Latest Papers in General Relativity

General Relativity And Quantum Cosmology

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Polysymplectic formulation for BF gravity with Immirzi parameter (1901.11532v1)

Jasel Berra-Montiel, Alberto Molgado, Angel Rodriguez-Lopez

2019-01-31

The polysymplectic formulation of the CMPR action, which is a BF-type formulation of General Relativity that involves an arbitrary Immirzi parameter, is performed. We implement a particular scheme within this covariant Hamiltonian approach to analyze the constraints that characterize the CMPR model. By means of the privileged -forms and the Poisson-Gerstenhaber bracket, inherent to the polysymplectic framework, the BF field equations associated to the CMPR action are obtained and, in consequence, the Einstein equations naturally emerge by solving the simplicity constraints of the theory. Further, from the polysymplectic analysis of the CMPR action the De Donder-Weyl Hamiltonian formulation of the Holst action is recovered, which is consistent with the Lagrangian analysis of this model as reported in the literature.

On the Structure of Rotating Charged Boson Stars (1901.11522v1)

Lucas G. Collodel, Burkhard Kleihaus, Jutta Kunz

2019-01-31

In this work we present full sets of solutions for rotating charged boson stars with different coupling values. By adopting local comoving coordinates, we are able to find expressions for the effective hydrodynamic quantities of the fluids as seen by this class of observers. We show that not only is the energy density non zero at the center, for the uncharged case it has a local maximum at the core from which it decreases until the point of local minimum where its variation is discontinuous. For the first time, it is reported how rotating boson stars, charged and uncharged, are completely anisotropic fluids featuring three different pressures. Furthermore, the character of the electromagnetic fields is analyzed.

On the necessity of phantom fields for solving the horizon problem in scalar cosmologies (1901.11511v1)

Davide Fermi, Massimo Gengo, Livio Pizzocchero

2019-01-31

We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Robertson-Walker spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field which can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and some minimal, additional requirements are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon appears.

Test of gravitomagnetism with satellites around the Earth (1810.11785v2)

Matteo Luca Ruggiero, Angelo Tartaglia

2018-10-28

We focus on the possibility of measuring the gravitomagnetic effects due to the rotation of the Earth, by means of a space-based experiment that exploits satellites in geostationary orbits. Due to the rotation of the Earth, there is an asymmetry in the propagation of electromagnetic signals in opposite directions along a closed path around the Earth. We work out the delays between the two counter-propagating beams for a simple configuration, and suggest that accurate time measurements could allow, in principle, to detect the gravitomagnetic effect of the Earth

Geodesic dual spacetime (1901.11506v1)

Wen-Du Li, Wu-Sheng Dai

2019-01-31

A duality between spacetime manifolds, the geodesic duality, is introduced. Two manifolds are geodesic dual, if the transformation between their metrics is also the transformation between their geodesics. That is, the transformation that transforms the metric to the metric of the dual manifold is also the transformation that transforms the geodesic to the geodesic of the dual manifold. On the contrary, for nondual spacetime manifolds, a geodesic is no longer a geodesic after the transformation between the metrics. We give a general result of the duality between spacetime manifolds with diagonal metrics. The geodesic duality of spherically symmetric spacetime are discussed for illustrating the concept. The geodesic dual spacetime of the Schwarzschild spacetime and the geodesic dual spacetime of the Reissner-Nordstr"om spacetime are presented.

Quartet-metric/multi-component gravity: scalar graviton as emergent dark substance (1811.12923v2)

Yury F. Pirogov

2018-11-30

In the general frameworks of an earlier introduced quartet-metric/multi-component gravity, a theory of a massive scalar graviton supplementing the massless tensor one is consistently deduced. The peculiarities of the scalar-graviton field compared to the canonical scalar one are demonstrated. The light scalar graviton is treated as an emergent dark substance of the Universe: dark matter and/or dark energy depending on the solution. The case with scalar graviton as dark energy responsible for the late-time accelerated expansion of the Universe is studied in more detail. In particular, it is shown that due to an attractor solution for the light scalar graviton there naturally emerges at the classical level a tiny nonzero effective cosmological constant, even in the absence of the Lagrangian one. The prospects of going beyond LCDM model per scalar graviton are shortly indicated.

Gravitational Lensing by Black Holes in Einsteinian Cubic Gravity (1810.10657v2)

Mohammad Bagher Jahani Poshteh, Robert B. Mann

2018-10-25

We investigate the predictions of Einsteinian Cubic Gravity (ECG) for the lensing effects due to supermassive black holes at the center of Milky Way and other galaxies. Working in the context of spherical symmetry, we obtain the metric function from a continued fraction method and find that both time delays and the angular positions of images considerably deviate from general relativity, as large as milliarcseconds. This suggests that observational tests of ECG are indeed feasible.

Shadow and Deflection Angle of Rotating Black Holes in Perfect Fluid Dark Matter with a Cosmological Constant (1810.04103v2)

Sumarna Haroon, Mubasher Jamil, Kimet Jusufi, Kai Lin, Robert B. Mann

2018-10-09

The presence of dark matter around a black hole remarkably affects its spacetime. We consider the effects of dark matter on the shadow of a new solution to the Einstein equations that describes a rotating black hole in the background of perfect dark matter fluid (PFDM), along with its extension to nonzero cosmological constant . Working in Boyer-Lindquist coordinates, we consider the effects of the PFDM parameter on the shadow cast by a black hole with respect to an observer at position . By applying the Gauss-Bonnet theorem to the optical geometry we find that notable distortions from a Kerr black hole can occur. We describe their dependence on and .

Distinguishing a Kerr-like black hole and a naked singularity in perfect fluid dark matter via precession frequencies (1812.01331v2)

Muhammad Rizwan, Mubasher Jamil, Kimet Jusufi

2018-12-04

We study a Kerr-like black hole and naked singularity in perfect fluid dark matter (PFDM). The critical value of spin parameter is presented to differentiate the black hole from naked singularity. It is seen that for any fixed value of dark matter parameter the rotating object is black hole if and naked singularity if . Also for the size of the black hole horizons decrease whereas for it increases. We also study spin precession frequency of a test gyroscope attached to stationary observer to differentiate a black hole from naked singularity in perfect fluid dark matter. For the black hole, spin precession frequency blows up as the observer reaches the central object while for naked singularity it remains finite except at the ring singularity. Moreover, we study Lense-Thirring precession for a Kerr-like black hole and geodetic precession for Schwarzschild black hole in perfect fluid dark matter. To this end, we have calculated the Kepler frequency (KF), the vertical epicyclic frequency (VEF), and the nodal plane precession frequency (NPPF). Our results show that, the PFDM parameter significantly affects those frequencies. This difference can be used by astrophysical observations in the near future to shed some light on the nature of dark matter.

No-hair theorem for spherically symmetric regular repulsive compact stars (1901.11415v1)

Yan Peng

2019-01-31

In a very interesting paper, Hod has proven that asymptotically flat spherically symmetric regular reflecting compact stars cannot support static scalar fields whose self-interaction potential is a monotonically increasing function of its argument. In this work, we generalize reflecting boundary conditions to repulsive boundary conditions. We show that there is still no static scalar hair behavior in the background of asymptotically flat spherically symmetric regular repulsive compact stars.

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