19 research outputs found
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 () 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
global monopole revisited
In this paper the 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 and , 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 . 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