Classical solutions and the energy-momentum tensor

A hyperelliptic two-meron solution of the massless scalar φ^N theory in $\text{n =}\text{2N}\text{(N ? 2)}$ Euclidean dimensions is given. This solution (which interpolates between the two-meron solution and the instanton solution of this theory) is used to illustrate several theory-independent statements which can be made about the energy-momentum tensor for instanton, meron and elliptic meron solutions of all scale invariant classical field theories.     

Massless versus massive Hawking radiation in AdS2 spacetime

We study massless and massive Hawking radiations on a two-dimensional AdS spacetime. For the massless case, the quantum stress-energy tensor of a massless scalar field on the AdS background is calculated, and the expected null radiation is obtained. However, for the massive case, the scattering analysis is performed in order to calculate the absorption and reflection coefficients which are related to statistical Hawking temperature. On the contrary to the massless case, we obtain a nonvanishing massive radiation.     

The Post-Minkowskian Limit of <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>)-gravity

We formally discuss the post-Minkowskian limit of f(R)-gravity without adopting conformal transformations but developing all the calculations in the original Jordan frame. It is shown that such an approach gives rise, in general, together with the standard massless graviton, to massive scalar modes whose masses are directly related to the analytic parameters of the theory. In this sense, the presence of massless gravitons only is a peculiar feature of General Relativity. This fact is never stressed enough and could have dramatic consequences in detection of gravitational waves. Finally the role of curvature stress-energy tensor of f(R)-gravity is discussed showing that it generalizes the so called Landau-Lifshitz tensor of General Relativity. The further degrees of freedom, giving rise to the massive modes, are directly related to the structure of such a tensor.     

Semiclassical gravitational effects near cosmic strings

The vacuum expectation value of the stress-energy tensor of an arbitrary collection of conformal massless free quantum fields (scalar, spinor, and vector) in the presence of a static, cylindrically symmetric cosmic string is found, up to an undetermined numerical constant. This quantum stress-energy tensor is then used as a source in the linearized semiclassical Einstein equations, which are solved to find the first-order (in Ł) corrections to the exterior metric of a static, cylindrically symmetric cosmic string. The main result is that, at first-order in Ł, the (r, ø) two-space is no longer a simple flat cone; to this order the (r, ø) two- space is a (linearized) hyperboloid, which asymptotically approaches the classical conical surface at large values of r. The asymptotic value of the deficit angle of the cone is unchanged, still being precisely 8πμ.     

Manifest chiral symmetry preservation in the 1N-expanded SU(N) Thirring model

By utilizing manifestly chiral-invariant auxiliary field operators, it is demonstrated that continuous chiral symmetry is preserved explicitly in the $\text{1}\text{N}$ expansion of 2-dimensional theories whose fermions have a Gross-Neveu type of potential. The effective lagrangian derived in the $\text{1}\text{N}$ limit describes a massless scalar field whose derivative coupling to the fermions vanishes as N → ∞, and a decoupled massive scalar field.     

Stress-energy tensor for parallel plate on background of conformally flat brane-world geometries and cosmological constant problem

In this paper, we calculate the stress-energy tensor for a quantized massless conformally coupled scalar field with a background of conformally flat brane-world geometries, where the scalar field satisfies Robin boundary conditions on two parallel plates. In the general case of Robin boundary conditions formulae are derived for the vacuum expectation values of the energy-momentum tensor. Further the surface energy per unit area is obtained. As an application of the general formulae we have considered the important special case of the AdS_{4 + 1} bulk; moreover the application to the Randall-Sundrum scenario is discussed. In this specific example for a certain choice of Robin coefficients, one could make the effective cosmological constant vanish.Received: 17 March 2004, Revised: 29 September 2004, Published online: 26 November 2004     

Generalised Kerr-Schild space-times

If V is a space-time with metric tensor g_μν admitting a null, geodesic shear free vector field l_μ, then one may determine a function H so that the spacetime $\text{V}\text{?}$ with metric g_μν = g_μν + 2Hl_μl_ν satisfies the Einstein field equations for various material sources, and for no sources. When V is Minkowski space, $\text{V}\text{?}$ is a Kerr-Schild space-time. In case V is a vacuum space-time, one may choose H so that the source is a null fluid with no pressure. In case V is a Robertson-Walker universe H may be chosen so that the source has a stress-energy tensor with one timelike proper vector and three spacelike ones. There are two equal proper values associated with the latter vectors and one which differs from these. The stress-energy tensor describing this source may be interpreted as representing a perfect fluid with anisotropic pressures or as one describing the sum of a perfect fluid with isotropic pressures and a presureless null fluid. Vaidya's Kerr metric in a cosmological background [Pramana8 (1977) 512–517] is discussed as is the metric representing an accelerating point mass in an expanding universe.     

Spontaneous symmetry breakdown and Regge intercepts

By Regge slope methods we exhibit a Nambu-Goldstone spontaneous symmetry breakdown and a Higgs mechanism inherent in a unit-intercept dual meson amplitude. Introducing an Abelian gauge group corresponding to conservation of a pseudocharge, Regge intercept shifts are calculated; to lowest order the leading Regge intercept of the Neveu-Schwarz model is lowered from $\text{α(0) = 1}\text{to}\text{α(0) =}\text{1}\text{2}$. Our method has the advantage over Bardakci's spurion method that massless scalar are not required at any stage.     

Plane symmetric self-gravitating fluids with pressure equal to energy density

Solutions of the Cauchy problem associated with the Einstein field equations which satisfy general initial conditions are obtained under the assumptions that (1) the source of the gravitational field is a perfect fluid with pressure,p, equal to energy density,w, and (2) the space-time admits the three parameter group of motions of the Euclidean plane, that is, the space-time is plane symmetric. The results apply to the situation where the source of the gravitational field is a massless scalar field since such a source has the same stress-energy tensor as an irrotational fluid withp=w. The relation between characteristic coordinates and comoving ones is discussed and used to interpret a number of special solutions. A solution involving a shock wave is discussed.     

Flavour and superunification

We consider the possibility of a non-trivial embedding of $\text{(}\text{10}\text{+}\text{5}\text{)}\text{SU}\text{(5)}$ families of spin if$\text{1}\text{2}$ left-handed fermions in a combination of irreducible massless supermultiplets of N extended supersymmetry. We demand the whole spectrum of spin $\text{1}\text{2}$ states to be anomaly free with respect to SU(N). This turns out to be a necessary condition for the absence of anomalies at the SU(5) level. We find two classes of models, with spin $\text{1}\text{2}$ fermions in SU(N) representations associated to one- and two-column Young tableaux, respectively, in which each irreducible massless multiplet occurs at most once. These two classes of models lead to a nontrivial family generation due to supersymmetry. For N = 8 extended supersymmetry, they give at most three and five families, respectively. The first class of models is more natural in the way it excludes SU(5) exotics. The same analysis is extended to the massless multiplets that can be obtained from bilinear composite fields of the (preonic) elementary fields of N extended supergravity. We prove that the generation of families requires the repetition of massless multiplets and that ($\text{10 +}\text{5}$) SU(5) families can only be generated in pairs. General properties of multilinear composite operators of the preonic fields are given and the rôle of massive representations to classify towers of operators with definite spin is pointed out.     

Trace anomaly of the stress-energy tensor for massless vector particles propagating in a general background metric

We reanalyze the problem of regularization of the stress-energy tensor for massless vector particles propagating in a general background metric, using covariant point separation techniques applied to the Hadamard elementary solution. We correct an error, pointed out by Wald, in the earlier formulation of Adler, Lieberman, and Ng, and find a stress-energy tensor trace anomaly agreeing with that found by other regularization methods.     

Weak Gravitational Wave and Casimir Energy of a Scalar Field

In this paper, we calculate the effect of a weak gravitational field on the Casimir force between two ideal plates subjected to a massless minimally coupled field. It is the aim of this work to study the Casimir energy under a weak perturbation of gravity. Moreover, the fluctuations of the stress-energy tensor for a scalar field in de Sitter space-time are computed as well.     

The charged scalar-graviton coupling to order α

The electrodynamical corrections of order α to the charged scalar-graviton vertex are calculated in quantum field theory using the improved energy-momentum tensor. It is found that the deviations from Newton's law are of the r^?2 type, exactly the same as for the spin-$\text{1}\text{2}$ case. Also, the trace of the energy-momentum tensor is found not to vanish, at the one-loop level, in the limit of zero-mass scalar particles.     

Higher spin vierbein gauge fermions and hypergravities

The actions describing any massless fermion of spin $\text{?}\text{5}\text{2}$ are given uniformly in terms of non-symmetric vierbein-like fields ψ_μã…ãs. Hamiltonian analysis of their dynamical content is performed explicitly for $\text{s =}\text{5}\text{2}\text{7}\text{2}$, and indicated for the general case. The consistency problems in gravitational coupling are discussed. For spin $\text{5}\text{2}$, they are equivalent to those in symmetric tensor-spinor formulation. The general spin case has the same structure: in Minkowski signature, the consistency requirements are too restrictive (vanishing Weyl tensor); their euclidean counterparts require self-duality in both gravity and matter.     

Massless composites with massive constituents

We construct model field theories in which a confining gauge interaction binds massive elementary fermions into massless composite particles. The massless composites are either Goldstone bosons or $\text{spin}\text{-}\text{1}\text{2}$ fermions. In these models, the manner in which exact chiral symmetries are realized changes at a critical value of the elementary fermion mass of order (e²/16π²)Λ, where Λ is the confinement scale and e is a weak gauge coupling.     

Soft spin 32 fermions require gravity and supersymmetry

If massless fermions of spin $\text{3}\text{2}$ have non-vanishing low-energy couplings, the fermions must have massless partners of spin 2, and all particles to which the fermions couple must display supersymmetry.     

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.