Can wormholes exist?

Renormalized vacuum expectation values of electromagnetic stress-energy tensor are calculated in the background spherically-symmetrical metric of the wormhole topology. The covariant geodesic point separation method of regularization is used. Violation of the weak energy condition at the throat of the wormhole takes place for a geometry sufficiently close to that of an infinitely long wormhole of constant radius irrespective of the detailed from of metric. This is an argument in favour of the possibility of the existence of a self-consistent wormhole in empty space maintained by vacuum field fluctuations in the wormhole's background.     

Some general properties of the renormalized stress-energy tensor for static quantum states on (<Emphasis Type="Italic">n</Emphasis> + 1)-dimensional spherically symmetric black holes

We study the renormalized stress-energy tensor (RSET) for static quantum states on (n + 1)-dimensional, static, spherically symmetric black holes. By solving the conservation equations, we are able to write the stress-energy tensor in terms of a single unknown function of the radial co-ordinate, plus two arbitrary constants. Conditions for the stress-energy tensor to be regular at event horizons (including the extremal and “ultra-extremal” cases) are then derived using generalized Kruskal-like co-ordinates. These results should be useful for future calculations of the RSET for static quantum states on spherically symmetric black hole geometries in any number of space-time dimensions.     

A MODEL OF CURVATURE,TORSION AND STRONG GRAVIEY

In this paper the stress-energy tensors of curvature and of torsion are introduced.We may derived a model of strong gravity from Einstein's equation with the stress-energy tensor of torsion,while Einstein's equation with the stressenergy tensor of curvature is an inconsistent equation.This conclusion is different from the Poincare gauge theories of gravitation,in which the curvature is directly proportional to the strong coupling.     

Stress-energy tensors and the Lichnerowicz Laplacian

To a critical point of a variational problem, we associate a divergence-free symmetric 2-tensor, called the stress-energy tensor. We calculate the Laplacian of this object as defined by Lichnerowicz. This has the property that it commutes with the divergence provided the Ricci curvature is covariantly constant. We deduce relations between different stress-energy tensors, discuss growth formulae and harmonic maps between spheres.     

Constraining conformal field theory with higher spin symmetry in four dimensions

We analyze the constraints on the general form and the singularity structure of the correlation functions of the symmetric, traceless and conserved stress-energy tensor implied by conformal invariance and higher spin symmetry in four dimensions. In particular, we show that all these correlation functions will have at most double pole singularities. We then compute the 4-, 5- and 6-point functions of the stress-energy tensor and find that they are linear combinations of the three free field expressions (scalar, fermion and Maxwell field). This is a strong indication that all such theories are essentially free.     

Cosmological Model with Ω M -Dependent Cosmological Constant

We investigate the evolution of the scale factor in a cosmological model in which the cosmological constant is given by the scalar arisen by the contraction of the stress-energy tensor.     

A multivector derivative approach to Lagrangian field theory

A new calculus, based upon the multivector derivative, is developed for Lagrangian mechanics and field theory, providing streamlined and rigorous derivations of the Euler-Lagrange equations. A more general form of Noether's theorem is found which is appropriate to both discrete and continuous symmetries. This is used to find the conjugate currents of the Dirac theory, where it improves on techniques previously used for analyses of local observables. General formulas for the canonical stress-energy and angular-momentum tensors are derived, with spinors and vectors treated in a unified way. It is demonstrated that the antisymmetric terms in the stress-energy tensor are crucial to the correct treatment of angular momentum. The multivector derivative is extended to provide a functional calculus for linear functions which is more compact and more powerful than previous formalisms. This is demonstrated in a reformulation of the functional derivative with respect to the metric, which is then used to recover the full canonical stress-energy tensor. Unlike conventional formalisms, which result in a symmetric stress-energy tensor, our reformulation retains the potentially important antisymmetric contribution.Supported by a SERC studentship.     

A local analytic characterization of Schwarzschild metrics

A local characterization of Schwarzschild metrics is made by showing the space-time is locally a 2 by 2 warped product and admitting a static reference frame on its certain open subsets under some assumptions on the global analytic structure and stress-energy tensor of the space-time, such as, assuming the existence of solutions to certain partial differential equations and the existence of a radiation stress-energy tensor consistent with these solutions on the space-time.     

An alternative f(R, T) gravity theory and the dark energy problem

Recently, a generalized gravity theory was proposed by Harko et al. where the Lagrangian density is an arbitrary function of the Ricci scalar R and the trace of the stress-energy tensor T, known as F(R,T) gravity. In their derivation of the field equations, they have not considered conservation of the stress-energy tensor. In the present work, we have shown that a part of the arbitrary function f(R,T) can be determined if we take into account of the conservation of stress-energy tensor, although the form of the field equations remain similar. For homogeneous and isotropic model of the universe the field equations are solved and corresponding cosmological aspects has been discussed. Finally, we have studied the energy conditions in this modified gravity theory both generally and a particular case of perfect fluid with constant equation of state.     

Quantum effects in the Gowdy T3 cosmology

The Gowdy T³ Cosmology is an exact solution to the vacuum Einstein equations interpreted to be a single polarization of gravitational waves propagating in an anisotropic, spatially inhomogeneous background. The classical behavior is reviewed and related to standard cosmological parameters. Canonical quantization of the dynamical degrees of freedom is reviewed. An adiabatic vacuum state is constructed. Adiabatic regularization is used to obtain non-divergent stress-energy tensor vacuum expectation values. Casimir energy terms due to T³ imposed discrete modes are evaluated. The vacuum expectation values are analyzed in early and late time limits and evaluated numerically. The regularized expectation value is used as a source for the classical background spacetime in the spirit of semi-classical gravity. An entirely vacuum expectation value source term produces essentially the time reverse of the classical evolution. Classical stress-energy added to the source restores the classical behavior at late times only. The combined system collapses from infinite to small but non-zero volume and reexpands. The classical singularity is replaced by a symmetric bounce.     

Puzzle of bulk conformal field theories at central charge c = 0

Nontrivial critical models in 2D with a central charge c=0 are described by logarithmic conformal field theories (LCFTs), and exhibit, in particular, mixing of the stress-energy tensor with a "logarithmic" partner under a conformal transformation. This mixing is quantified by a parameter (usually denoted b), introduced in Gurarie [Nucl. Phys. B546, 765 (1999)]. The value of b has been determined over the last few years for the boundary versions of these models: b(perco)=-5/8 for percolation and b(poly)=5/6 for dilute polymers. Meanwhile, the existence and value of b for the bulk theory has remained an open problem. Using lattice regularization techniques we provide here an "experimental study" of this question. We show that, while the chiral stress tensor has indeed a single logarithmic partner in the chiral sector of the theory, the value of b is not the expected one; instead, b=-5 for both theories. We suggest a theoretical explanation of this result using operator product expansions and Coulomb gas arguments, and discuss the physical consequences on correlation functions. Our results imply that the relation between bulk LCFTs of physical interest and their boundary counterparts is considerably more involved than in the nonlogarithmic case.     

An “interior” of the Kerr metric

We show that the Kerr solution may be interpreted as arising form a thin rigidly rotating disk with a regular interior and a singular rim. The surface stress-energy tensor in the interior of the disk represents a fictitious material with positive isotropic stress and zero energy density.     

Wormhole Geometries in f(R,T) Gravity

We study wormhole solutions in the framework of f(R,T) gravity where R is the scalar curvature, and T is the trace of the stress-energy tensor of the matter. We have obtained the shape function of the wormhole by specifying an equation of state for the matter field and imposing the flaring out condition at the throat. We show that in this modified gravity scenario, the matter threading the wormhole may satisfy the energy conditions, so it is the effective stress-energy that is responsible for violation of the null energy condition.     

Comparison of Exact and Perturbative Results for Two Metrics

We compare the exact and perturbative results in two metrics and show that the spurious effects due to the perturbation method do not survive for physically relevant quantities such as the vacuum expectation value of the stress-energy tensor.     

The cosmic acceleration due to an alternative interpretation of the cosmological constant

In common discussions, the cosmological constant is identified with vacuum energy. The idea here is to identify it with the Lorentz-invariant scalar arisen by the contraction of the stress-energy tensor of ordinary matter which represents a form of the quintessence. We describe a quintessence cosmological scenario within the Friedmann universe and confront it with data.     

Comments on the Stress-Energy Tensor Operator in Curved Spacetime

 The technique based on a *-algebra of Wick products of field operators in curved spacetime, in the local covariant version proposed by Hollands and Wald, is strightforwardly generalized in order to define the stress-energy tensor operator in curved globally hyperbolic spacetimes. In particular, the locality and covariance requirement is generalized to Wick products of differentiated quantum fields. Within the proposed formalism, there is room to accomplish all of the physical requirements provided that known problems concerning the conservation of the stress-energy tensor are assumed to be related to the interface between the quantum and classical formalism. The proposed stress-energy tensor operator turns out to be conserved and reduces to the classical form if field operators are replaced by classical fields satisfying the equation of motion. The definition is based on the existence of convenient counterterms given by certain local Wick products of differentiated fields. These terms are independent from the arbitrary length scale (and any quantum state) and they classically vanish on solutions of the Klein-Gordon equation. Considering the averaged stress-energy tensor with respect to Hadamard quantum states, the presented definition turns out to be equivalent to an improved point-splitting renormalization procedure which makes use of the nonambiguous part of the Hadamard parametrix only that is determined by the local geometry and the parameters which appear in the Klein-Gordon operator. In particular, no extra added-by-hand term g _αβQ and no arbitrary smooth part of the Hadamard parametrix (generated by some arbitrary smooth term ``ω ₀ ') are involved. The averaged stress-energy tensor obtained by the point-splitting procedure also coincides with that found by employing the local ζ-function approach whenever that technique can be implemented. Received: 24 September 2001/Accepted: 14 May 2002 Published online: 22 November 2002     

Spurious Effects in Perturbative Calculations

We show spurious effects in perturbativecalculations due to different orderings of inhomogeneousterms while computing corrections to Green functions fortwo different metrics. These effects are not carried over to physically measurable quantities likethe renormalized value of the vacuum expectation valueof the stress-energy tensor.     

The renormalized stress-energy tensor and its associated operators for photons in curved four-dimensional space–time

We establish expressions for the renormalized stress-energy (or energy-momentum) tensor and its associated operators relative to photons as a Klein–Gordon field of non-zero rest-mass particles (with gravitational interaction) in curved four-dimensional space–time.