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At least 289 records · Page 16Linked to original sources
arXiv2013

Holographic geometries for condensed matter applications

Holographic modeling of strongly correlated many-body systems motivates the study of novel spacetime geometries where the scaling behavior of quantum critical systems is encoded into spacetime symmetries. Einstein-Dilaton-Maxwell theory has planar black brane solutions that exhibit Lifshitz scaling and in some cases hyperscaling violation. Entanglement entropy and Wilson loops in the dual field theory are studied by inserting simple geometric probes involving minimal surfaces into the black brane geometry. Coupling to background matter fields leads to interesting low-energy behavior in holographic models, such as U(1) symmetry breaking and emergent Lifshitz scaling.

gr-qc↗
arXiv2013

The Lens-Thirring effect in the anomalistic period of celestial bodies

In the weak field and slow motion approximation, the general relativistic field equations are linearized, resembling those of the electromagnetic theory. In a way analogous to that of a moving charge generating a magnetic field, a mass energy current can produce a gravitomagnetic field. In this contribution, the motion of a secondary celestial body is studied under the influence of the gravitomagnetic force generated by a spherical primary. More specifically, two equations are derived to approximate the periastron time rate of change and its total variation over one revolution (i.e., the difference between the anomalistic period and the Keplerian period). Kinematically, this influence results to an apsidal motion. The aforementioned quantities are numerically estimated for Mercury, the companion star of the pulsar PSR 1913 plus 16, the companion planet of the star HD 80606 and the artificial Earth satellite GRACE A. The case of the artificial Earth satellite GRACE A is also considered, but the results present a low degree of reliability from a practical standpoint.

gr-qc↗
arXiv2013

Black Holes in Supergravity with Applications to String Theory

This thesis is devoted to the study of black hole solutions in ungauged four-dimensional extended Supergravity. We characterize the most general spherically symmetric and static black-hole solution of ungauged Supergravity, and use the result to study the hidden conformal symmetries of Supergravity black holes, obtaining the full Virasoro algebra of the dual conformal field theory. We obtain also all the supersymmetric black-hole metrics of all extended Supergravities using the properties of the groups of Type E7. We introduce the H-F.G.K. formalism, which simplifies the construction of non-supersymmetric black-hole solutions in N=2 Supergravity, and apply it to a class of theories corresponding to Type-IIA String Theory compactified on a Calabi-Yau threefold. As a result we obtain the so-called "quantum" black holes, which only exist when certain quantum corrections (perturbative or non-perturbative, depending on the solution) are included in the prepotential. For the case of non-perturbative black holes, we elaborate on the potential consequences of the appearance on the solution of multi-valued functions in relation to the no-hair theorem for four-dimensional black holes. This thesis also contains the relevant mathematical background to formulate extended ungauged Supergravity in four-dimensions, that is, Special Kähler Geometry and homogeneous spaces, as well as a brief introduction to the mathematical structure of four-dimensional extended ungauged Supergravity.

hep-th↗
arXiv2013

Metrics for multi-detector template placement in searches for short-duration nonprecessing inspiral gravitational-wave signals

Using the family of multi-detector F-statistic metrics for short duration, nonprecessing inspiral signals, we derive a marginalized metric that is directly applicable to the problem of generating template banks for coincident and coherent multi-detector searches for gravitational-waves. This metric is compared to other average metrics, such as that proposed for the case of searches associated with continuous signals from rotating neutron stars. We show how the four-dimensional metric can be separated into two two-dimensional metrics associated with the sky and mass parameter subspaces, allowing the creation of separate template banks for these subspaces. Finally, we present an algorithm for computing the mass space metric associated with both coincident and coherent multi-detector targeted or all-sky searches for short duration, nonprecessing inspiral gravitational-wave signals.

gr-qc↗
arXiv2013

A new geometrical approach to metric-affine gravity

Here we consider a metric-affine theory of gravity in which the gravitational Lagrangian is the scalar curvature. The matter action is allowed to depend also on the torsion and the nonmetricity, which are considered as the field variables together with the metric. We find the field equations of the theory and show that they reduce to Einstein equations in vacuum

gr-qc↗
arXiv2013

Relativistic disks with two charged perfect fluids components

A method to describe exact solutions of the Einstein-Maxwell field equations in terms of relativistic thin disks constituted by two perfect charged fluids is presented. Describing the surface of the disk as a single charged fluid we find explicit expressions for the rest energies, the pressures and the electric charge densities of the two fluids. An explicit example is given. The particular case of the thin disks composed by two charged perfect fluids with barotropic equation of state is also presented.

gr-qc↗
arXiv2013

Constant Mean Curvature Slices of the Reissner-Nordström Spacetime

In order to specify a foliation of spacetime by spacelike hypersurfaces we need to place some restriction on the initial data and from this derive a way to calculate the lapse function $α$ which measures the proper time and interval between neighbouring hypersurfaces along the normal direction. Here we study a prescribed slicing known as the Constant Mean Curvature (CMC) slicing. This slicing will be applied to the Reissner-Nordström metric and the resultant slices will be investigated and then compared to those of the extended Schwarzschild solution.

gr-qc↗
arXiv2013

Measurement of Quantum Geometry Using Laser Interferometry

New quantum degrees of freedom of space-time, originating at the Planck scale, could create a coherent indeterminacy and noise in the transverse position of massive bodies on macroscopic scales. An experiment is under development at Fermilab designed to detect or rule out a transverse position noise with Planck spectral density, using correlated signals from an adjacent pair of Michelson interferometers. A detection would open an experimental window on quantum space-time.

gr-qc↗
arXiv2013

Describing SGRs/AXPs as fast and magnetized white dwarfs

Over the last decade, observational evidence has mounted that SGRs/AXPs belong to a particular class of pulsars. Furthermore, fast and very magnetic white dwarfs have been observed, and recently two SGRs with low magnetic fields $B\sim(10^{12}-10^{13})$ G, namely SGR 0418+5729 and Swift J1822.3-1606 were discovered with a period of $P\sim9.08$ s and $P\sim8.44$ s, respectively. These new discoveries opens the question of the nature of SGRs/AXPs, emerging alternative scenarios, in particular the white dwarf (WD) pulsar model. These astronomical observations have based an alternative description of the SGRs/AXPs expressed on rotating highly magnetized and very massive WDs. In this new description, several observational properties are easy understood and well explained as a consequence of the large radius of a massive white dwarf that manifests a new scale of mass density, moment of inertia, rotational energy, and magnetic dipole moment in comparison with the case of neutron stars. In this contribution, we will show that these recent discoveries of SGRs with low magnetic field share some properties with the recent detected fast WD pulsar AE Aquarii, and also with RXJ 0648.0-4418, and EUVE J0317-855, supporting the understanding of at least these SGRs with low-B as belonging to a class of very fast and magnetic massive WDs. Furthermore, these recent astronomical observations suggest that we should revisit the real nature of AXP/SGRs: are they really magnetars or very fast and massive white dwarfs?

astro-ph.SR↗
arXiv2013

Asymptotic Expansions and Bondi Positivity in Higher Dimensional Relativity

The positivity of the Bondi mass has been proven in 4 dimensions, but in higher dimensions the issue remains open. The formalism of the present paper has been developed to investigate this and is well suited to the higher dimensional case. At null infinity, we make asymptotic expansions of the metric components in conformal Gaussian null coordinates, and use the vacuum Einstein equations to solve for the expansion coefficients. We find simple coordinate formulae for the Bondi mass, news and flux in terms of the expansion coefficients of the metric components. We also make expansions of the generator of an asymptotic symmetry and obtain expressions for its expansion coefficients in terms of those of the metric components. We make a spinorial asymptotic expansion of a solution to the Dirac equation in the four dimensional case, and use it to give a clean proof that the Bondi mass is positive in four dimensions. The generalisation to higher dimensions has been investigated, but not yet resolved: indeed, the spinor mass appears to be divergent in the higher dimensional case, because of terms which appear to be singular in the limit at infinity. Such apparently singular terms are common in many of our calculations, but can generally be shown to be zero before the limit is taken. However, it is not yet clear this can be done in the case of the spinor mass in higher dimensions, so further investigation is required.

gr-qc↗
arXiv2013

Why there is no noon-midnight red shift in the GPS

Although the effects of solar (and lunar) gravitational potentials on the frequencies of orbiting Global Positioning System (GPS) clocks are actually no more than a few parts in 10^(15), a naïve calculation appears to show that such effects are much larger, and depend on whether the orbiting clock is between the earth and the sun, or on the side of the earth opposite to the sun. Consequently questions about whether such effects have been properly accounted for in the GPS continue to arise. This issue has been discussed in a misleading way in terms of cancellations arising from a second-order Doppler shift in the literature for almost 50 years. The purpose of this article is to provide a correct argument, based on fundamental relativity principles, so that one may understand in a simple way why the effects of external solar system bodies on orbiting or earth-bound clocks in the GPS are so small. The relativity of simultaneity plays a crucial role in these arguments.

gr-qc↗
arXiv2013

Three dimensional stationary cyclic symmetric Einstein-Maxwell solutions; energy, mass, momentum, and algebraic tensors characteristics

The main purpose of this contribution is to determine physical and geometrical characterizations of whole classes of stationary cyclic symmetric gravitational fields coupled to Maxwell electromagnetic fields within the $(2+1)$-dimensional gravity. The physical characterization is based on the determination of the local and global energy-momentum-mass quantities using the Brown-York approach. As far as to the algebraic-geometrical characterization is concerned, the eigenvalue problem for the electromagnetic field, energy-momentum and Cotton tensors is solved and their types are established. The families of Einstein-Maxwell solutions to be considered are: all uniform electromagnetic solutions possessing electromagnetic fields with vanishing covariant derivatives (stationary uniform and spinning Clement classes), all fields having constant electromagnetic field and energy-momentum tensors' invariants (Kamata-Koikawa solutions), the whole classes of hybrid electromagnetic Ayon-Cataldo-Garcia solutions, a new family of stationary electromagnetic solutions, the electrostatic and magnetostatic solutions with Peldan limit, the Clement spinning charged metric, the Martinez-Teitelboim-Zanelli black hole solution, and Dias-Lemos electromagnetic solution.

gr-qc↗
arXiv2013

Tests of the Lorentz and CPT Symmetries at the Planck Energy Scale with X-Ray and Gamma-Ray Observations

X-ray and gamma-ray observations of astrophysical objects at cosmological distances can be used to probe the energy dependence of the speed of light with high accuracy and to search for violations of Lorentz invariance and CPT symmetry at the Planck energy scale. In this conference contribution, we discuss these searches in the theoretical framework of the Standard-Model Extension. We present new limits on the dispersion relation governed by operators of mass dimension d=5 and d=6, and we discuss avenues for future progress.

astro-ph.CO↗
arXiv2013

Relativity on two-dimensional spacetimes

Lorentz transformation on two-dimensional spacetime is obtained without assumption of linearity. To obtain this, we use the invariance of wave equations, which is recently proved to be equivalent to the causality preservation.

gr-qc↗
arXiv2013

Black-hole lattices

The construction of black-hole lattices, first attempted by Richard Lindquist and John Wheeler in 1957, has recently been tackled with renewed interest, as a test bed for studying the behavior of inhomogeneities in the context of the backreaction problem. In this contribution, I discuss how black-hole lattices can help shed light on two important issues, and illustrate the conclusions reached so far in the study of these systems.

gr-qc↗
arXiv2013

Hyperbolicity of Force-Free Electrodynamics

We analyze the equations of relativistic magnetized plasma dynamics in the limiting case that electromagnetic stress-energy is dominant over pressure and rest mass energy density. The naive formulation of these equations is shown to be not hyperbolic. Modifying the equations by terms that vanish for all physical solutions, we obtain a symmetric hyperbolic evolution system, which should exhibit improved numerical behavior.

gr-qc↗
arXiv2013

Mixing of gravitons with photons in primordial magnetic fields

Here I discuss the conversion of relic gravitons into photons in large scale cosmological magnetic fields. It is shown that the conversion probability is quite large at the post recombination epoch with a rather large density of formed photons. The produced electromagnetic radiation could make a substantial contribution to the cosmic extragalactic background light and even explain the cosmic x ray background excess.

astro-ph.CO↗