In the last 30 days

• Open/close Physical time and other conceptual issues of quantum gravity on the example of loop quantum cosmology

  Wojciech Kamiński et al  2009 Class. Quantum Grav. 26 035012 Tag this article

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  Several conceptual aspects of quantum gravity (QG) are studied on the example of the homogeneous isotropic loop quantum cosmology (LQC) model. In particular: (i) the proper time of the comoving observers is shown to be a quantum operator and a quantum spacetime metric tensor operator is derived. (ii) Solutions of the quantum scalar constraint for two different choices of the lapse function are compared and contrasted. In particular it is shown that in the case of a model with massless scalar field and cosmological constant Λ, the physical Hilbert spaces constructed for two choices of lapse are the same for Λ < 0 while they are significantly different for Λ > 0. (iii) The mechanism of the singularity avoidance is analyzed via detailed studies of an energy density operator, whose essential spectrum was shown to be an interval , where . (iv) The relation between the kinematical and the physical quantum geometry is discussed on the level of relation between observables.

• Open/close What is a particle?

  Daniele Colosi and Carlo Rovelli  2009 Class. Quantum Grav. 26 025002 Tag this article

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  Theoretical developments related to gravitational interaction have questioned the notion of particle in quantum field theory (QFT). For instance, uniquely defined particle states do not exist in general, in QFT on a curved spacetime. More generally, particle states are difficult to define in a background-independent quantum theory of gravity. These difficulties have led some to suggest that in general QFT should not be interpreted in terms of particle states, but rather in terms of eigenstates of local operators. Still, it is not obvious how to reconcile this view with the empirically-observed ubiquitous particle-like behavior of quantum fields, apparent for instance in experimental high-energy physics, or 'particle' physics. Here we offer an element of clarification by observing that already in flat space there exist—strictly speaking—two distinct notions of particles: globally defined n-particle Fock-states and local particle states. The last describes the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are appropriately large, global and local particle states converge in a weak topology (but not in norm). This observation has little relevance for flat-space theories—it amounts to a reminder that there are boundary effects in realistic detectors—but is relevant for gravity. It reconciles the two points of view mentioned above. More importantly, it provides a definition of the local particle state that remains well defined even when the conventional global particle states are not defined. This definition plays an important role in quantum gravity.

• Open/close Torsion formulation of gravity

  M A Lledó and L Sommovigo  2010 Class. Quantum Grav. 27 065014 Tag this article

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  We explain precisely what it means to have a connection with torsion as a solution of the Einstein equations. While locally the theory remains the same, the new formulation allows for topologies that would have been excluded in the standard formulation of gravity. In this formulation it is possible to couple arbitrary torsion to gauge fields without breaking the gauge invariance.

• Open/close A counterexample to a recent version of the Penrose conjecture

  Alberto Carrasco and Marc Mars  2010 Class. Quantum Grav. 27 062001 Tag this article

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  By considering suitable axially symmetric slices on the Kruskal spacetime, we construct a counterexample to a recent version of the Penrose inequality in terms of so-called generalized apparent horizons.

• Open/close Positioning systems in Minkowski spacetime: from emission to inertial coordinates

  Bartolomé Coll et al  2010 Class. Quantum Grav. 27 065013 Tag this article

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  The coordinate transformation between emission coordinates and inertial coordinates in Minkowski spacetime is obtained for arbitrary configurations of the emitters. It appears that a positioning system always generates two different coordinate domains, namely, the front and the back emission coordinate domains. For both domains, the corresponding covariant expression of the transformation is explicitly given in terms of the emitter world lines. This task requires the notion of the orientation of an emitter configuration. The orientation is shown to be computable from the emission coordinates for the users of a 'central' region of the front emission coordinate domain. Other spacetime regions associated with the emission coordinates are also outlined.

• Open/close Accelerated expansion of a universe containing a self-interacting Bose–Einstein gas

  Germán Izquierdo and Jaime Besprosvany  2010 Class. Quantum Grav. 27 065012 Tag this article

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  Acceleration of the universe is obtained from a model of non-relativistic particles with a short-range attractive interaction, at low enough temperature to produce a Bose–Einstein condensate. Conditions are derived for negative-pressure behavior. In particular, we show that a phantom-accelerated regime at the beginning of the universe solves the horizon problem, consistently with nucleosynthesis.

• Open/close Three types of superpotentials for perturbations in the Einstein-Gauss-Bonnet gravity

  A N Petrov  2010 Class. Quantum Grav. 27 069801 Tag this article

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  The full text of this article is available in the PDF provided.

• Open/close Boundary terms unbound! Holographic renormalization of asymptotically linear dilaton gravity

  Robert B Mann and Robert McNees  2010 Class. Quantum Grav. 27 065015 Tag this article

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  A variational principle is constructed for gravity coupled to an asymptotically linear dilaton and a p-form field strength. This requires the introduction of appropriate surface terms—also known as 'boundary counterterms'—in the action. The variation of the action with respect to the boundary metric yields a boundary stress tensor, which is used to construct conserved charges that generate the asymptotic symmetries of the theory. In most cases a minimal set of assumptions leads to a unique set of counterterms. However, for certain examples we find families of actions that depend on one or more continuous parameters. We show that the conserved charges and the value of the on-shell action are always independent of these parameters.

• Open/close Regularity of Cauchy horizons in S² × S¹ Gowdy spacetimes

  Jörg Hennig and Marcus Ansorg  2010 Class. Quantum Grav. 27 065010 Tag this article

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  We study general S ² × S ¹ Gowdy models with a regular past Cauchy horizon and prove that a second (future) Cauchy horizon exists, provided that a particular conserved quantity J is not zero. We derive an explicit expression for the metric form on the future Cauchy horizon in terms of the initial data on the past horizon and conclude the universal relation A _p A _f = (8π J) ² where A _p and A _f are the areas of past and future Cauchy horizons respectively.

• Open/close Neutrino oscillations in the Kerr–Newman spacetime

  Jun Ren and Cheng-Min Zhang  2010 Class. Quantum Grav. 27 065011 Tag this article

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  The mass neutrino oscillation in the Kerr–Newman (K–N) spacetime is studied in the plane θ = θ ₀, and general equations of the oscillation phases are given. The effect of the rotation and electric charge on the phase is presented. Then, we consider three special cases. (1) The neutrinos travel along the geodesics with angular momentum L = aE in the equatorial plane. (2) The neutrinos travel along the geodesics with L = 0 in the equatorial plane. (3) The neutrinos travel along the radial geodesics in the direction θ = 0. Finally, we calculate the proper oscillation length in the K–N spacetime. The effect of the gravitational field on the oscillation length is embodied in the gravitational red shift factor. When the neutrino travels out of the gravitational field, a blue shift of the oscillation length takes place. We discuss the variation of the oscillation length influenced by the gravitational field strength, the rotation a ² and charge Q.