148 research outputs found
A Rotating Charged Black Hole Solution in f(R) Gravity
In the context of f(R) theories of gravity, we address the problem of finding
a rotating charged black hole solution in the case of constant curvature. The
new metric is obtained by solving the field equations and we show that the
behavior of it is typical of a rotating charged source. In addition, we analyze
the thermodynamics of the new black hole. The results ensures that the
thermodynamical properties in f(R) gravities are qualitatively similar to those
of standard General Relativity.Comment: 9 pages, no figure
A f(R) gravity without cosmological constant
In this work we consider the possibility of describing the current evolution
of the universe, without the introduction of any cosmological constant or dark
energy (DE), by modifying the Einstein-Hilbert (EH) action. In the context of
the f(R) gravities within the metric formalism, we show that it is possible to
find an action without cosmological constant which exactly reproduces the
behavior of the EH action with cosmological constant. In addition the f(R)
action is analytical at the origin having Minkowski and Schwarzschild as vacuum
solutions. The found f(R) action is highly non-trivial and must be written in
terms of hypergeometric functions but, in spite of looking somewhat artificial,
it shows that the cosmological constant, or more generally the DE, is not a
logical necessity.Comment: 5 pages, no figure
Some cosmological and astrophysical aspects of modified gravity theories
This thesis will try to contribute to the understanding of open issues in
cosmology by considering f(R) and brane-world theories. In Chapter 1, we shall
summarize the main features of f(R) gravities in the metric formalism and we
shall introduce both the notion of brane excitations, the branons, and the
brane-skyrmions. We shall finish the chapter by providing some insight about
the possibility of mini black holes detection in the LHC as a signature for the
validity of these modified gravity theories.
The Chapter 2 will deal with f(R) theories able to mimic Einstein-Hilbert
plus cosmological constant solutions and f(R) theories will be shown to be able
to mimic the cosmological evolution generated by any perfect fluid with
constant equation of state.
The Chapter 3 will be devoted to the computation of cosmological
perturbations for f(R) theories. Special attention will be paid here to obtain
a completely general differential equation for the evolution of perturbations
and its particularization for the so-called sub-Hubble scales will be
explicitly shown.
In the Chapter 4 we shall focus on the study of black holes in f(R) gravities
in an arbitrary number of dimensions. With this purpose we shall study constant
curvature solutions for f(R) theories as well as perturbative solutions around
the standard SAdS geometry. An important part of this chapter will be then
devoted to the thermodynamics of SAdS black holes in f(R) theories.
In the Chapter 5 we will thoroughly study brane-skyrmions. In this context,
the recent claim of detection of an unexpected feature in the CMB, referred to
as the cold spot, will be explained as a topological defect on the brane in
complete agreement with those calculations in the literature that tried to
explain that cold spot as a texture of a NLSM.
Main conclusions are summarized all together in Chapter 6 .Comment: PhD. thesis, April 2010, Complutense University of Madrid. 129 pages.
Preface, five chapters and conclusions. Two appendice
Modified spinning black holes
In the context of f(R) modified gravity theories we determine that the black
holes existence is determined by the sign of a parameter dependent of the mass,
the charge, the spin and the scalar curvature. We obtain the different
astrophysical objects derived from the presence of different horizons.
Thermodynamics for this kind of black holes is studied for negative values of
the curvature, revealing that we can distinguish between two kinds of BH: fast
and slow.Comment: 4 pages, 6 figures, Contribution to the ERE2011 Spanish Relativity
Meeting, Madrid 201
On the collapse in fourth order gravities
The gravitational collapse in fourth order theories of gravity defined by an
arbitrary action of the scalar curvature shows significant deviations with
General Relativity. The presence of a new scalar mode produces a higher initial
contraction that favors the reduction of the collapsing time. However,
depending on the particular model, there are fundamental differences when the
modifications to the General Relativity collapse leave the linear regime. These
analyses can be used to exclude an important region of the parameter space
associated with alternative gravitational models.Comment: 4 pages, 2 figures, Contribution to the Proceedings of Spanish
Relativity Meeting ERE2011, Madrid 201
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