55 research outputs found
Nonsingular black holes in nonlinear gravity coupled to Euler-Heisenberg electrodynamics
We study static, spherically symmetric black holes supported by
Euler-Heisenberg theory of electrodynamics and coupled to two different
modified theories of gravity. Such theories are the quadratic model and
Eddington-inspired Born-Infeld gravity, both formulated in metric-affine
spaces, where metric and affine connection are independent fields. We find
exact solutions of the corresponding field equations in both cases,
characterized by mass, charge, the Euler-Heisenberg coupling parameter and the
modified gravity one. For each such family of solutions, we characterize its
horizon structure and the modifications in the innermost region, finding that
some subclasses are geodesically complete. The singularity regularization is
achieved under two different mechanisms: either the boundary of the manifold is
pushed to an infinite affine distance, not being able to be reached in finite
time by any geodesic, or the presence of a wormhole structure allows for the
smooth extension of all geodesics overcoming the maximum of the potential
barrier.Comment: 14 pages, 8 figures, revtex4-1 style. v2: some new discussion and
minor corrections. Version to appear in Phys. Rev.
Non-topological solitons in field theories with kinetic self-coupling
We investigate some fundamental features of a class of non-linear
relativistic lagrangian field theories with kinetic self-coupling. We focus our
attention upon theories admitting static, spherically symmetric solutions in
three space dimensions which are finite-energy and stable. We determine general
conditions for the existence and stability of these non-topological soliton
solutions. In particular, we perform a linear stability analysis that goes
beyond the usual Derrick-like criteria. On the basis of these considerations we
obtain a complete characterization of the soliton-supporting members of the
aforementioned class of non-linear field theories. We then classify the family
of soliton-supporting theories according to the central and asymptotic
behaviors of the soliton field, and provide illustrative explicit examples of
models belonging to each of the corresponding sub-families. In the present work
we restrict most of our considerations to one and many-components scalar
models. We show that in these cases the finite-energy static spherically
symmetric solutions are stable against charge-preserving perturbations,
provided that the vacuum energy of the model vanishes and the energy density is
positive definite. We also discuss briefly the extension of the present
approach to models involving other types of fields, but a detailed study of
this more general scenario will be addressed in a separate publication.Comment: 5 pages, 1 figure, revtex4, minor corrections adde
From fundamental physics to tests with compact objects in metric-affine theories of gravity
This work provides a short but comprehensible overview of some relevant
aspects of metric-affine theories of gravity in relation to the physics and
astrophysics of compact objects. We shall highlight the pertinence of this
approach to supersede General Relativity on its strong-field regime, as well as
its advantages and some of its difficulties. Moreover, we shall reflect on the
present and future opportunities to testing its predictions with relativistic
and non-relativistic stars, black holes, and other exotic horizonless compact
objects.Comment: 12 double column pages, 2 figures, Contribution to Selected Papers of
the Fifth Amazonian Symposium on Physics, accepted in IJMP
Einstein-Cartan-Dirac gravity with symmetry breaking
Einstein-Cartan theory is an extension of the standard formulation of General
Relativity where torsion (the antisymmetric part of the affine connection) is
non-vanishing. Just as the space-time metric is sourced by the stress-energy
tensor of the matter fields, torsion is sourced via the spin density tensor,
whose physical effects become relevant at very high spin densities. In this
work we introduce an extension of the Einstein-Cartan-Dirac theory with an
electromagnetic (Maxwell) contribution minimally coupled to torsion. This
contribution breaks the gauge symmetry, which is suggested by the
possibility of a torsion-induced phase transition in the early Universe,
yielding new physics in extreme (spin) density regimes. We obtain the
generalized gravitational, electromagnetic and fermionic field equations for
this theory, estimate the strength of the corrections, and discuss the
corresponding phenomenology. In particular, we briefly address some
astrophysical considerations regarding the relevance of the effects which might
take place inside ultra-dense neutron stars with strong magnetic fields
(magnetars).Comment: 15 double column pages; v2: removed one section and added content to
other sections. Version accepted for publication on EPJ
Soliton solutions in relativistic field theories and gravitation
We report on some recent results on a class of relativistic lagrangian field
theories supporting non-topological soliton solutions and their applications in
the contexts of Gravitation and Cosmology. We analyze one and many-components
scalar fields and gauge fields.Comment: Latex, 4 pages, 1 figure. Talk given at 30th Spanish Relativity
Meeting (ERE 2007): Relativistic Astrophysics and Cosmology, Puerto de la
Cruz, Tenerife, Spain, 10-14 Sep 200
Junction conditions in Palatini gravity
We work out the junction conditions for gravity formulated in
metric-affine (Palatini) spaces using a tensor distributional approach. These
conditions are needed for building consistent models of gravitating bodies with
an interior and exterior regions matched at some hypersurface. Some of these
conditions depart from the standard Darmois-Israel ones of General Relativity
and from their metric counterparts. In particular, we find that the
trace of the stress-energy momentum tensor in the bulk must be continuous
across the matching hypersurface, though its normal derivative need not to. We
illustrate the relevance of these conditions by considering the properties of
stellar surfaces in polytropic models, showing that the range of equations of
state with potentially pathological effects is shifted beyond the domain of
physical interest. This confirms, in particular, that neutron stars and white
dwarfs can be safely modelled within the Palatini framework.Comment: 7 pages; some additions in conclusions and references' list. Version
accepted for publication on Class. Quant. Gra
Born-Infeld inspired modifications of gravity
General Relativity has shown an outstanding observational success in the
scales where it has been directly tested. However, modifications have been
intensively explored in the regimes where it seems either incomplete or signals
its own limit of validity. In particular, the breakdown of unitarity near the
Planck scale strongly suggests that General Relativity needs to be modified at
high energies and quantum gravity effects are expected to be important. This is
related to the existence of spacetime singularities when the solutions of
General Relativity are extrapolated to regimes where curvatures are large. In
this sense, Born-Infeld inspired modifications of gravity have shown an
extraordinary ability to regularise the gravitational dynamics, leading to
non-singular cosmologies and regular black hole spacetimes in a very robust
manner and without resorting to quantum gravity effects. This has boosted the
interest in these theories in applications to stellar structure, compact
objects, inflationary scenarios, cosmological singularities, and black hole and
wormhole physics, among others. We review the motivations, various
formulations, and main results achieved within these theories, including their
observational viability, and provide an overview of current open problems and
future research opportunities.Comment: 212 pages, Review under press at Physics Report
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