54 research outputs found
A dual null formalism for the collapse of fluids in a cosmological background
In this work we revisit the definition of Matter Trapping Surfaces (MTS)
introduced in previous investigations and show how it can be expressed in the
so-called dual null formalism developed for Trapping Horizons (TH). With the
aim of unifying both approaches, we construct a 2+2 threading from the 1+3
flow, and thus isolate one prefered spatial direction, that allows
straightforward translation into a dual nul subbasis, and to deduce the
geometric apparatus that follows. We remain as general as possible, reverting
to spherical symmetry only when needed, and express the MTS conditions in terms
of 2-expansion of the flow, then in purely geometric form of the dual null
expansions. The Raychadhuri equations that describe both MTS and TH are written
and interpreted using the previously defined gTOV (generalized
Tolman-Oppenheimer-Volkov) functional introduced in previous work. Further
using the Misner-Sharp mass and its previous perfect fluid definition, we
relate the spatial 2-expansion to the fluid pressure, density and acceleration.
The Raychaudhuri equations also allows us to define the MTS dynamic condition
with first order differentials so the MTS conditions are now shown to be all
first order differentials. This unified formalism allows one to realise that
the MTS can only exist in normal regions, and so it can exist only between
black hole horizons and cosmological horizons. Finally we obtain a relation
yielding the sign, on a TH, of the non-vanishing null expansion which
determines the nature of the TH from fluid content, and flow characteristics.
The 2+2 unified formalism here investigated thus proves a powerful tool to
reveal, in the future extensions, more of the very rich and subtle relations
between MTS and TH.Comment: 10pp 1 fig. corrected for equation labels, cross listing correcte
Energy conditions in modified gravity
We consider generalized energy conditions in modified theories of gravity by
taking into account the further degrees of freedom related to scalar fields and
curvature invariants. The latter are usually recast as generalized {\it
geometrical fluids} that have different meanings with respect to the standard
matter fluids generally adopted as sources of the field equations. More
specifically, in modified gravity the curvature terms are grouped in a tensor
and a coupling that can be reorganized in effective
Einstein field equations, as corrections to the energy-momentum tensor of
matter. The formal validity of such inequalities does not assure some basic
requirements such as the attractive nature of gravity, so that the energy
conditions have to be considered in a wider sense.Comment: 4 pages. V2: 5 pages; version to appear in Physics Letters B. V3:
typo in Eq. (4) correcte
Spherically symmetric models: separating expansion from contraction in models with anisotropic pressures
We investigate spherically symmetric spacetimes with an anisotropic fluid and
discuss the existence and stability of a dividing shell separating expanding
and collapsing regions. We find that the dividing shell is defined by a
relation between the pressure gradients, both isotropic and anisotropic, and
the strength of the fields induced by the Misner-Sharpe mass inside the
separating shell and by the pressure fluxes. This balance is a generalization
of the Tolman-Oppenheimer- Volkoff equilibrium condition which defines a local
equilibrium condition, but conveys also a non- local character given the
definition of the Misner-Sharpe mass. We present a particular solution with
dust and radiation that provides an illustration of our results.Comment: 4pp Towards New Paradigms: Proceeding Of The Spanish Relativity
Meeting 2011. AIP Conference Proceedings, Volume 1458, pp. 487-490 (2012).
Published in AIP Conf.Proc. 1458 (2011) 487-49
Generalized energy conditions in Extended Theories of Gravity
Theories of physics can be considered viable if the initial value problem and
the energy conditions are formulated self-consistently. The former allow a
uniquely determined dynamical evolution of the system, and the latter guarantee
that causality is preserved and that "plausible" physical sources have been
considered. In this work, we consider the further degrees of freedom related to
curvature invariants and scalar fields in Extended Theories of Gravity (ETG).
These new degrees of freedom can be recast as effective perfect fluids that
carry different meanings with respect to the standard matter fluids generally
adopted as sources of the field equations. It is thus somewhat misleading to
apply the standard general relativistic energy conditions to this effective
energy-momentum, as the latter contains the matter content and a geometrical
quantity, which arises from the ETG considered. Here, we explore this subtlety,
extending on previous work, in particular, to cases with the contracted Bianchi
identities with diffeomorphism invariance and to cases with generalized
explicit curvature-matter couplings, which imply the non-conservation of the
energy-momentum tensor. Furthermore, we apply the analysis to specific ETGs,
such as scalar-tensor gravity, gravity and modified Gauss-Bonnet
gravity. Interesting results appear such as matter that may exhibit unusual
thermodynamical features, for instance, and gravity that retains its attractive
character in the presence of negative pressures; or alternatively, we verify
that repulsive gravity may occur for standard matter.Comment: 12 pages, version accepted for publication in Phys.Rev.
Gravitational induced particle production through a nonminimal curvature-matter coupling
We consider the possibility of a gravitationally induced particle production
through the mechanism of a nonminimal curvature-matter coupling. An interesting
feature of this gravitational theory is that the divergence of the
energy-momentum tensor is nonzero. As a first step in our study we reformulate
the model in terms of an equivalent scalar-tensor theory, with two arbitrary
potentials. By using the formalism of open thermodynamic systems, we interpret
the energy balance equations in this gravitational theory from a thermodynamic
point of view, as describing irreversible matter creation processes. The
particle number creation rates, the creation pressure, and the entropy
production rates are explicitly obtained as functions of the scalar field and
its potentials, as well as of the matter Lagrangian. The temperature evolution
laws of the newly created particles are also obtained. The cosmological
implications of the model are briefly investigated, and it is shown that the
late-time cosmic acceleration may be due to particle creation processes.
Furthermore, it is also shown that due to the curvature--matter coupling,
during the cosmological evolution a large amount of comoving entropy is also
produced.Comment: 15 pages; accepted for publication in the European Physical Journal
An anti-Schwarzshild solution: wormholes and scalar-tensor solutions
We investigate a static solution with an hyperbolic nature, characterised by
a pseudo-spherical foliation of space. This space-time metric can be perceived
as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs
to the class of static vacuum solutions termed "a degenerate static solution of
class A". In the present work we review its fundamental features, discuss the
existence of generalised wormholes, and derive its extension to scalar-tensor
gravity theories in general.Comment: 3 pages, contribution to the proceedings of the Spanish Relativity
Meeting-ERE200
What if Newton's Gravitational Constant was negative?
In this work, we seek a cosmological mechanism that may define the sign of
the effective gravitational coupling constant, {\em G}. To this end, we
consider general scalar-tensor gravity theories as they provide the field
theory natural framework for the variation of the gravitational coupling. We
find that models with a quadratic potential naturally stabilize the value of
{\em G} into the positive branch of the evolution and further, that de Sitter
inflation and a relaxation to General Relativity is easily attained.Comment: 16 pages, 16 figure
Testing -Free f(Q) Cosmology
We study a model of Symmetric Teleparallel gravity that is able to account
for the current accelerated expansion of the universe without the need for dark
energy component. We investigate this model by making use of dynamical system
analysis techniques to identify the regions of the parameter space with viable
cosmologies and constrain it using type Ia supernova (SnIa), cosmic microwave
background (CMB) data and make forecasts using standard siren (SS) events. We
conclude that this model is disfavored with respect to CDM and
forthcoming standard siren events can be decisive in testing the viability of
the model.Comment: 9 pages, 3 figure
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