Hasil untuk "gr-qc"

Antonio C. Gutiérrez-Piñeres

A recent paper proposes a static ellipsoidal space-time metric that reduces to Schwarzschild. After examining the corrected line element using Maple's Differential geometry package, we find that the Einstein tendor and Ricci scalar are non-zero, and the metric yields non-zero pressures when interpreted as an anisotropic fluid. Thus, it does not represent a vacuum solution. We also checked a cited ellipsoidal metric from the literature and found it likewise fails to satisfy the vacuum Einstein equations.

Roberto A Sussman, Carlo Alberto Mantica, Luca Guido Molinari et al.

We address in this article the criticism in a recently submitted article by Clement and Noiucer (arXiv:2312.17662 [gr-qc]) on "Cotton Gravity" (CG), a gravity theory alternative to General Relativity. These authors claim that CG is "not predictive" for producing "too many" spherically symmetric vacuum solutions, while taking the Bianchi I vacuum as test case they argue that geometric constraint on the Cotton tensor lead to an undetermined problem, concluding in the end that CG "is not a physical theory". We provide arguments showing that this critique is incorrect and misrepresents the theory.

Marcelo M. Amaral, Raymond Aschheim, Laurenţiu Bubuianu et al.

The goal of this work is to elaborate on new geometric methods of constructing exact and parametric quasiperiodic solutions for anamorphic cosmology models in modified gravity theories, MGTs, and general relativity, GR. There exist previously studied generic off-diagonal and diagonalizable cosmological metrics encoding gravitational and matter fields with quasicrystal like structures, QC, and holonomy corrections from loop quantum gravity, LQG. We apply the anholonomic frame deformation method, AFDM, in order to decouple the (modified) gravitational and matter field equations in general form. This allows us to find integral varieties of cosmological solutions determined by generating functions, effective sources, integration functions and constants. The coefficients of metrics and connections for such cosmological configurations depend, in general, on all spacetime coordinates and can be chosen to generate observable (quasi)-periodic/ aperiodic/ fractal / stochastic / (super) cluster / filament / polymer like (continuous, stochastic, fractal and/or discrete structures) in MGTs and/or GR. In this work, we study new classes of solutions for anamorphic cosmology with LQG holonomy corrections. Such solutions are characterized by nonlinear symmetries of generating functions for generic off--diagonal cosmological metrics and generalized connections, with possible nonholonomic constraints to Levi-Civita configurations and diagonalizable metrics depending only on a time like coordinate. We argue that anamorphic quasiperiodic cosmological models integrate the concept of quantum discrete spacetime, with certain gravitational QC-like vacuum and nonvacuum structures. And, that of a contracting universe that homogenizes, isotropizes and flattens without introducing initial conditions or multiverse problems.

Sean T. McWilliams

We respond to a recent criticism from Zaslavskii (arXiv:1301.3429v2 [gr-qc]) of our article, which appeared in Phys. Rev. Lett. 110 011102 (2013). We clarify the points of apparent disagreement, none of which relate to the central result of our original paper, which is the demonstration that the diverging center-of-masss energy of collisions in Kerr spacetime, known as the "BSW effect", can never be observed in nature. To emphasize this result, we present it in a more concise way which completely avoids any of the questions brought up by Zaslavskii.

Yasha Neiman

We argue that finite-region observables in quantum gravity are best approached in terms of boundary data on null hypersurfaces. This has far-reaching effects on the basic notions of classical and quantum mechanics, such as Hamiltonians and canonical conjugates. Such radical properties are not unexpected in finite-region quantum gravity. We are thus motivated to reformulate field theory in terms of null boundary data. As a starting point, we consider the on-shell action functional for classical field theory in finite null-bounded regions. Closed-form results are obtained for free scalars and for Maxwell fields. The action of classical gravity is also discussed, to the extent possible without solving the field equations. These action functionals exhibit non-locality and, in special cases, a "holographic" reduction of the degrees of freedom. Also, they cannot be used to define global charges. Whereas for ordinary field theory these are just artifacts of a restrictive formalism, in quantum gravity they are expected to be genuine features. This further supports a connection between quantum gravity and null-boundary observables. In our treatment of the GR action, we identify a universal imaginary term that reproduces the Bekenstein entropy formula.

F. Felber

Hilbert's 1917 discovery of reversed gravitational acceleration is discussed, and the connection to arXiv:1102.2870v2 [gr-qc] is explained.

M. Ghaderi, A. S. Mahoonak, M. Aalami et al.

Hans Halvorson

S. Hollands

Liu Zhao

This paper is withdrawn because it was already known in the literature that the single function metric ansatz invalidates the effective action method. (Refs: gr-qc/0306114, gr-qc/0408067,gr-qc/0404120, gr-qc/0506014, 0705.1669, thanks to S. Deser for providing the information.)

Y. Pirogov, I. Y. Polev

In a previous article due to one of the present authors (YFP), an extension to General Relativity, violating general covariance to the residual unimodular one, was proposed. As a manifestation of such a violation, there appears the (massive) scalar graviton in addition to the massless tensor one. The former was proposed as a candidate on the dark matter in the Universe. In a subsequent article (Yu. F. Pirogov, MPLA 24, 3239, 2009;arXiv:0909.3311 [gr-qc]), an application of the extension was developed. Particularly, a regular solution to the static spherically symmetric equations in empty space was studied by means of analytical methods. This solution was proposed as a prototype model for the galaxy soft-core dark halos, with the coherent scalar-graviton field as dark matter. The present report is a supplement to the aforementioned article. The statements of the latter are verified and visualized by means of numerical analysis and symbolic calculations. The nice validity of analytical results is found.

Fariborz Riahi

B. Saha, V. Rikhvitsky

The study of a self-consistent system of nonlinear spinor and Bianchi type I gravitational fields in the presence of a viscous fluid and a Λ term, with the spinor field nonlinearity being some arbitrary functions of the invariants I and J constructed from bilinear spinor forms S and P, generates a multi-parametric system of ordinary differential equations (Saha 2005 Rom. Rep. Phys. 57 7, Saha 2007 Preprint gr-qc/0703085 (Astrophys. Space Sci. at press)). A qualitative analysis of the system in question has been thoroughly carried out. A complete qualitative classification of the mode of the evolution of the universe given by the corresponding dynamic system has been illustrated.

N. A. Lemos, G. A. Monerat, E. V. C. Silva et al.

[gr-qc/0611029] contains a valid criticism of the numerical precisionof the results reported in a recent paper of ours [Phys. Rev. D 73, 044022 (2006)], as well as freshideas on how to characterize a quantum cosmological singularity. However, we argue that, contraryto what is suggested in the Comment, the quantum cosmological models we studied show hardlyany sign of singular behavior.

Akihiro Ishibashi

A higher (even spacetime) dimensional generalization of the Bondi energy has recently been proposed by gr-qc/0304054 within the framework of conformal infinity and Hamiltonian formalizm. The gauge condition employed in gr-qc/0304054 to derive the Bondi energy expression is, however, peculiar in the sense that cross-sections of null infinity specified by that gauge are anisotropic and in fact non-compact. For this reason, that gauge is difficult to use for explicit computations of the Bondi energy in general, asymptotically flat radiative spacetimes. Also it is not clear, under that gauge condition, whether apparent difference between the expressions of higher dimensional Bondi energy and the 4-dimensional one is due to the choice of gauges or qualitatively different nature of higher dimensional gravity from 4-dimensional gravity. In this paper, we consider instead, Gaussian null conformal gauge as one of more natural gauge conditions that admit a global specification of background structure with compact, spherical cross-sections of null infinity. Accordingly, we modify the previous definition of higher dimensional news tensor so that it becomes well-defined in the Gaussian null conformal gauge and derive, for vacuum solutions, the expression for the Bondi energy-momentum in the new gauge choice, which takes a universal form in arbitrary (even spacetime) dimensions greater than or equal to four.

Johannes Brunnemann, David Rideout

The properties of the Volume operator in Loop Quantum Gravity, as constructed by Ashtekar and Lewandowski, are analyzed for the first time at generic vertices of valence greater than four. The present analysis benefits from the general simplified formula for matrix elements of the Volume operator derived in gr-qc/0405060, making it feasible to implement it on a computer as a matrix which is then diagonalized numerically. The resulting eigenvalues serve as a database to investigate the spectral properties of the volume operator. Analytical results on the spectrum at 4-valent vertices are included. This is a companion paper to arXiv:0706.0469, providing details of the analysis presented there.

Paul Frampton

In a recently archived paper by Zhang\cite{Zhang}, it is claimed that before turnaround in a cyclic model two unexpected events happen: (1) black holes cease to contract and begin to expand; (2) separated causal patches start to reconnect. We show that both conclusions are erroneous and result from the author's choice of variables.

A. Kirilyuk

Universe structure emerges in the unreduced, complex-dynamic interaction process with the simplest initial configuration (two attracting homogeneous fields, quant-ph/9902015). The unreduced interaction analysis gives intrinsically creative cosmology, describing the real, explicitly emerging world structure with dynamic randomness on each scale. Without imposing any postulates or entities, we obtain physically real space, time, elementary particles with their detailed structure and intrinsic properties, causally complete and unified version of quantum and relativistic behaviour, the origin and number of naturally unified fundamental forces, and classical behaviour emergence in closed systems (gr-qc/9906077). Main problems of standard cosmology and astrophysics are consistently solved in this extended picture (without introduction of any additional entities), including those of quantum cosmology and gravity, entropy growth and time, "hierarchy" of elementary particles, "anthropic" difficulties, space-time flatness, and "missing" ("dark") mass and energy. The observed universe structure and laws can be presented as manifestations of the universal symmetry (conservation) of complexity providing the unified, irregular, but exact (never "broken") Order of the World (physics/0404006).

F. Finster

The Dirac sea is calculated in an expansion around the light cone. The method is to analyze the perturbation expansion for the Dirac sea in position space. This leads to integrals over expressions containing distributions which are singular on the light cone. We derive asymptotic formulas for these "light cone integrals" in terms of line integrals over the external potential and its partial derivatives. The calculations are based on the perturbation expansion for the Dirac sea in the preprint gr-qc/9606040 and yield the formulas listed in the appendix of this preprint. The results can be obtained easier with a combination of calculations in position and momentum space (see the corresponding preprints on the hep-th server). Therefore the calculations are preliminary and will remain unpublished; they are intended as reference for people who encounter similar mathematical problems.

V. Mashkevich

This paper is a continuation of the papers [gr-qc/9409010, gr-qc/9505034]. A revision of the Einstein equation shows that its dynamic incompleteness, contrary to a popular opinion, cannot be circumvented by so-called coordinate conditions. Gravidynamics, i.e., dynamics for gravitational potentials $g_{\mu\nu}$ is advanced, which differs from geometrodynamics of general relativity in that the former is based on a projected Einstein equation. Cosmic gravidynamics, due to a global structure of spacetime, is complete. The most important result is a possibility of the closed universe with a density below the critical one. Keywords: general relativity, cosmic time, cosmic space, Einstein equation, quantum, metric, indeterministic