Table of contents

Classical and Quantum Gravity(CQG) welcomes articles on experimental gravitation. This includes articles on instrumentation that is mostly of interest for gravitational experiments, as long as the relationship to gravitation is clearly explained in the paper. Experimental authors should also take into account that the readership of CQG is broad, consisting of theorists and experimentalists. It is therefore beneficial to include in the introduction a summary that places the findings in a meaningful context for such a readership.

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String cosmology is revisited from a cosmological viewpoint of the holographic principle put forward by 't Hooft, and by Fischler and Susskind. It is shown that the holography principle requires the existence of a `graceful exit' mechanism, which renders the universe non-singular by connecting pre- and post-big-bang phases smoothly. It is proven that a flat universe is consistent with the holography principle only if it starts with an absolutely cold and vacuous state and particle entropy is produced during the `graceful exit' period. An open universe can always satisfy the holography principle no matter what the initial state of the universe is.

We present a model for the dark matter in spiral galaxies, which is a result of a static and axial symmetric exact solution of the Einstein-dilaton theory. We suppose that dark matter is a scalar field endowed with a scalar potential. We obtain that (a) the effective energy density goes like 1/(r² +r_c² ) and (b) the resulting circular velocity profile of test particles is in good agreement with the observed one.

A new approach to the inverse-scattering technique of Alekseev is presented which permits real-pole soliton solutions of the Ernst equations to be considered. This is achieved by adopting distinct real poles in the scattering matrix and its inverse. For the case in which the electromagnetic field vanishes, some explicit solutions are given using a Minkowski seed metric. The relation with the corresponding soliton solutions that can be constructed using the Belinskii-Zakharov inverse-scattering technique is determined.

We present a proof of the positive mass theorem for black holes in Einstein-Maxwell axion-dilaton gravity, which is the low-energy limit of heterotic string theory. We show that the total mass of a spacetime containing a black hole is greater than or equal to the square root of the sum of the squares of the adequate dilaton-electricand dilaton-magneticcharges.

We analyse the scalar radiation emitted from a source rotating around a Schwarzschild black hole using the framework of quantum field theory at the tree level. We show that for relativistic circular orbits the emitted power is about 20-30% smaller than what would be obtained in Minkowski spacetime. We also show that most of the emitted energy escapes to infinity. Our formalism can readily be adapted to investigate similar processes.

We study the Hamiltonian and constraints of a spherically symmetric dilaton gravity model. We find the ADM mass of the solution representing the Schwarzschild black hole in thermal equilibrium with the Hawking radiation.

A special subset of generalized fractional superstring models extending those of Argyres et alis studied. This subset concerns models based on SU_K (K )/U (1)^{K -1}Gepner parafermions. It is shown that there exists a remarkable link between generalized fractional superstrings based on SU_K (K )/U (1)^{K -1}Wess-Zumino-Witten theory with K= 2, 3 and 5, and the associative division algebras. These models have critical dimensions 10, 2 × 5 and 4 × 3, respectively, and are in one-to-one correspondence with real, Kähler, and hyper-Kähler target spaces. Moreover, we obtain field-theoretical realizations of c₀= 4 super-W₃and c₀= 12 super-W₅symmetries based on the K= 3 and 5 parafermions. It is also shown that the conformal anomaly of the parafermion ghosts of the worldsheet fractional supersymmetry is C_paraghost= 15 - K² .

In this paper we fill a necessary gap in order to realize an explicit comparison between the Kaluza-Klein spectra of supergravity compactified on AdS₄× X  ⁷and superconformal field theories living on the worldvolume of M2-branes. On the algebraic side we consider the superalgebra Osp ( |  4) and we study the double interpretation of its unitary irreducible representations either as supermultiplets of particle states in the bulk or as a conformal superfield on the boundary. On the Lagrangian field theory side we construct, using rheonomy rather than superfield techniques, the generic form of an = 2, d= 3 gauge theory. Indeed, the superconformal multiplets are supposed to be composite operators in a suitable gauge theory.

We consider gravitational wave modes in the Friedmann-Robertson-Walker metrics in a de Sitter phase and show that the state space splits into many unitarily inequivalent representations of the canonical commutation relations. Non-unitary time evolution is described as a trajectory in the space of the representations. The generator of time evolution is related to the entropy operator. The thermodynamic arrow of time is shown to point in the same direction as the cosmological arrow of time. The vacuum is a two-mode SU (1,1) squeezed state of thermo-field dynamics. The link between an expanding geometry, squeezing and thermal properties is exhibited.

A five-dimensional black hole (M₅ ) is investigated in the type IIB superstring theory compactified on S  ¹× T  ⁴ . This corresponds to AdS₃× S  ³× T  ⁴in the near horizon with asymptotically flat space. Here the harmonic gauge is introduced to decouple the mixing between the dilaton and others. On the other hand, we obtain the BTZ black hole (AdS₃× S  ³× T  ⁴ ) as the non-dilatonic solution. We calculate the grey-body factor of the dilaton as a test scalar both for a 5D black hole (M₅× S  ¹× T  ⁴ ) and the BTZ black hole (AdS₃× S  ³× T  ⁴ ). The result of the BTZ black hole agrees with the grey-body factor of the dilaton in the dilute gas approximation of a 5D black hole.

The apparent shapes of various Kerr-Newman spacetimes are plotted. For this purpose the geometry of closed photon orbits is studied, forming a subset of the bifurcation set A₄ , well known in elementary catastrophe theory. One additional result is that the cosmic censorship hypothesis guarantees that the spacetime casts a shadow, whereas naked ring singularities enable the visibility of the `anti-world' of negative radii rthrough its interior. Moreover, the optical restrictions to the observability of the shadow of an astronomical black hole are inferred.

I study a stochastic approach for warm inflation considering backreaction of the metric with the fluctuations of the matter field. This formalism takes into account the local inhomogeneities of the spacetime in a globally flat Friedmann-Robertson-Walker metric. The stochastic equations for the fluctuations of the matter field and metric are obtained. Finally, the dynamics for the amplitude of these fluctuations in a power-law expansion for the universe are examined.

A rigidity theorem that applies to smooth electrovacuum spacetimes which represent either (A) an asymptotically flat stationary black hole or (B) a cosmological spacetime with a compact Cauchy horizon ruled by closed null geodesics was given in a recent paper by Friedrich et al(1999 Commun. Math. Phys.204691-707). Here we enlarge the framework of the corresponding investigations by allowing the presence of other types of matter fields. In the first part the matter fields are involved merely implicitly via the assumption that the dominant energy condition is satisfied. In the second part Einstein-Klein-Gordon (EKG), Einstein-[non-Abelian]-Higgs (E[nA]H), Einstein-[Maxwell]-Yang-Mills-dilaton (E[M]YMd) and Einstein-Yang-Mills-Higgs (EYMH) systems are studied. The black hole event horizon or, respectively, the compact Cauchy horizon of the considered spacetimes is assumed to be a smooth non-degenerate null hypersurface. It is proved that there exists a Killing vector field in a one-sided neighbourhood of the horizon in EKG, E[nA]H, E[M]YMd and EYMH spacetimes. This Killing vector field is normal to the horizon, moreover, the associated matter fields are also shown to be invariant with respect to it. The presented results provide generalizations of the rigidity theorems of Hawking (for case A) and of Moncrief and Isenberg (for case B) and, in turn, they strengthen the validity of both the black hole rigidity scenario and the strong cosmic censor conjecture of classical general relativity.

In this paper, we show that all flat gravitational instantons have a spin structure and most of the flat gravitational instantons have a complex structure. We also prove that a gravitational instanton has an almost-complex structure if and only if its Euler characteristic is divisible by four.

Every (one-polarization) cylindrical wave solution of vacuum general relativity is completely determined by a corresponding axisymmetric solution to the free scalar wave equation on an auxiliary (2 + 1)-dimensional flat spacetime. The physical metric at radius Ris determined by the energy, (R ), of the scalar field in a box (in the flat spacetime) of radius R . In a recent work, among other important results, Ashtekar and Pierri have introduced a strategy to study the quantum geometry in this system, through a regularized quantum counterpart of (R ). We show that this regularized object is a densely defined symmetric operator, thereby correcting an error in their proof of this result. We argue that it admits a self-adjoint extension and show that the operator, unlike its classical counterpart, is not positive.

The Robertson-Walker spacetimes are conformally flat and so are conformally invariant under the action of the Lie group SO (4,2), the conformal group of Minkowski spacetime. We find a local coordinate transformation allowing the Robertson-Walker metric to be written in a manifestly conformally flat form for all values of the curvature parameter kcontinuously and use this to obtain the conformal Killing vectors of the Robertson-Walker spacetimes directly from those of the Minkowski spacetime. The map between the Minkowski and Robertson-Walker spacetimes preserves the structure of the Lie algebra so (4,2). Thus the conformal Killing vector basis obtained does not depend upon k , but has the disadvantage that it does not contain explicitly a basis for the Killing vector subalgebra. We present an alternative set of bases that depend (continuously) on kand contain the Killing vector basis as a sub-basis (these are compared with a previously published basis). In particular, bases are presented which include the Killing vectors for all Robertson-Walker spacetimes with additional symmetry, including the Einstein static spacetimes and the de Sitter family of spacetimes, where the basis depends on the Ricci scalar R

The low-energy (bosonic `heterotic') string theory is interpreted as a universal limit of the Kaluza-Klein reduction when the dimension of an internal space goes to infinity. We show that such an approach is helpful in obtaining classical solutions of the string model. As a particular application, we obtain new exact static solutions for the two-dimensional effective string model. They turn out to be in agreement with the generalized no-hair conjecture, in complete analogy with the four- and higher-dimensional Einstein theory of gravity.

We study the self-forces acting on static scalar and electric test charges in the spacetime of a Schwarzschild black hole. The analysis is based on a direct, local calculation of the self-forces via mode decomposition and on two independent regularization procedures: a spatially extended particle model method and on a mode-sum regularization prescription. In all cases we find excellent agreement with the known exact results.

Recent work concerning the electrostatic equilibrium of charged masses in general relativity is examined. Various points of criticism are addressed and a clarification of terminology is made.