Hasil untuk "gr-qc"

A. A. Araújo Filho, Iarley P. Lobo

The particle creation via quantum tunneling was recently calculated for the Schwarzschild non-commutative black hole solution in Ref. [Phys. Lett. B 848 (2024) 138335, e-Print: 2310.02445 [gr-qc]]. Nevertheless, it contains inconsistencies in the calculations that need to be properly corrected. In particular, the event horizon was incorrectly determined in that work, which affected all the subsequent calculations. Moreover, the same issue have been repeated elsewhere by the same authors in Refs. [1-4].

M A. M. Smith, Vasileios Paschalidis, Gabriele Bozzola

In a previous paper, arXiv:2411.11960 [gr-qc], we initiated a study of high-energy interactions of charged binary black holes near the scattering threshold, focusing on zoom-whirl orbits. In this second paper in our series, we focus on merger remnant properties and energetics with new simulations of equal-mass, equal-charge, nonspinning binary black holes with variable impact parameter. We find near-extremal merger remnants with Kerr-Newman parameter reaching $Υ_f = 0.97$, and observe that the maximum $Υ_f$ increases monotonically with $λ$ for a fixed initial Lorentz factor. We find that binaries with larger $λ$ radiate less total energy despite having stronger electromagnetic emission. The maximum energy radiated by a binary in our study is $31\%$ of its gravitational mass. Increasing $λ$ has little effect on the maximum angular momentum radiated, which was $\approx 72\%$ of the spacetime total angular momentum for each $λ$ explored here. Lastly, we provide additional evidence for the universality with the irreducible mass that we discovered in arXiv:2411.11960 [gr-qc]. The black hole horizon areal radius determines a fundamental, gauge-invariant length scale governing BH interactions near the scattering threshold.

Antonio De Felice, Andreas Doll, Shinji Mukohyama

We propose a modified gravity theory that propagates only two local gravitational degrees of freedom and that does not have an Einstein frame. According to the classification in JCAP 01 (2019) 017 [arXiv:1810.01047 [gr-qc]], this is a type-II minimally modified gravity theory. The theory is characterized by the gravitational constant $G_{\rm N}$ and a function $V(φ)$ of a non-dynamical auxiliary field $φ$ that plays the role of dark energy. Once one fixes a homogeneous and isotropic cosmological background, the form of $V(φ)$ is determined and the theory no longer possesses a free parameter or a free function, besides $G_{\rm N}$. For $V'(φ) = 0$ the theory reduces to general relativity (GR) with $G_N$ being the Newton's constant and $V=const.$ being the cosmological constant. For $V'(φ) \ne 0$, it is shown that gravity behaves differently from GR but that GR with $G_{\rm N}$ being the Newton's constant is recovered for weak gravity at distance and time scales sufficiently shorter than the scale associated with $V(φ)$. Therefore this theory provides the simplest framework of cosmology in which deviations from GR can be tested by observational data.

S. Katore, A. Y. Shaikh

J. Klusoň

This short paper is devoted to the Hamiltonian formulation of the conformal decomposition of the gravitational field that was performed in [gr-qc/0501092]. We also analyze the gauge fixed form of the theory when we fix the conformal symmetry by imposing the condition root g = 1.

Dipo Mahto, Ved Prakash, U. Prasad et al.

B. Chen 陈, Bo 波 Ning 宁, J. Zhang 张

Mariafelicia De Laurentis, Rosario De Rosa, Fabio Garufi et al.

In this paper we compare the effects of different theories of gravitation on the apsidal motion of a sample of Eccentric Eclipsing Detached Binary stars. The comparison is performed by using the formalism of the Post-Newtonian parametrization to calculate the theoretical advance at periastron and compare it to the observed one, after having considered the effects of the structure and rotation of the involved stars. A variance analysis on the results of this comparison, shows that no significant difference can be found due to the effect of the different theories under test with respect to the standard General Relativity. It will be possible to observe differences, as we would expect, by checking the observed period variation on a much larger lapse of time. It can also be noticed from our results, that f(R) theory is the nearest to GR with respect to the other tested theories.

D. Reddy, R. Naidu, B. Satyanarayana

A. Amani, H. Farahani

L. Glinka

Draft of book strictly based on the author research results presented in the arXiv papers: 0906.3827[gr-qc], 0906.3825[gr-qc], 0905.3916[hep-ph], 0902.4811[hep-ph], 0902.2829[hep-th], 0812.0551[hep-th], 0809.5216[gr-qc], 0808.1035[gr-qc], 0804.3516[gr-qc], 0803.1533[gr-qc], 0801.4157[gr-qc], 0712.2769[hep-th], 0712.1674[gr-qc], 0711.1380[gr-qc], 0707.3341[gr-qc], gr-qc/0612079.

N. Kiriushcheva, S. V. Kuzmin

We demonstrate that from the first order formulation of the Einstein– Cartan action it is possible to derive the basic differential identity that leads to translational invariance of the action in the tangent space. The transformations of fields is written explicitly for both the first and second order formulations and the group properties of transformations are studied. This, combined with the preliminary results from the Hamiltonian formulation (Kiriushcheva and Kuzmin in arXiv:0907.1553 [gr-qc]), allows us to conclude that without any modification, the Einstein–Cartan action in any dimension higher than two possesses not only rotational invariance but also a form of translational invariance in the tangent space. We argue that not only a complete Hamiltonian analysis can unambiguously give an answer to the question of what a gauge symmetry is, but also the pure Lagrangian methods allow us to find the same gauge symmetry from the basic differential identities.

N. Kiriushcheva, S. V. Kuzmin

Based on our preliminary analysis of the Hamiltonian formulation of the first order Einstein-Cartan action (arXiv:0902.0856 [gr-qc] and arXiv:0907.1553 [gr-qc]) we derive the Darboux coordinates, which are a unique and uniform change of variables that preserve equivalence with the original action in all spacetime dimensions higher than two. Considerable simplification of the Hamiltonian formulation using the Darboux coordinates, as compared with the direct analysis, is explicitly demonstrated. Even an incomplete Hamiltonian analysis in combination with known symmetries of the Einstein-Cartan action and the equivalence of Hamiltonian and Lagrangian formulations allows us to unambiguously conclude that the unique gauge invariances generated by the first class constraints are translation and rotation in the tangent space. Diffeomorphism invariance, though a manifest invariance of the action, is not generated by the first class constraints of the theory.

Francisco S. N. Lobo

The authors of the paper ''On Anisotropic Dark Energy Stars'', arXiv:0803.2508 [gr-qc], consider the equations of state $p_r=ωρ$ and $p_t=ωρ$, ignoring the fact that this implies an isotropic pressure, which places strict restrictions on the values of $ω$. The authors then argue for an anisotropic pressure fluid throughout their work, and apply these equations of state to the energy conditions, consequently obtaining incorrect intervals for the parameter $ω$. This procedure invalidates their criticisms on the following paper: Class.Quant.Grav. 23, 1525 (2006).

S. Carlip

This is a reply to a recent comment (hep-th/9910158) by Park and Ho on a paper (hep-th/9812013) that describes a derivation of black hole entropy from horizon conformal field theory. The criticism of Park and Ho is partially correct - the original paper did not give the complete surface term in the Hamiltonian - but it does not affect the conclusions, which have been checked using a somewhat different, manifestly covariant formalism (gr-qc/9906126).

A. Abbassi

The Schwarzschild-deSitter metric is the known solution of Einstein field equations with cosmological constant term for vacuum spherically symmetric space around a point mass M. Recently it has been reported that in a $Lamda$-dominant world the Schwarzschild type coordinate systems are disqualified by redshift-magnitude test as a proper frame of reference(gr-qc/9812092). We derive the solution in a FRW type coordinate system which is qualified according to the mentioned test. Asymptotically it approachs to the non-static form of deSitter metric. The obtained metric is transformable to Schwarzschild-deSitter metric. It is an analytic function of $r$ for all values except $r=0$which is singular. This is carried out with no making use of Eddington-Finkelstein coordinates and without entering any cross term in the metric.

R. Livine

We study the generalised constrained BF theory described in gr-qc/0102073 in order to introduce the Immirzi parameter in spin foam models. We show that the resulting spin foam model is still based on simple representations and that the generalised BF action is simply a deformation of the Barrett-Crane model. The Immirzi parameter doesn't change the representations used in the spin foam model, so it doesn't affect the geometry of the model. However we show how it may still appear as a factor in the area spectrum.

E. Abdalla, R. Mohayaee, M. B. Ribeiro

This paper seeks to check the validity of the "apparent fractal conjecture" (Ribeiro 2001ab: gr-qc/9909093, astro-ph/0104181), which states that the observed power-law behaviour for the average density of large-scale distribution of galaxies arises when some observational quantities, selected by their relevance in average density profile determination, are calculated along the past light cone. Implementing these conditions in the proposed set of observational relations profoundly changes the behaviour of many observables in the standard cosmological models. In particular, the average density becomes observationally inhomogeneous, even in the spatially homogeneous spacetime of standard cosmology, change which was already analysed by Ribeiro (1992b, 1993, 1994, 1995: astro-ph/9910145) for a non-perturbed model. Here we derive observational relations in a perturbed Einstein-de Sitter cosmology by means of the perturbation scheme proposed by Abdalla and Mohayaee (1999: astro-ph/9810146), where the scale factor is expanded in power series to yield perturbative terms. The differential equations derived in this perturbative context, and other observables necessary in our analysis, are solved numerically. The results show that our perturbed Einstein-de Sitter cosmology can be approximately described by a decaying power-law like average density profile, meaning that the dust distribution of this cosmology has a scaling behaviour compatible with the power-law profile of the density-distance correlation observed in the galaxy catalogues. These results show that, in the context of this work, the apparent fractal conjecture is correct.

B. Carter, B. Leith, S. Ng et al.

The unexpected dimness of Type Ia supernovae at redshifts z >~ 1 has over the past 7 years been seen as an indication that the expansion of the universe is accelerating. A new model cosmology, the "fractal bubble model", has been proposed by one of us [gr-qc/0503099], based on the idea that our observed universe resides in an underdense bubble remnant from a primordial epoch of cosmic inflation, together with a new solution for averaging in an inhomogeneous universe. Although there is no cosmic acceleration, it is claimed that the luminosity distance of type Ia supernovae data will nonetheless fit the new model, since it mimics a Milne universe at low redshifts. In this paper the hypothesis is tested statistically against the available type Ia supernovae data by both chi-square and Bayesian methods. While the standard model with cosmological constant Omega_Lambda = 1-Omega_m is favoured by a Bayesian analysis with wide priors, the comparison depends strongly on the priors chosen for the density parameter, Omega_m. The fractal bubble model gives better agreement generally for Omega_m<0.2. It also gives reasonably good fits for all the range, Omega_m=0.01-0.55, allowing the possibility of a viable cosmology with just baryonic matter, or alternatively with both baryonic matter and additional cold dark matter.

Sergei M Kopeikin, E. Fomalont

Abstract Recent papers by Samuel [S. Samuel, Phys. Rev. Lett. 90 (2003) 231101; S. Samuel, Int. J. Mod. Phys. D 13 (2004) 1753] declared that the linearized post-Newtonian v / c effects are too small to have been measured in the recent experiment involving Jupiter and quasar J 0842 + 1845 [S.M. Kopeikin, Astrophys. J. Lett. 556 (2001) L1; E.B. Fomalont, S.M. Kopeikin, in: E. Ros, R.W. Porcas, A.P. Lobanov, J.A. Zensus (Eds.), Proc. 6th European VLBI Network Symp., MPIfR, Bonn, 2002 pp. 49–52, gr-qc/0206022; E.B. Fomalont, S.M. Kopeikin, Astrophys. J. 598 (2003) 704] that was used to measure the ultimate speed of gravity defined as a fundamental constant entering in front of each time derivative of the metric tensor in the Einstein gravity field equations. We describe our Lorentz-invariant formulation of the Jovian deflection experiment and confirm that v / c effects are do observed, as contrasted to the erroneous claim by Samuel, and that they vanish if and only if the speed of gravity is infinite.