Massless Dirac Perturbations in a Consistent Model of Loop Quantum Gravity Black Hole: Quasinormal Modes and Particle Emission Rates

We consider perturbations of the massless Dirac field in the background of a black hole solution found by Bodendorfer, Mele, and M\"{u}nch (BMM), using a polymerization technique that furnishes contributions inspired by Loop Quantum Gravity (LQG) Theory. Using the sixth order WKB method, we analyzed its quasinormal modes for several modes, multipole numbers and the two classes of BMM black holes. We also considered the potential that governs these perturbations to analyze the bound on the Greybody Factor (GF) due the emission rates of particles. As results, we found that the Loop Quantum Gravity parameters are responsible for raising the potential and the real and imaginary parts of the quasinormal frequencies and decrease the bound on the Greybody Factor for the two classes of black holes (with more prominent effects for the de-amplification case, which is compatible with previous analyses done for other fields).Comment: 24 pages, 14 figures. Added references. v2: fixed typos, improved the discussion on the stability of the 6th order WKB method and on the derivation of the bound on the greybody factor. Matches the published versio

Tsallis holographic dark energy in the Brans-Dicke cosmology

Using the Tsallis generalized entropy, holographic hypothesis and also considering the Hubble horizon as the IR cutoff, we build a holographic model for dark energy and study its cosmological consequences in the Brans-Dicke framework. At first, we focus on a non-interacting universe, and thereinafter, we study the results of considering a sign-changeable interaction between the dark sectors of the cosmos. Our investigations show that, compared with the flat case, the power and freedom of the model in describing the cosmic evolution is significantly increased in the presence of the curvature. The stability analysis also indicates that, independent of the universe curvature, both the interacting and non-interacting cases are classically unstable. In fact, both the classical stability criterion and an acceptable behavior for the cosmos quantities, including the deceleration and density parameters as well as the equation of state, are not simultaneously obtainable.Comment: Accepted version, Eur. Phys. J. C (2018

Quantum-spacetime effects on nonrelativistic Schr\"odinger evolution

The last three decades have witnessed the surge of quantum gravity phenomenology in the ultraviolet regime as exemplified by the Planck-scale accuracy of time-delay measurements from highly energetic astrophysical events. Yet, recent advances in precision measurements and control over quantum phenomena may usher in a new era of low-energy quantum gravity phenomenology. In this study, we investigate relativistic modified dispersion relations (MDRs) in curved spacetime and derive the corresponding nonrelativistic Schr\"odinger equation using two complementary approaches. First, we take the nonrelativistic limit, and canonically quantise the result. Second, we apply a WKB-like expansion to an MDR-inspired deformed relativistic wave equation. Within the area of applicability of single-particle quantum mechanics, both approaches imply equivalent results. Surprisingly, we recognise in the generalized uncertainty principle (GUP), the prevailing approach in nonrelativistic quantum gravity phenomenology, the MDR which is least amenable to low-energy experiments. Consequently, importing data from the mentioned time-delay measurements, we constrain the linear GUP up to the Planck scale and improve on current bounds to the quadratic one by 17 orders of magnitude. MDRs with larger implications in the infrared, however, can be tightly constrained in the nonrelativistic regime. We use the ensuing deviation from the equivalence principle to bound some MDRs, for example the one customarily associated with the bicrossproduct basis of the $\kappa$-Poincar\'e algebra, to up to four orders of magnitude below the Planck scale.Comment: 34 pages, one figur

Gravitational Aharonov-Bohm Effect for a Spinor Part icle

We consider a spinor particle in the background spacetime generated by a cosmic string. Some physical effects associated with the non-trivial conical topology of this spacetime are investigated. Topological defects of spacetime can be characterized by a spacetime metric with nu11 RiemannChristoffel curvature tensor everywhere except on the defects, that is, by conic type of curvature singularities. Recent attempts to marry the grand unified theories of particle physics with general relativistic models of the early evolution of the universe have predicted the existente of such topological defects. One example of these topological defects are the cosmic stringsI1] which appear naturally in gauge theories with spontaneous symmetry breaking. Cosmic strings are expected to be created during the phase transitions. Some may still exist and may even be observable; others may have collapsed long ago, and have served as the seeds of the galaxies [112]. Straight cosmic strings are long and exceedingly fine objects. Their thickness is comparable to the Compton wavelength h/m (in units c = 1) of typical particles when the string was formed and their tension were enormous, numerically equal to their linear mass density times the square of the speed of light. The tension, for example, for grand unified strings with mass per unit length on the order of 1 0~~~/ c m would be 1 0~~ dynes. The line element of the spacetime described by an infinite, straight and static cylindrically symmetric cosmic string12], lying along the z-ais, is given by [' ] in a cylindrical coordinate system (t, p, cp, z ) with p 2 O and O 5 cp < 2a, the hypersurface cp = O and cp = 27r being identified. The parameter a is related to the linear mass density p of the string by a = 1 -4p. This metric describes the spacetime which is locally flat (for p # 0) but has conelike singularity at p = O with the angle deficit 81rp. Then, the spacetime around an infinite straight and static cosmic string is locally flat but of course not globally flat; it does not differ from Minkowski spacetime locally, it does differ globally. The local flatness of the spacetime surrounding a straight cosrnic string means that there is no local gravity due to the string. There is no Newtonian gravitational potencial around the string and consequently a particle placed at rest in the spacetime of a cosmic string will not be attracted to it, even though the string may have a linear mass density on the order of 1022g/cm. There is no Newtonian gravitational potential around the string, however we have some very interesting gravitational effects associated with the non-trivial topology of the space-like sections around the cosrnic string. Among these effects, a cosrnic string can act as a gravitational l e n~ [~] and can induce a repulsive force on an electric charge at restc4]. Others effects include pair production by a high energy photon when it is placed in the spacetime around a cosmic stringL5] and a gravitational analog~e [~] of the electromagnetic AharonovBohm effectF7]. Clearly a11 those gravitational effects of a cosmic string are due to global (topological) features of this spacetime. In this paper we study some effects of the global features of the spacetime of a straight cosmic string on a spinor particle. To do this we use the Dirac equation in covariant form. Let us consider a spinor quantum particle imbedded in a classical background gravitational field. Its behavior is described by the covariant Dirac equation where yp(x) are the generalized Dirac matrices and are given in terrns of the standard flat spacetime gammas y(') by the relation where e( (x) are vierbeins defined by the relations a

The Inverse Problem of Analog Gravity Systems

Analog gravity models of black holes and exotic compact objects provide a unique opportunity to study key properties of such systems in controlled laboratory environments. In contrast to astrophysical systems, analog gravity systems can be prepared carefully and their dynamical aspects thus investigated in unprecedented ways. While gravitational wave scattering properties of astrophysical compact objects are more connected to quasi-normal modes, laboratory experiments can also access the transmission and reflection coefficients, which are otherwise mostly relevant for Hawking radiation related phenomena. In this work, we report two distinct results. First, we outline a semi-classical, non-parametric method that allows for the reconstruction of the effective perturbation potential from the knowledge of transmission and reflection coefficients for certain types of potentials in the Schr\"odinger wave equation admitting resonant tunneling. Second, we show how to use our method by applying it to an imperfect draining vortex, which has been suggested as analog of extreme compact objects. Although the inverse problem is in general not unique, choosing physically motivated assumptions and requiring the validity of semi-classical theory, we demonstrate that the method provides efficient and accurate results.Comment: 11 pages, 7 figure

Effects of a string cloud on the criticality and efficiency of AdS black holes as heat engines

We study the black hole thermodynamics in the presence of a string cloud matter distribution, considering a work term due to a variable cosmological "constant" in arbitrary dimensions. Then, we explore the criticality of the system and the behavior of the black hole as a heat engine in the context of general relativity and metric f(R) gravity.Comment: 12 pages, 10 figures. Matches published versio