symmetric solutions in a four dimensional low energy limit of string theory, the Einstein-Maxwell-dilaton gravity. The methods allow us to generate exact string backgrounds from known solutions to the equations of Einstein or Einstein gravity coupled to a massless scalar field. We also give and analyze explicit examples of such solutions. It is shown that they are free of curvature singularities,(quasi)regular on the axis of symmetry, asymptotically flat and describe nonrotating cosmic strings interacting with gravitational, dilaton and electromagnetic waves.     

Stability analysis and observational measurement in 5-D Brans–Dicke cosmology

This Letter is a study of the effects of higher dimensional gravity and Brans–Dicke (BD) scalar field on cosmic acceleration in 5-D BD cosmological model. We assume a flat cosmological model in which the matter content of the universe is either cold dark matter or radiation. In a framework to study attractor solutions in the phase space we simultaneously constrain the model parameters with the observational data for distance modulus. The phase space analysis illustrates that the universe begins from an unstable state in the past and eventually reaches an asymptotically stable state (attractor). We examine the model by performing Hubble parameter test in addition to statefinder diagnosis. We also reconstruct the equation of state parameter, the scale factor in 3-D space and along extra dimension. The results show that due to the presence of extra dimension and Brans–Dicke scalar field in the model, the universe undergoes a period of acceleration.     

Null infinity, H-space and equations of motion

We discuss the existence, arising by analogy to that in algebraically special space-times, of a unique asymptotically shear-free congruence in any asymptotically flat space-time. Associated with it is a unique complex analytic curve in H-space. The surprising potential physical significance of this curve is discussed. Fifth Award in the 2006 Essay Competition of the Gravity Research Foundation.     

The static gravitational field near spatial infinity I

A static solution to Einstein's equations, which is asymptotically flat near spatial infinity, is shown to be asymptotically Schwarzschildian. As an application of this result we prove a technical lemma on the existence of asymptotically flat harmonic coordinates near infinity.     

Thermodynamics of black holes in anti-de Sitter space

The Einstein equations with a negative cosmological constant admit black hole solutions which are asymptotic to anti-de Sitter space. Like black holes in asymptotically flat space, these solutions have thermodynamic properties including a characteristic temperature and an intrinsic entropy equal to one quarter of the area of the event horizon in Planck units. There are however some important differences from the asymptotically flat case. A black hole in anti-de Sitter space has a minimum temperature which occurs when its size is of the order of the characteristic radius of the anti-de Sitter space. For larger black holes the red-shifted temperature measured at infinity is greater. This means that such black holes have positive specific heat and can be in stable equilibrium with thermal radiation at a fixed temperature. It also implies that the canonical ensemble exists for asymptotically anti-de Sitter space, unlike the case for asymptotically flat space. One can also consider the microcanonical ensemble. One can avoid the problem that arises in asymptotically flat space of having to put the system in a box with unphysical perfectly reflecting walls because the gravitational potential of anti-de Sitter space acts as a box of finite volume.     

Rotating Solutions of Einstein-Maxwell-Dilaton Gravity with Unusual Asymptotics

We study electrically charged, dilaton black holes, which possess infinitesimal angular momentum in the presence of one or two Liouville type potentials. These solutions are neither asymptotically flat nor (anti)-de Sitter. Some properties of the solutions are discussed.     

Distorting metric with vanishing twist and divergence

In linear relativity, we obtain a gravitational field having irrotational and shearing geodesic rays with vanishing expansion for asymptotically flat vacuum space-time. Such a field is not allowed by field equations in nonlinear theory.     

Induced Gravity Theory from Noether Symmetry

We discuss the functional form of the potential and the non-minimal coupling in the scalar tensor gravity (induced gravity) theories as allowed by the Noether symmetry in the spatially homogenous and isotropic spacetime background. The solution of the field equations (for k=0) are presented by using the results obtained from the Noether symmetry. It has been observed that the potential and the coupling function obtained from the Noether symmetric approach do not satisfy the continuity equation for k=± 1. Finally we present an inflationary solution that goes over to the Einstein's gravity asymptotically as t .     

Mass and angular-momentum inequalities for axi-symmetric initial data sets. II. Angular momentum

We extend the validity of Dain’s angular-momentum inequality to maximal, asymptotically flat, initial data sets on a simply connected manifold with several asymptotically flat ends which are invariant under a U(1) action and which admit a twist potential.     

均匀弱磁场中的Kasuya双荷外部引力场

利用Ernst方法,在Harrison变换为线性变换的条件下,得到Kasuya荷电荷磁的外部引力场解,解中所含常量B₀表示一均匀弱磁场,该解不要求q=-2B₀J.当r趋于∞时,度规接近Melvin’s磁宇宙的渐进行为.该度规是非渐近平直的,在均匀外磁场下这是合理的结果. 关键词： Ernst方法 Kasuya双荷 磁场 引力场     

A New Parametrization for Tetrad Gravity

A new version of tetrad gravity in globally hyperbolic, asymptotically flat at spatial infinity spacetimes with Cauchy surfaces diffeomorphic to R ³ is obtained by using a new parametrization of arbitrary cotetrads to define a set of configurational variables to be used in the ADM metric action. Seven of the fourteen first class constraints have the form of the vanishing of canonical momenta. A comparison is made with other models of tetrad gravity and with the ADM canonical formalism for metric gravity.     

Noether Symmetry Study in General Scalar Tensor Theory

We present the gravitational coupling function and potential function in a General Scalar Tensor Theory from Noether symmetry approach. Some exact cosmological solutions are presented in the spatially flat, homogeneous and isotropic background. The solutions are exponentially expanding without any graceful exit from inflation. Also the solutions asymptotically reduce to Einstein's theory with finite value of the coupling function supporting the earlier claim of Santiago et al. Further it is shown that in a particular case the effective Newtonian constant is determined by the effective potential of the scalar field.     

On the stability of flat space

It is shown that: (1) there exists, near flat space, a neighborhood of nonsingular asymptotically flat weak field solutions of the initial value equations of General Relativity; the solutions have physically appropriate generality; and (2) this neighborhood is complete and flat space is stable; every geometry representing a weak perturbation and satisfying the varied constraints is tangent to the space of solutions.     

Black Hole Formation from a Complete Regular Past

An open problem in general relativity has been to construct an asymptotically flat solution to a reasonable Einstein-matter system containing a black hole and yet causally geodesically complete to the past, containing no white holes. We construct such a solution in this paper–in fact a family of such solutions, stable in a suitable sense–where matter is described by a self-gravitating scalar field.     

Instability of flat spacetime in semiclassical gravity

The semiclassical theory of gravity is considered in which an asymptotically flat background metric is coupled to quantized matter. We show that, in general, there are modes with spacelike wave vectors for small metric fluctuations around flat spacetime. Besides the usual axioms of quantum field theory in flat spacetime, the proof rests on the existence of a hard trace anomaly in the energy-momentum tensor due to matter self-couplings. Two possible interpretations of the result are discussed.