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Published: 15 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa924f

Abstract:
In this paper we expand upon our previous work Coley et al (2016 Class. Quantum Grav. 33 215010) by using the entire family of Bianchi type V stiff fluid solutions as seed solutions of the Stephani transformation. Among the new exact solutions generated, we observe a number of important physical phenomena. The most interesting phenomenon is exact solutions with intersecting spikes. Other interesting phenomena are solutions with saddle states and a close-to-FL epoch.
, , David Serrano-Blanco
Published: 8 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa924a

Abstract:
We analyse the behaviour of the MacDowell–Mansouri action with internal symmetry group under the De Donder–Weyl Hamiltonian formulation. The field equations, known in this formalism as the De Donder–Weyl equations, are obtained by means of the graded Poisson–Gerstenhaber bracket structure present within the De Donder–Weyl formulation. The decomposition of the internal algebra allows the symmetry breaking , which reduces the original action to the Palatini action without the topological term. We demonstrate that, in contrast to the Lagrangian approach, this symmetry breaking can be performed indistinctly in the polysymplectic formalism either before or after the variation of the De Donder–Weyl Hamiltonian has been done, recovering Einstein's equations via the Poisson–Gerstenhaber bracket.
Published: 25 February 2011
Classical and Quantum Gravity, Volume 28; https://doi.org/10.1088/0264-9381/28/6/065013

Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa91f6

Abstract:
Generic resolution of singularities and geodesic completeness in the loop quantization of Bianchi-II spacetimes with arbitrary minimally coupled matter is investigated. Using the effective Hamiltonian approach, we examine two available quantizations: one based on the connection operator and second by treating extrinsic curvature as connection via gauge fixing. It turns out that for the connection based quantization, either the inverse triad modifications or imposition of weak energy condition is necessary to obtain a resolution of all strong singularities and geodesic completeness. In contrast, the extrinsic curvature based quantization generically resolves all strong curvature singularities and results in a geodesically complete effective spacetime without inverse triad modifications or energy conditions. In both the quantizations, weak curvature singularities can occur resulting from divergences in pressure and its derivatives at finite densities. These are harmless events beyond which geodesics can be extended. Our work generalizes previous results on the generic resolution of strong singularities in the loop quantization of isotropic, Bianchi-I and Kantowski–Sachs spacetimes.
Published: 9 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa91f4

Abstract:
The proper vertex amplitude is derived from the Engle-Pereira-Rovelli-Livine vertex by restricting to a single gravitational sector in order to achieve the correct semi-classical behaviour. We apply the proper vertex to calculate a cosmological transition amplitude that can be viewed as the Hartle-Hawking wavefunction. To perform this calculation we deduce the integral form of the proper vertex and use extended stationary phase methods to estimate the large-volume limit. We show that the resulting amplitude satisfies an operator constraint whose classical analogue is the Hamiltonian constraint of the Friedmann-Robertson-Walker cosmology. We find that the constraint dynamically selects the relevant family of coherent states and demonstrate a similar dynamic selection in standard quantum mechanics. We investigate the effects of dynamical selection on long-range correlations.
, Luisa G Jaime
Published: 20 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa91f5

Abstract:
We derive a class of non-static inhomogeneous dust solutions in gravity described by the Lemaître–Tolman–Bondi (LTB) metric. The field equations are fully integrated for all parameter subcases and compared with analogous subcases of LTB dust solutions of GR. Since the solutions do not admit regular symmetry centres, we have two possibilities: (i) a spherical dust cloud with angle deficit acting as the source of a vacuum Schwarzschild-like solution associated with a global monopole, or (ii) fully regular dust wormholes without angle deficit, whose rest frames are homeomorphic to the Schwarzschild–Kruskal manifold or to a 3d torus. The compatibility between the LTB metric and generic ansatzes furnishes an 'inverse procedure' to generate LTB solutions whose sources are found from the geometry. While the resulting fluids may have an elusive physical interpretation, they can be used as exact non-perturbative toy models in theoretical and cosmological applications of theories.
, Joydip Mitra, Soumitra Sengupta
Published: 15 December 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa91f7

Abstract:
Fermion localization in a braneworld model in presence of dilaton coupled higher curvature Gauss–Bonnet bulk gravity is discussed. It is shown that the lowest mode of left handed fermions can be naturally localized on the visible brane due to the dilaton coupled higher curvature term without the necessity of any external localizing bulk field.
James Healy, , , Manuela Campanelli
Published: 6 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa91b1

Abstract:
The RIT numerical relativity group is releasing a public catalog of black-hole-binary waveforms. The initial release of the catalog consists of 126 recent simulations that include precessing and non precessing systems with mass ratios $q=m_1/m_2$ in the range $1/6\leq q\leq1$. The catalog contains information about the initial data of the simulation, the waveforms extrapolated to infinity, as well as information about the peak luminosity and final remnant black hole properties. These waveforms can be used to independently interpret gravitational wave signals from laser interferometric detectors and the remnant properties to model the merger of binary black holes from initial configurations.
Published: 29 October 2004
Classical and Quantum Gravity, Volume 21, pp 5245-5251; https://doi.org/10.1088/0264-9381/21/22/015

Published: 29 October 2004
Classical and Quantum Gravity, Volume 21, pp 5233-5243; https://doi.org/10.1088/0264-9381/21/22/014

E Buffenoir, , K Noui,
Published: 29 October 2004
Classical and Quantum Gravity, Volume 21, pp 5203-5220; https://doi.org/10.1088/0264-9381/21/22/012

Ofer Aharony, Joseph Marsano, , Toby Wiseman
Published: 29 October 2004
Classical and Quantum Gravity, Volume 21, pp 5169-5191; https://doi.org/10.1088/0264-9381/21/22/010

Published: 7 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa9151

Abstract:
We study a large family of metric-affine theories with a projective symmetry, including non-minimally coupled matter fields which respect this invariance. The symmetry is straightforwardly realised by imposing that the connection only enters through the symmetric part of the Ricci tensor, even in the matter sector. We leave the connection completely free (including torsion), and obtain its general solution as the Levi-Civita connection of an auxiliary metric, showing that the torsion only appears as a projective mode. This result justifies the widely used condition of setting vanishing torsion in these theories as a simple gauge choice. We apply our results to some particular cases considered in the literature, including the so-called Eddington-inspired-Born–Infeld theories among others. We finally discuss the possibility of imposing a gauge fixing where the connection is metric compatible, and comment on the genuine character of the non-metricity in theories where the two metrics are not conformally related.
Marcus Khuri,
Published: 5 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa9154

Abstract:
We show that near-horizon geometries in the presence of a positive cosmological constant cannot exist with ring topology. In particular, de Sitter black rings with vanishing surface gravity do not exist. Our result relies on a known mathematical theorem which is a straightforward consequence of a type of energy condition for a modified Ricci tensor, similar to the curvature-dimension conditions for the m-Bakry-Émery-Ricci tensor.
, , Takahiro Miyamoto, , Koki Okutomi, Yoshinori Fujii, Hiroki Tanaka, Mark A Barton, Ryutaro Takahashi, , et al.
Published: 4 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa90e3

Abstract:
KAGRA is a 3-km cryogenic interferometric gravitational wave telescope located at an underground site in Japan. In order to achieve its target sensitivity, the relative positions of the mirrors of the interferometer must be finely adjusted with attached actuators. We have developed a model to simulate the length control loops of the KAGRA interferometer with realistic suspension responses and various noises for mirror actuation. Using our model, we have designed the actuation parameters to have sufficient force range to acquire lock as well as to control all the length degrees of freedom without introducing excess noise.
Published: 30 November 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa90e7

Abstract:
We prove uniqueness of the near-horizon geometries arising from degenerate Kerr black holes within the collection of nearby vacuum near-horizon geometries.
Published: 2 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa9039

Abstract:
The (generalized) Rainich conditions are algebraic conditions which are polynomial in the (mixed-component) stress-energy tensor. As such they are logically distinct from the usual classical energy conditions (NEC, WEC, SEC, DEC), and logically distinct from the usual Hawking-Ellis (Segre-Plebanski) classification of stress-energy tensors (type I, type II, type III, type IV). There will of course be significant inter-connections between these classification schemes, which we explore in the current article. Overall, we shall argue that it is best to view the (generalized) Rainich conditions as a refinement of the classical energy conditions and the usual Hawking-Ellis classification.
Chen-Yu Liu, , Chi-Yong Lin
Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa903b

Abstract:
We examine the dynamics of a neutral particle around a Kerr–Newman black hole, and in particular focus on the effects of the charge of the spinning black hole on the motion of the particle. We first consider the innermost stable circular orbits (ISCO) on the equatorial plane. It is found that the presence of the charge of the black hole leads to the effective potential of the particle with stronger repulsive effects as compared with the Kerr black hole. As a result, the radius of the ISCO decreases as charge Q of the black hole increases for a fixed value of the black hole's angular momentum a. We then consider a kick on the particle from its initial orbit out of the equatorial motion. The perturbed motion of the particle will eventually be bounded, or unbounded so that it escapes to spatial infinity. Even more, the particle will likely be captured by the black hole. Thus we analytically and numerically determine the parameter regions of the corresponding motions, in terms of the initial radius of the orbital motion and the strength of the kick. The comparison will be made with the motion of a neutral particle in the Kerr black hole.
Published: 2 October 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa903c

Abstract:
McVittie spacetimes embed the Schwarzschild(-(anti) de Sitter) spacetime in an isotropic FLRW background universe. We study the global structure of McVittie spacetimes with spatially non-flat FLRW backgrounds. We extend the definition of such spacetimes, previously given only for the flat and open cases, to the closed case. We revisit this definition and show how it gives rise to a unique spacetime (given the FLRW background, the mass parameter M and the cosmological constant Λ) in the open and flat cases. In the closed case, an additional free function of the cosmic time arises. We derive some basic results on the metric, curvature and matter content of McVittie spacetimes and derive a representation of the line element that makes the study of their global properties possible. In the closed case (independently of the free function mentioned above), the spacetime is confined (at each instant of time) to a region bounded by a minimum and a maximum area radius, and is bounded either to the future or to the past by a scalar curvature singularity. This allowed region only exists when the background scale factor is above a certain minimum. In the open case, radial null geodesics originate in finite affine time in the past at a boundary formed by the union of the Big Bang of the FLRW background and a non-singular hypersurface of varying causal character. In the case of eternally expanding open universes, we show that black holes are ubiquitous: ingoing radial null geodesics extend in finite affine time to a hypersurface that forms the boundary of the region from which photons can escape to future null infinity. We revisit the spatially flat McVittie spacetimes, and show that the black hole interpretation holds also in the case of a vanishing cosmological constant, contrary to a previous claim of ours.
Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa903f

Abstract:
We study a sufficient condition for proving the stability of a black hole when the master equation for linear perturbation takes the form of the Schrödinger equation. If the potential contains a small negative region, the S-deformation method is usually used to show the non-existence of an unstable mode. However, in some cases, it is hard to find an appropriate deformation function analytically because the only way found so far to find it is by trial-and-error. In this paper, we show that it is easy to find a regular deformation function by numerically solving the differential equation such that the deformed potential vanishes everywhere, when the spacetime is stable. Even if the spacetime is almost marginally stable, our method still works. We also discuss a simple toy model which can be solved analytically, and show that the condition for the non-existence of a bound state is the same as that for the existence of a regular solution for the differential equation in our method. From these results, we conjecture that our criteria is also a necessary condition.
Published: 20 February 2006
Classical and Quantum Gravity, Volume 23, pp 1721-1761; https://doi.org/10.1088/0264-9381/23/5/016

Abstract:
We compute the one loop fermion self-energy for massless Dirac + Einstein in the presence of a locally de Sitter background. We employ dimensional regularization and obtain a fully renormalized result by absorbing all divergences with BPHZ counterterms. An interesting technical aspect of this computation is the need for a noninvariant counterterm owing to the breaking of de Sitter invariance by our gauge condition. Our result can be used in the quantum-corrected Dirac equation to search for inflation-enhanced quantum effects from gravitons, analogous to those which have been found for massless, minimally coupled scalars.
Published: 15 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8fe2

Abstract:
We introduce the extended Freudenthal–Rosenfeld–Tits magic square based on six algebras: the reals , complexes , ternions , quaternions , sextonions and octonions . The sextonionic row/column of the magic square appeared previously and was shown to yield the non-reductive Lie algebras, , , , and , for and respectively. The fractional ranks are used to denote the semi-direct extension of the simple Lie algebra in question by a unique (up to equivalence) Heisenberg algebra. The ternionic row/column yields the non-reductive Lie algebras, , , , and , for and respectively. The fractional ranks here are used to denote the semi-direct extension of the semi-simple Lie algebra in question by a unique (up to equivalence) nilpotent 'Jordan' algebra. We present all possible real forms of the extended magic square. It is demonstrated that the algebras of the extended magic square appear quite naturally as the symmetries of supergravity Lagrangians. The sextonionic row (for appropriate choices of real forms) gives the non-compact global symmetries of the Lagrangian for the maximal , magic and magic non-supersymmetric theories, obtained by dimensionally reducing the parent theories on a circle, with the graviphoton left undualised. In particular, the extremal intermediate non-reductive Lie algebra (which is not a subalgebra of ) is the non-compact global symmetry algebra of , supergravity as obtained by dimensionally reducing , supergravity with symmetry on a circle. On the other hand, the ternionic row (for appropriate choices of real forms) gives the non-compact global symmetries of the Lagrangian for the maximal , magic and magic non-supersymmetric theories, as obtained by dimensionally reducing the parent theories on a circle. In particular, the Kantor–Koecher–Tits intermediate non-reductive Lie algebra is the non-compact global symmetry algebra of , supergravity as obtained by dimensionally reducing , supergravity with symmetry on a circle.
Published: 11 December 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa8fb3

Abstract:
We illustrate and examine diverse approaches to the quantum matter–gravity system which refer to the Born–Oppenheimer (BO) method. In particular we first examine a quantum geometrodynamical approach introduced by other authors in a manner analogous to that previously employed by us, so as to include back reaction and non-adiabatic contributions. On including such effects it is seen that the unitarity violating effects previously found disappear. A quantum loop space formulation (based on a hybrid quantisation, polymer for gravitation and canonical for matter) also refers to the BO method. It does not involve the classical limit for gravitation and has a highly peaked initial scalar field state. We point out that it does not resemble in any way our traditional BO approach. Instead it does resemble an alternative, canonically quantised, non BO approach which we have also previously discussed.
, Fabien Casse, Philippe Grandclement, Eric Gourgoulhon, Frederic Dauvergne
Published: 28 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8fb5

Abstract:
We present two-dimensional general relativistic hydrodynamics simulations of free-falling gas clouds onto rotating boson stars (BS). Those objects consist of a complex scalar field coupled to gravity. BS are interesting as black hole (BH) mimickers. By this, one means that they are very compact objects but without any event horizon. It is then expected that the physics around BS is different than the one around BH. In this paper, we consider two BS configurations and study the trajectories and internal properties of infalling gas clouds, varying their initial positions. We follow the various disruption phases of the cloud until the formation, in some cases, of a gas torus in the inner region of the BS . We then discuss the cloud capture process by BS and the torus formation. We find that the characteristic time for torus formation increases when the initial distance between of the cloud and the BS decreases.
Published: 27 September 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa8f7a

Abstract:
This paper presents the recent version of the Lunar Laser Ranging (LLR) analysis model at the Institut für Erdmessung (IfE), Leibniz Universität Hannover and highlights a few tests of Einstein's theory of gravitation using LLR data. Investigations related to a possible temporal variation of the gravitational constant, the equivalence principle, the PPN parameters β and γ as well as the geodetic precession were carried out. The LLR analysis model was updated by gravitational effects of the Sun and planets with the Moon as extended body. The higher-order gravitational interaction between Earth and Moon as well as effects of the solid Earth tides on the lunar motion were refined. The basis for the modeled lunar rotation is now a 2-layer core/mantle model according to the DE430 ephemeris. The validity of Einstein's theory was studied using this updated analysis model and a LLR data set from 1970 to January 2015. Within the estimated accuracies, no deviations from Einstein's theory are detected. A relative temporal variation of the gravitational constant is estimated as $\dot{G}$/G0=(7.1±7.6)×10-14 yr-1, the test of the equivalence principle gives Δ(mg/mi)EM=(-3±5)×10-14 and the Nordtvedt parameter η=(-0.2±1.1)×10-4, the PPN-parameters β and γ are determined as β-1=(-4.5±5.6)×10-5 and γ-1=(-1.2±1.2)×10-4 and the geodetic precession is confirmed within 0.09 %. The results for selected relativistic parameters are obtained by introducing constraints from a LLR solution without estimating relativistic quantities. The station coordinates are constrained for the estimation of $\dot{G}$/G0, β and γ, the initial value of the core rotation vector is constrained to a reasonable model value for the estimation of $\dot{G}$/G0 and geodetic precession. A constrained z-component of the initial lunar velocity is used for the estimation of the geodetic precession.
, , Behrouz Mirza
Published: 27 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8f7c

Abstract:
Lorentz gauge theory (LGT) is a feasible candidate for theory of quantum gravity in which routine field theory calculations can be carried out perturbatively without encountering too many divergences. In LGT spin of matter also gravitates. The spin-generated gravity is expected to be extremely stronger than that generated by mass and could be explored in current colliders. In this article the observable signals of the theory in an electron-positron collider is investigated. We specifically study pair annihilation into two gravitons, and LGT corrections to processes like $e^-+e^+\rightarrow \mu^-+\mu^+$ and $e^-+e^+\rightarrow e^-+e^+$.
, Bianca Dittrich
Published: 26 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8f24

Abstract:
One of the most pressing issues for loop quantum gravity and spin foams is the construction of the continuum limit. In this paper, we propose a systematic coarse-graining scheme for three-dimensional lattice gauge models including spin foams. This scheme is based on the concept of decorated tensor networks, which have been introduced recently. Here we develop an algorithm applicable to gauge theories with non-Abelian groups, which for the first time allows for the application of tensor network coarse-graining techniques to proper spin foams. The procedure deals efficiently with the large redundancy of degrees of freedom resulting from gauge symmetry. The algorithm is applied to 3D spin foams defined on a cubical lattice which, in contrast to a proper triangulation, allows for non--trivial simplicity constraints. This mimics the construction of spin foams for 4D gravity. For lattice gauge models based on a finite group we use the algorithm to obtain phase diagrams, encoding the continuum limit of a wide range of these models. We find phase transitions for various families of models carrying non--trivial simplicity constraints.
, Jason Koeller,
Published: 26 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8f2c

Abstract:
The quantum null energy condition (QNEC) is a conjectured bound on components $(T_{kk} = T_{ab} k^a k^b$) of the stress tensor along a null vector $k^a$ at a point $p$ in terms of a second $k$-derivative of the von Neumann entropy $S$ on one side of a null congruence $N$ through $p$ generated by $k^a$. The conjecture has been established for super-renormalizeable field theories at points $p$ that lie on a bifurcate Killing horizon with null tangent $k^a$ and for large-N holographic theories on flat space. While the Koeller-Leichenauer holographic argument clearly yields an inequality for general $(p,k^a)$, more conditions are generally required for this inequality to be a useful QNEC. For $d\le 3$, for arbitrary backgroud metric we show that the QNEC is naturally finite and independent of renormalization scheme when the expansion $\theta$ of $N$ at the point $p$ vanishes. This is consistent with the original QNEC conjecture which required $\theta$ and the shear $\sigma_{ab}$ to satisfy $\theta |_p= \dot{\theta}|_p =0$, $\sigma_{ab}|_p=0$. But for $d=4,5$ more conditions than even these are required. In particular, we also require the vanishing of additional derivatives and a dominant energy condition. In the above cases the holographic argument does indeed yield a finite QNEC, though for $d\ge6$ we argue these properties to fail even for weakly isolated horizons (where all derivatives of $\theta, \sigma_{ab}$ vanish) that also satisfy a dominant energy condition. On the positive side, a corrollary to our work is that, when coupled to Einstein-Hilbert gravity, $d \le 3$ holographic theories at large $N$ satisfy the generalized second law (GSL) of thermodynamics at leading order in Newton's constant $G$. This is the first GSL proof which does not require the quantum fields to be perturbations to a Killing horizon.
Richard H Price,
Published: 26 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8f29

Abstract:
The recent detection of gravitational waves has generated interest in alternatives to the black hole interpretation of sources. A subset of such alternatives involves a prediction of gravitational wave echoes''. We consider two aspects of possible echoes: First, general features of echoes coming from spacetime reflecting conditions. We find that the detailed nature of such echoes does not bear any clear relationship to quasi-normal frequencies. Second, we point out the pitfalls in the analysis of local reflecting walls'' near the horizon of rapidly rotating black holes.
David Langlois, Hongguang Liu, ,
Published: 26 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8f2f

Abstract:
Recently, Chamseddine and Mukhanov introduced a higher-derivative scalar-tensor theory which leads to a modified Friedmann equation allowing for bouncing solutions. As we note in the present work, this Friedmann equation turns out to reproduce exactly the loop quantum cosmology effective dynamics for a flat isotropic and homogeneous space-time. We generalize this result to obtain a class of scalar-tensor theories, belonging to the family of mimetic gravity, which all reproduce the loop quantum cosmology effective dynamics for flat, closed and open isotropic and homogeneous space-times.
Published: 26 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa89f4

Abstract:
Gravitoelectromagnetism (GEM) as a theory for gravity has been developed similar to the electromagnetic field theory. A weak field approximation of Einstein theory of relativity is similar to GEM. This theory has been quantized. Traditional Bhabha scattering, electron–positron scattering, is based on quantized electrodynamics theory. Usually the amplitude is written in terms of one photon exchange process. With the development of quantized GEM theory, the scattering amplitude will have an additional component based on an exchange of one graviton at the lowest order of perturbation theory. An analysis will provide the relative importance of the two amplitudes for Bhabha scattering. This will allow an analysis of the relative importance of the two amplitudes as the energy of the exchanged particles increases.
Steven Johnston
Published: 17 September 2015
Classical and Quantum Gravity, Volume 32; https://doi.org/10.1088/0264-9381/32/19/195020

Abstract:
The causal set approach to quantum gravity models spacetime as a discrete structure—a causal set. Recent research has led to causal set models for the retarded propagator for the Klein–Gordon equation and the d'Alembertian operator. These models can be compared to their continuum counterparts via a sprinkling process. It has been shown that the models agree exactly with the continuum quantities in the limit of an infinite sprinkling density—the continuum limit. This paper obtains the correction terms for these models for sprinkled causal sets with a finite sprinkling density. These correction terms are an important step towards testable differences between the continuum and discrete models that could provide evidence of spacetime discreteness.
, M Brüggen, C Gheller, S Hackstein, D Wittor, P M Hinz
Published: 7 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8e60

Abstract:
The origin of extragalactic magnetic fields is still poorly understood. Based on a dedicated suite of cosmological magneto-hydrodynamical simulations with the ENZO code we have performed a survey of different models that may have caused present-day magnetic fields in galaxies and galaxy clusters. The outcomes of these models differ in cluster outskirts, filaments, sheets and voids and we use these simulations to find observational signatures of magnetogenesis. With these simulations, we predict the signal of extragalactic magnetic fields in radio observations of synchrotron emission from the cosmic web, in Faraday rotation, in the propagation of ultra high energy cosmic rays, in the polarized signal from fast radio bursts at cosmological distance and in spectra of distant blazars. In general, primordial scenarios in which present-day magnetic fields originate from the amplification of weak (≤) uniform seed fields result in more homogeneous and relatively easier to observe magnetic fields than astrophysical scenarios, in which present-day fields are the product of feedback processes triggered by stars and active galaxies. In the near future the best evidence for the origin of cosmic magnetic fields will most likely come from a combination of synchrotron emission and Faraday rotation observed at the periphery of large-scale structures.
Published: 19 December 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa8e70

Abstract:
We analyze the stability of the Cauchy horizon associated with a globally naked, shell-focussing singularity arising from the complete gravitational collapse of a spherical dust cloud. In a previous work, we have studied the dynamics of spherical test scalar fields on such a background. In particular, we proved that such fields cannot develop any divergences which propagate along the Cauchy horizon. In the present work, we extend our analysis to the more general case of test fields without symmetries and to linearized gravitational perturbations with odd parity. To this purpose, we first consider test fields possessing a divergence-free stress-energy tensor satisfying the dominant energy condition, and we prove that a suitable energy norm is uniformly bounded in the domain of dependence of the initial slice. In particular, this result implies that free-falling observers co-moving with the dust particles measure a finite energy of the field, even as they cross the Cauchy horizon at points lying arbitrarily close to the central singularity. Next, for the case of Klein–Gordon fields, we derive point-wise bounds from our energy estimates which imply that the scalar field cannot diverge at the Cauchy horizon, except possibly at the central singular point. Finally, we analyze the behaviour of odd-parity, linear gravitational and dust perturbations of the collapsing spacetime. Similarly to the scalar field case, we prove that the relevant gauge-invariant combinations of the metric perturbations stay bounded away from the central singularity, implying that no divergences can propagate in the vacuum region. Our results are in accordance with previous numerical studies and analytic work in the self-similar case.
, Brian O’Reilly, Mario Diaz
Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8e6b

Abstract:
Noise produced by light being scattered from objects limited the sensitivity of the laser interferometer gravitational-wave observatory (LIGO) during the observation period O1. This scattering noise followed a defined model relative to the object's motion from which light is being scattered. A method based on the Hilbert–Huang transform was developed to identify the scattering surfaces. In this document, we present the efficiency of our method identifying scattering objects in LIGO.
Rodrigo Eyheralde, Miguel Campiglia, Rodolfo Gambini,
Published: 15 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8e30

Abstract:
We study Hawking radiation on the quantum space-time of a collapsing null shell. We use the geometric optics approximation as in Hawking's original papers to treat the radiation. The quantum space-time is constructed by superposing the classical geometries associated with collapsing shells with uncertainty in their position and mass. We show that there are departures from thermality in the radiation even though we are not considering a back reaction. One recovers the usual profile for the Hawking radiation as a function of frequency in the limit where the space-time is classical. However, when quantum corrections are taken into account, the profile of the Hawking radiation as a function of time contains information about the initial state of the collapsing shell. More work will be needed to determine whether all the information can be recovered. The calculations show that non-trivial quantum effects can occur in regions of low curvature when horizons are involved, as is proposed in the firewall scenario, for instance.
Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8e31

Abstract:
We apply a master equation approximation with dynamical coarse graining to a pair of detectors interacting with a scalar field. By solving the master equation numerically, we investigate the evolution of negativity between comoving detectors in de Sitter space. For a massless conformal scalar field with conformal vacuum, it is found that a pair of detectors can perceive entanglement beyond the Hubble horizon scale if the initial separation of detectors is sufficiently small. At the same time, violation of the Bell–Clauser–Horne–Shimony–Holt inequality on the super-horizon scale is also detected. For a massless minimal scalar field with Bunch–Davies vacuum, on the other hand, the entanglement decays within Hubble time scale, owing to the quantum noise caused by particle creations in de Sitter space, and the entanglement on the super-horizon scale cannot be detected.
Published: 21 September 2017
Classical and Quantum Gravity, Volume 35; https://doi.org/10.1088/1361-6382/aa8e2e

Abstract:
In this work we present a no-hair theorem which discards the existence of four-dimensional asymptotically flat, static and spherically symmetric or stationary axisymmetric, non-trivial black holes in the frame of f (R) gravity under the metric formalism. Here we show that our no-hair theorem also can discard asymptotic de Sitter stationary and axisymmetric non-trivial black holes. The novelty is that this no-hair theorem is built without resorting to known mapping between f(R) gravity and scalar-tensor theory. Then, an advantage will be that our no-hair theorem applies as well to metric f (R) models that cannot be mapped to scalar-tensor theory.
Karen Schulze-Koops, ,
Published: 18 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8d46

Abstract:
We study the propagation of light bundles in non-empty spacetime, as most of the Universe is filled by baryonic matter in the form of a (dilute) plasma. Here we restrict to the case of a cold (i.e., pressureless) and non-magnetised plasma. Then the influence of the medium on the light rays is encoded in the spacetime dependent plasma frequency. Our result for a general spacetime generalises the Sachs equations to the case of a cold plasma Universe. We find that the reciprocity law (Etherington theorem), the relation that connects area distance with luminosity distance, is modified. Einstein's field equation is not used, i.e., our results apply independently of whether or not the plasma is self-gravitating. As an example, our findings are applied to a homogeneous plasma in a Robertson-Walker spacetime. We find small modifications of the cosmological redshift of frequencies and of the Hubble law.
, , Frederic H Vincent, Philippe Grandclement, Eric Gourgoulhon
Published: 18 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8d39

Abstract:
The second-generation beam combiner at the VLT, GRAVITY, observes the stars orbiting the compact object located at the center of our galaxy, with an unprecedented astrometric accuracy of 10 $\mu$as. The nature of this compact source is still unknown since black holes are not the only candidates explaining the 4 million solar masses at the Galactic center. Boson stars are such an alternative model to black holes. This paper focuses on the study of trajectories of stars orbiting a boson star and a Kerr black hole. We put in light strong differences between orbits obtained in both metrics when considering stars with sufficiently close pericenters to the compact object, typically $\lesssim 30~M$. Discovery of closer stars to the Galactic center than the S2 star by the GRAVITY instrument would thus be a powerful tool to possibly constrain the nature of the central source.
Published: 27 May 2014
Classical and Quantum Gravity, Volume 31; https://doi.org/10.1088/0264-9381/31/12/125008

Abstract:
In a seminal paper, Kaminski et al for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of loop quantum gravity whose Hilbert space contains all these graphs. This makes it finally possible to investigate the question whether any of the presently considered spin foam models yields a rigging map for any of the presently defined Hamiltonian constraint operators. We postulate a rigging map by summing over all abstract spin foams with arbitrary but given boundary graphs. The states induced on the boundary of these spin foams can then be identified with elements in the gauge invariant Hilbert space of the canonical theory. Of course, such a sum over all spin foams is potentially divergent and requires a regularization. Such a regularization can be obtained by introducing specific cut-offs and a weight for every single foam. Such a weight could be for example derived from a generalized formal group field theory allowing for arbitrary interaction terms. Since such a derivation is, however, technical involved we forgo to present a strict derivation and assume that there exist a weight satisfying certain natural axioms, most importantly a gluing property. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
, Usha Kulshreshtha, Daya Shankar Kulshreshtha
Published: 1 August 2014
Classical and Quantum Gravity, Volume 31; https://doi.org/10.1088/0264-9381/31/16/167001

Abstract:
We study boson shells and boson stars in a theory of a complex scalar field coupled to the gauge field and Einstein gravity with the potential . This could be considered either as a theory of a massive complex scalar field coupled to an electromagnetic field and gravity in a conical potential, or as a theory in the presence of a potential that is an overlap of a parabolic and conical potential. Our theory has a positive cosmological constant . Boson stars are found to come in two types, having either ball-like or shell-like charge density. We studied the properties of these solutions and also determined their domains of existence for some specific values of the parameters of the theory. Similar solutions have also been obtained by Kleihaus, Kunz, Laemmerzahl and List, in a V-shaped scalar potential.
James M Cordes
Published: 4 November 2013
Classical and Quantum Gravity, Volume 30; https://doi.org/10.1088/0264-9381/30/22/224002

, Luciano Rezzolla
Published: 19 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8d98

Abstract:
In grid-based codes that provide the combined solution of the Einstein equations and of relativistic hydrodynamics, the history of the fluid is not simple to track, especially when compared with particle-based codes. The use of tracers, namely massless particles that are advected with the flow, represents a simple and effective way to solve this problem. Yet, the use of tracers in numerical relativity is far from being settled and several issues, such as the impact of different placements in time and space of the tracers, or the relation between the placement and the description of the underlying fluid, have not yet been addressed. In this paper we present the first detailed discussion of the use tracers in numerical-relativity simulations focussing on both unbound material -- such as the one needed as input for nuclear-reaction networks calculating r-process nucleosynthesis in mergers of neutron stars -- and on bound material -- such as the one in the core of the object produced from the merger of two neutron stars. In particular, when interested in unbound matter, we have evaluated different placement schemes that could be used to initially distribute the tracers and how well their predictions match those obtained when using information from the actual fluid flow. Countering our naive expectations, we found that the most effective method does not rely on the rest-mass density distribution nor on the fluid that is unbound, but simply distributes tracers uniformly in rest-mass density. This prescription leads to the closest matching with the information obtained from the hydrodynamical solution. When considering bound matter, we demonstrate that tracers can provide insight into the fine details of the fluid motion as they can be used to track the evolution of fluid elements or to calculate the variation of quantities that are conserved along streamlines of adiabatic flows.
Published: 19 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8da8

Abstract:
Recently a consistent non-perturbative quantization of the Schwarzschild interior resulting in a bounce from black hole to white hole geometry has been obtained by loop quantizing the Kantowski-Sachs vacuum spacetime. As in other spacetimes where the singularity is dominated by the Weyl part of the spacetime curvature, the structure of the singularity is highly anisotropic in the Kantowski-Sachs vacuum spacetime. As a result the bounce turns out to be in general asymmetric creating a large mass difference between the parent black hole and the child white hole. In this manuscript, we investigate under what circumstances a symmetric bounce scenario can be constructed in the above quantization. Using the setting of Dirac observables and geometric clocks we obtain a symmetric bounce condition which can be satisfied by a slight modification in the construction of loops over which holonomies are considered in the quantization procedure. These modifications can be viewed as quantization ambiguities, and are demonstrated in three different flavors which all lead to a non-singular black to white hole transition with identical masses. Our results show that quantization ambiguities can mitigate or even qualitatively change some key features of physics of singularity resolution. Further, these results are potentially helpful in motivating and constructing symmetric black to white hole transition scenarios.
, , Guillaume Faye, Francesco Haardt,
Published: 19 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8da5

Abstract:
We highlight some subtleties that affect naive implementations of quadrupolar and octupolar gravitational waveforms from numerically-integrated trajectories of three-body systems. Some of those subtleties arise from the requirement that the source be contained in its "coordinate near zone" when applying the standard PN formulae for gravitational-wave emission, and from the need to use the non-linear Einstein equations to correctly derive the quadrupole emission formula. We show that some of these subtleties were occasionally overlooked in the literature, with consequences for published results. We also provide prescriptions that lead to correct and robust predictions for the waveforms computed from numerically-integrated orbits.
J Aasi, B P Abbott, R Abbott, T Abbott, M R Abernathy, T Accadia, , K Ackley, C Adams, T Adams, et al.
Published: 5 August 2014
Classical and Quantum Gravity, Volume 31; https://doi.org/10.1088/0264-9381/31/16/165014

Published: 14 December 2012
Classical and Quantum Gravity, Volume 30; https://doi.org/10.1088/0264-9381/30/1/013001

Published: 15 September 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8d06

Abstract:
The covariant Hamiltonian formulation for general relativity is studied in terms of self-dual variables on a manifold with an internal and lightlike boundary. At this inner boundary, new canonical variables appear: a spinor and a spinor-valued two-form that encode the entire intrinsic geometry of the null surface. At a two-dimensional cross-section of the boundary, quasi-local expressions for the generators of two-dimensional diffeomorphisms, time translations, and dilatations of the null normal are introduced and written in terms of the new boundary variables. In addition, a generalisation of the first-law of black-hole thermodynamics for arbitrary null surfaces is found, and the relevance of the framework for non-perturbative quantum gravity is stressed and explained.
Published: 13 November 2017
Classical and Quantum Gravity, Volume 34; https://doi.org/10.1088/1361-6382/aa8cf9

Abstract:
On the path towards quantum gravity we find friction between temporal relations in quantum mechanics (QM) (where they are fixed and field-independent), and in general relativity (where they are field-dependent and dynamic). This paper aims to attenuate that friction, by encoding gravity in the timeless configuration space of spatial fields with dynamics given by a path integral. The framework demands that boundary conditions for this path integral be uniquely given, but unlike other approaches where they are prescribed—such as the no-boundary and the tunneling proposals—here I postulate basic principles to identify boundary conditions in a large class of theories. Uniqueness arises only if a reduced configuration space can be defined and if it has a profoundly asymmetric fundamental structure. These requirements place strong restrictions on the field and symmetry content of theories encompassed here; shape dynamics is one such theory. When these constraints are met, any emerging theory will have a Born rule given merely by a particular volume element built from the path integral in (reduced) configuration space. Also as in other boundary proposals, Time, including space-time, emerges as an effective concept; valid for certain curves in configuration space but not assumed from the start. When some such notion of time becomes available, conservation of (positive) probability currents ensues. I show that, in the appropriate limits, a Schrödinger equation dictates the evolution of weakly coupled source fields on a classical gravitational background. Due to the asymmetry of reduced configuration space, these probabilities and currents avoid a known difficulty of standard WKB approximations for Wheeler DeWitt in minisuperspace: the selection of a unique Hamilton–Jacobi solution to serve as background. I illustrate these constructions with a simple example of a full quantum gravitational theory (i.e. not in minisuperspace) for which the formalism is applicable, and give a formula for calculating gravitational semi-classical relative probabilities in it.
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