Nonminimal global monopole

In this paper, we investigate the gravitational effects of a global monopole that couples nonminimally to gravity. Considering a coupling parameter of arbitrary strength, we have obtained an analytical solution for the field equations of the model in the asymptotic region outside the monopole's core, thus generalizing previous results. Within the weak coupling regime, we have also analyzed how this matter-geometry interplay affects some physical properties of the monopole, such as its radius and the mass enclosed in its core, which are explicitly computed and expressed in terms of the parameter of the theory. Using the Hahari-Loust\'o toy model, we have also found that the modification of general relativity (GR) may render a positive sign to the monopole mass, which means a global monopole able to exert attractive gravitational force on matter particles surrounding it. Next, we have studied some properties of a hairy black hole carrying such a global monopole charge. In this vein, we verified that the deviation from GR may provide such a spacetime with both event and Cauchy horizons, thus emulating a Reissner-Nordstr\"om framework. Finally, we have performed a study of the geodesic motion of timelike particles around this black hole by setting the conditions for stable circular orbits. We also examined the gravitational bending experienced by photons traveling through this geometry, taking into account finite-distance corrections on the deflection angle. Additionally, we make an order-of-magnitude estimate for this extra contribution considering the light bending caused by Sagittarius A*.Comment: V4: 22 Pages, 6 figures; Text improved, typos corrected, results unchanged. Matches the published versio

Degeneracy between warm and coupled cold dark matter: A clarifying note

Wei et al [PRD 88, 043510 (2013)] have proposed the existence of a cosmological degeneracy between warm dark matter (WDM), modified gravity and coupled cold dark matter (CDM) cosmologies at both the background expansion and the growth of density perturbation levels, i.e., corresponding cosmological data would not be able to differentiate such scenarios. Here, we will focus on the specific indistinguishability between a warm dark matter plus cosmological constant ($\Lambda$) and coupled scalar field-CDM scenarios. Although the statement of Wei et al is true for very specific conditions we present a more complete discussion on this issue and show in more detail that these models are indeed distinguishable. We show that the degeneracy breaks down since coupled models leave a specific signature in the redshift space distortion data which is absent in the uncoupled warm dark matter cosmologies. Furthermore, we complement our claim by providing the reasons which suggest that even at nonlinear level a breaking of such apparent equivalence is also expected.Comment: 6 pages, 1 Figure; revised version, section IV extended and enriched with further discussion, title altered, typos corrected and new references added. Version published in PR

f(R)f(R) global monopole revisited

In this paper the $f(R)$ global monopole is reexamined. We provide an exact solution for the modified field equations in the presence of a global monopole for regions outside its core, generalizing previous results. Additionally, we discuss some particular cases obtained from this solution. We consider a setup consisting of a possible Schwarzschild black hole that absorbs the topological defect, giving rise to a static black hole endowed with a monopole's charge. Besides, we demonstrate how the asymptotic behavior of the Higgs field far from the monopole's core is shaped by a class of spacetime metrics which includes those ones analyzed here. In order to assess the gravitational properties of this system, we analyse the geodesic motion of both massive and massless test particles moving in the vicinity of such configuration. For the material particles we set the requirements they have to obey in order to experience stable orbits. On the other hand, for the photons we investigate how their trajectories are affected by the gravitational field of this black hole.Comment: 16 pages, 1 figure and 1 table. Minor changes to match published version in EPJ

Global monopole as a generator of a bulk-brane structure in Brans-Dicke bulk gravity

We investigate a braneworld model generated by a global monopole in the context of Brans-Dicke gravity. After solving the dynamical equations we found a model capable to alleviate the so-called hierarchy problem. The obtained framework is described by a hybrid compactification scheme endowed with a seven-dimensional spacetime, in which the brane has four non-compact dimensions and two curled extra dimensions. The relevant aspects of the resulting model are studied and the requirements to avoid the well known seesaw-like behavior are discussed. We show that under certain conditions it is possible to circumvent such a pathological behavior that characterizes most of the models that exhibit hybrid compactification. Lastly, we deepen our analysis by considering possible extensions of this model to a setup with multiple branes and orbifold-like extra dimension. For this, we compute the consistency conditions to be obeyed by this more general configuration as predicted by the braneworld sum rules formalism. This study indicates the possibility of exclusively positive brane tensions in the model.Comment: 10 pages, 1 figure. Improved version with more discussion added to the Introduction, section II and conclusion. A new plot was added as well as new references. Matches version accepted in EPJ

The Brans-Dicke-Rastall theory

We formulate a theory combining the principles of a scalar-tensor gravity and Rastall's proposal of a violation of the usual conservation laws. We obtain a scalar-tensor theory with two parameters $\omega$ and $\lambda$, the latter quantifying the violation of the usual conservation laws. The only exact spherically symmetric solution is that of Robinson-Bertotti besides Schwarzschild solution. A PPN analysis reveals that General Relativity results are reproduced when $\lambda = 0$. The cosmological case displays a possibility of deceleration/acceleration or acceleration/deceleration transitions during the matter dominated phase depending on the values of the free parameters.Comment: 17 pages, 3 figure