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Roldao da Rocha

Publications and source records attributed to Roldao da Rocha.

At least 73 records · Page 4Linked to original sources
arXiv2016

New Spinor Fields on Lorentzian 7-Manifolds

This paper deals with the classification of spinor fields according to the bilinear covariants in 7 dimensions. The previously investigated Riemannian case is characterized by either one spinor field class, in the real case of Majorana spinors, or three non-trivial classes in the most general complex case. In this paper we show that by imposing appropriate conditions on spinor fields in 7d manifolds with Lorentzian metric, the formerly obtained obstructions for new classes of spinor fields can be circumvented. New spinor fields classes are then explicitly constructed. In particular, on 7-manifolds with asymptotically flat black hole background, these spinors can define a generalized current density which further defines a time Killing vector at the spatial infinity.

hep-th↗
arXiv2016

Dark Spinors Hawking Radiation in String Theory Black Holes

The Hawking radiation spectrum of Kerr-Sen axion-dilaton black holes is derived, in the context of dark spinors tunnelling across the horizon. Since a black hole has a well defined temperature, it should radiate in principle all the standard model particles, similar to a black body at that temperature. We investigate the tunnelling of mass dimension one spin-1/2 dark fermions, that are beyond the standard model and are prime candidates to the dark matter. Their interactions with the standard model matter and gauge fields are suppressed by at least one power of unification scale, being restricted just to the Higgs field and to the graviton likewise. The tunnelling method for the emission and absorption of mass dimension one particles across the event horizon of Kerr-Sen axion-dilaton black holes is shown here to provide further evidence for the universality of black hole radiation, further encompassing particles beyond the standard model.

gr-qc↗
arXiv2015

The Minimal Geometric Deformation Approach Extended

The minimal geometric deformation approach was introduced in order to study the exterior space-time around spherically symmetric self-gravitating systems, like stars or similar astrophysical objects as well, in the Randall-Sundrum brane-world framework. A consistent extension of this approach is developed here, which contains modifications of both the time component and the radial component of a spherically symmetric metric. A modified Schwarzschild geometry is obtained as an example of its simplest application, and a new solution potentially useful to describe stars in the brane-world is also presented.

gr-qc↗
arXiv2015

Information content in $F(R)$ brane models with non-constant curvature

In this work we investigate the entropic information-measure in the context of braneworlds with non-constant curvature. The braneworld entropic information is studied for gravity modified by the squared of the Ricci scalar, besides the usual Einstein-Hilbert term. We showed that the minimum value of the brane configurational entropy provides a stricter bound on the parameter that is responsible for the $F(R)$ model to differ from the Einstein-Hilbert standard one. Our results are moreover consistent to a negative bulk cosmological constant.

hep-th↗
arXiv2015

False Vacuum Transitions - Analytical Solutions and Decay Rate Values

In this work we show a class of oscillating configurations for the evolution of the domain walls in Euclidean space. The solutions are obtained analytically. Phase transitions are achieved from the associated fluctuation determinant, by the decay rates of the false vacuum.

hep-th↗
arXiv2015

D-Oscillons in the Standard Model-Extension

In this work we investigate the consequences of the Lorentz symmetry violation on extremely long-living, time-dependent, and spatially localized field configurations, named oscillons. This is accomplished in ($D+1$) dimensions for two interacting scalar field theories in the so-called Standard Model-Extension context. We show that $D$-dimensional scalar field lumps can present a typical size $R_{\min }\ll R_{KK}$, where $R_{KK}$ is the associated length scale of extra dimensions in Kaluza-Klein theories. Here, the size $R_{\min }$ is shown to strongly depend on the terms that control the Lorentz violation of the theory. This implies either contraction or dilation of the average radius $R_{\min}$, and a new rule for its composition, likewise. Moreover, we show that the spatial dimensions for existence of oscillating lumps have an upper limit, opening new possibilities to probe the existence of a $D$ -dimensional oscillons at TeV energy scale. Moreover, in a cosmological scenario with Lorentz symmetry breaking, we argue that in the early Universe with an extremely high energy density and a strong Lorentz violation, the typical size $R_{\min }$ was highly dilated. With the expansion and subsequent cooling of the Universe, we propose that it passed through a phase transition towards a Lorentz symmetry, wherein $R_{\min }$ tends to be compact.

hep-th↗
arXiv2015

Questing for Algebraic Mass Dimension One Spinor Fields

This work deals with new classes of spinors of mass dimension one in Minkowski spacetime. In order to accomplish it, the Lounesto classification scheme and the inversion theorem are going to be used. The algebraic framework shall be revisited by explicating the central point performed by the Fierz aggregate. Then the spinor classification is generalized in order to encompass the new mass dimension one spinors. The spinor operator is shown to play a prominent role to engender the new mass dimension one spinors, accordingly.

hep-th↗
arXiv2015

Classical Tests of General Relativity: Brane-World Sun from Minimal Geometric Deformation

We consider a solution of the effective four-dimensional brane-world equations, obtained from the General Relativistic Schwarzschild metric via the principle of Minimal Geometric Deformation, and investigate the corresponding signatures stemming from the possible existence of a warped extra dimension. In particular, we derive bounds on an extra-dimensional parameter, closely related with the fundamental gravitational length, from the experimental results of the classical tests of General Relativity in the Solar system.

gr-qc↗
arXiv2015

Charge varying sine-Gordon deformed defects

Sine-Gordon deformed defects that exhibit unusual phenomenological features on the topological charge are investigated. The possibility of a smooth and continuous transition between topological (non null charge) and non-topological (null charge) scenarios of deformed defects supported by sine-Gordon structures is evinced by the analytical calculation of topological charges and localized energy distributions. By describing cyclic deformation chains, we show that a triggering sine-Gordon model simultaneously supports kink and lump-like defects, whose topological mass values are closed by trigonometric or hyperbolic successive deformations. In spite of preserving analytical closure relations constraining the topological masses of $3$-and $4$-cyclically deformed defects, the deformation chains produce kinks and lumps which exhibit non-monotonic behavior and extra inflection points, respectively. The outcome of our analysis suggests that cyclic deformations create novel scenarios of physical and mathematical applicability of defect structures supported by the sine-Gordon theory.

hep-th↗
arXiv2015

Configurational Entropy in Brane-world Models: A New Approach to Stability

In this work we investigate the entropic information on thick brane-worlds scenarios and its consequences. The brane-world entropic information is studied for the sine-Gordon model is and hence the brane-world entropic information measure is shown an accurate way for providing the most suitable values for the bulk AdS curvature. Besides, the brane-world configurational entropy is employed to demonstrate a high organisational degree in the structure of the system configuration, for large values of a parameter of the sine-Gordon model but the one related to the AdS curvature. The Gleiser and Stamatopoulos procedure is finally applied in order to achieve a precise correlation between the energy of the system and the brane-world configurational entropy.

hep-th↗
arXiv2015

Information-Entropic for Travelling Solitons in Lorentz and CPT Breaking Systems

In this work we group three research topics apparently disconnected, namely solitons, Lorentz symmetry breaking and entropy. Following a recent work [Phys. Lett. B 713 (2012) 304], we show that it is possible to construct in the context of travelling wave solutions a configurational entropy measure in functional space, from the field configurations. Thus, we investigate the existence and properties of travelling solitons in Lorentz and CPT breaking scenarios for a class of models with two interacting scalar fields. Here, we obtain a complete set of exact solutions for the model studied which display both double and single-kink configurations. In fact, such models are very important in applications that include Bloch branes, Skyrmions, Yang-Mills, Q-balls, oscillons and various superstring-motivated theories. We find that the so-called Configurational Entropy (CE) for travelling solitons, which we name as travelling Configurational Entropy (TCE), shows that the best value of parameter responsible to break the Lorentz symmetry is one where the energy density is distributed equally around the origin. In this way, the information-theoretical measure of travelling solitons in Lorentz symmetry violation scenarios opens a new window to probe situations where the parameters responsible for breaking the symmetries are random. In this case, the TCE selects the best value.

hep-th↗
arXiv2015

Regular Bulk Solutions in Brane-worlds with Inhomogeneous Dust and Generalized Dark Radiation

From the dynamics of a brane-world with matter fields present in the bulk, the bulk metric and the black string solution near the brane are generalized, when both the dynamics of inhomogeneous dust/generalized dark radiation on the brane-world and inhomogeneous dark radiation in the bulk as well are considered -- as exact dynamical collapse solutions. Based on the analysis on the inhomogeneous static exterior of a collapsing sphere of homogeneous dark radiation on the brane, the associated black string warped horizon is studied, as well as the 5D bulk metric near the brane. Moreover, the black string and the bulk are shown to be more regular upon time evolution, for suitable values for the dark radiation parameter in the model, by analyzing the physical soft singularities.

gr-qc↗
arXiv2014

Holographic Dark Energy Models and Higher Order Generalizations in Dynamical Chern-Simons Modified Gravity

Dark Energy models are here investigated and studied in the framework of the Chern-Simons modified gravity model. We bring into focus the Holographic Dark Energy (HDE) model with Granda-Oliveros cut-off, the Modified Holographic Ricci Dark Energy (MHRDE) model and, moreover, a model with higher derivatives of the Hubble parameter as well. The relevant expressions of the scale factor a(t) for a Friedmann-Robertson-Walker Universe are derived and studied, and in this context, the evolution of the scale factor is shown to be similar to that one displayed by the modified Chaplygin gas in two of the above models.

gr-qc↗
arXiv2014

Gauge Fixing in the Maxwell Like Gravitational Theory in Minkowski Spacetime and in the Equivalent Lorentzian Spacetime

In a previous paper we investigate a Lagrangian field theory for the gravitational field (which is there represented by a section g^a of the orthonormal coframe bundle over Minkowski spacetime. Such theory, under appropriate conditions, has been proved to be equivalent to a Lorentzian spacetime structure, where the metric tensor satisfies Einstein field equations. Here, we first recall that according to quantum field theory ideas gravitation is described by a Lagrangian theory of a possible massive graviton field (generated by matter fields and coupling also to itself) living in Minkowski spacetime. The graviton field is moreover supposed to be represented by a symmetric tensor field h carrying the representations of spin two and zero of the Lorentz group. Such a field, then (as it is well known), must necessarily satisfy the gauge condition given by Eq.(3) below. Next, we introduce an ansatz relating h to the 1-form fields g^a. Then, using the Clifford bundle formalism we derive, from our Lagrangian theory, the exact wave equation for the graviton and investigate the role of the gauge condition given by Eq.(3) in obtaining a reliable conservation law for the energy-momentum tensor of the gravitational plus the matter fields in Minkowski spacetime. Finally we ask the question: does Eq.(3) fix any gauge condition for the field g of the effective Lorentzian spacetime structure that represents the field h in our theory? We show that no gauge condition is fixed a priory, as is the case in General Relativity. Moreover we investigate under which conditions we may fix Logunov gauge condition.

math-ph↗
arXiv2014

Spherically Symmetric Thick Branes Cosmological Evolution

Spherically symmetric time-dependent solutions for the 5D system of a scalar field canonically coupled to gravity are obtained and identified as an extension of recent results obtained by Ahmed, Grzadkowskia and Wudkab. The corresponding cosmology of models with regularized branes generated by such a 5D scalar field scenario is also investigated. It has been shown that the anisotropic evolution of the warp factor and consequently the Hubble like parameter are both driven by the radial coordinate on the brane, which leads to an emergent thick brane-world scenario with spherically symmetric time dependent warp factor. Meanwhile, the separability of variables depending on fifth dimension, y, which is exhibited by the equations of motion, allows one to recover the extra dimensional profiles obtained in the literature, namely the extra dimensional part of the scale (warp) factor and the scalar field dependence on y. Therefore, our results are mainly concerned with the time dependence of a spherically symmetric warp factor. Besides evincing possibilities for obtaining asymmetric stable brane-world scenarios, the extra dimensional profiles here obtained can also be reduced to those ones investigated in the literature.

gr-qc↗
arXiv2014

Spinor Fields Classification in Arbitrary Dimensions and New Classes of Spinor Fields on 7-Manifolds

A classification of spinor fields according to the associated bilinear covariants is constructed in arbitrary dimensions and metric signatures, generalizing Lounesto's 4D spinor field classification. In such a generalized classification a basic role is played by the geometric Fierz identities. In 4D Minkowski spacetime the standard bilinear covariants can be either null or non-null -- with the exception of the current density which is invariably different from zero for physical reasons -- and sweep all types of spinor fields, including Dirac, Weyl, Majorana and more generally flagpoles, flag-dipoles and dipole spinor fields. To obtain an analogous classification in higher dimensions we use the Fierz identities, which constrain the covariant bilinears in the spinor fields and force some of them to vanish. A generalized graded Fierz aggregate is moreover obtained in such a context simply from the completeness relation. We analyze the particular and important case of Riemannian 7-manifolds, where the Majorana spinor fields turn out to have a quite special place. In particular, at variance with spinor fields in 4D Minkowski spacetime that are classified in six disjoint classes, spinors in Riemannian 7-manifolds are shown to be classified, according to the bilinear covariants: (a) in just one class, in the real case of Majorana spinors; (b) in four classes, in the most general case. Much like new classes of spinor fields in 4D Minkowski spacetime have been evincing new possibilities in physics, we think these new classes of spinor fields in seven dimensions are, in particular, potential candidates for new solutions in the compactification of supergravity on a seven-dimensional manifold and its exotic versions.

hep-th↗