1,160 research outputs found
Semiclassical geons as solitonic black hole remnants
We find that the end state of black hole evaporation could be represented by
non-singular and without event horizon stable solitonic remnants with masses of
the order the Planck scale and up to 16 units of charge. Though these objects
are locally indistinguishable from spherically symmetric, massive electric (or
magnetic) charges, they turn out to be sourceless geons containing a wormhole
generated by the electromagnetic field. Our results are obtained by
interpreting semiclassical corrections to Einstein's theory in the first-order
(Palatini) formalism, which yields second-order equations and avoids the
instabilities of the usual (metric) formulation of quadratic gravity. We also
discuss the potential relevance of these solutions for primordial black holes
and the dark matter problem.Comment: 9 pages, 1 figur
Microscopic wormholes and the geometry of entanglement
It has recently been suggested that Einstein-Rosen (ER) bridges can be
interpreted as maximally entangled states of two black holes that form a
complex Einstein-Podolsky-Rosen (EPR) pair. This relationship has been dubbed
as the ER = EPR correlation. In this work, we consider the latter conjecture in
the context of quadratic Palatini theory. An important result, which stems from
the underlying assumptions about the geometry on which the theory is
constructed, is the fact that all the charged solutions of the quadratic
Palatini theory possess a wormhole structure. Our results show that spacetime
may have a foam-like microstructure with wormholes generated by fluctuations of
the quantum vacuum. This involves the spontaneous creation/annihilation of
entangled particle-antiparticle pairs, existing in a maximally entangled state
connected by a non-traversable wormhole. Since the particles are produced from
the vacuum and therefore exist in a singlet state, they are necessarily
entangled with one another. This gives further support to the ER=EPR claim.Comment: 5 pages. V2: minor changes and references adde
Hybrid modified gravity unifying local tests, galactic dynamics and late-time cosmic acceleration
The non-equivalence between the metric and Palatini formalisms of
gravity is an intriguing feature of these theories. However, in the recently
proposed hybrid metric-Palatini gravity, consisting of the superposition of the
metric Einstein-Hilbert Lagrangian with an term constructed \`{a}
la Palatini, the "true" gravitational field is described by the interpolation
of these two non-equivalent approaches. The theory predicts the existence of a
light long-range scalar field, which passes the local constraints and affects
the galactic and cosmological dynamics. Thus, the theory opens new
possibilities for a unified approach, in the same theoretical framework, to the
problems of dark energy and dark matter, without distinguishing a priori matter
and geometric sources, but taking their dynamics into account under the same
standard.Comment: 8 pages. Received an Honorable Mention in the Gravity Research
Foundation Essay Contest 2013. V2: references added; version to appear in the
International Journal of Modern Physics
Palatini wormholes and energy conditions from the prism of General Relativity
Wormholes are hypothetical shortcuts in spacetime that in General Relativity
unavoidably violate all of the pointwise energy conditions. In this paper, we
consider several wormhole spacetimes that, as opposed to the standard
\emph{designer} procedure frequently employed in the literature, arise directly
from gravitational actions including additional terms resulting from
contractions of the Ricci tensor with the metric, and which are formulated
assuming independence between metric and connection (Palatini approach). We
reinterpret such wormhole solutions under the prism of General Relativity and
study the matter sources that thread them. We discuss the size of violation of
the energy conditions in different cases, and how this is related to the same
spacetimes when viewed from the modified gravity side.Comment: 13 pages, 7 figures. Significant changes mainly in introduction and
conclusions. Accepted for publication in Eur. Phys. J.
The Cauchy problem in hybrid metric-Palatini f(X)-gravity
The well-formulation and the well-posedness of the Cauchy problem is
discussed for {\it hybrid metric-Palatini gravity}, a recently proposed
modified gravitational theory consisting of adding to the Einstein-Hilbert
Lagrangian an term constructed {\it \`{a} la} Palatini. The theory can
be recast as a scalar-tensor one predicting the existence of a light long-range
scalar field that evades the local Solar System tests and is able to modify
galactic and cosmological dynamics, leading to the late-time cosmic
acceleration. In this work, adopting generalized harmonic coordinates, we show
that the initial value problem can always be {\it well-formulated} and,
furthermore, can be {\it well-posed} depending on the adopted matter sources.Comment: 7 page
Dynamical generation of wormholes with charged fluids in quadratic Palatini gravity
The dynamical generation of wormholes within an extension of General
Relativity (GR) containing (Planck's scale-suppressed) Ricci-squared terms is
considered. The theory is formulated assuming the metric and connection to be
independent (Palatini formalism) and is probed using a charged null fluid as a
matter source. This has the following effect: starting from Minkowski space,
when the flux is active the metric becomes a charged Vaidya-type one, and once
the flux is switched off the metric settles down into a static configuration
such that far from the Planck scale the geometry is virtually indistinguishable
from that of the standard Reissner-Nordstr\"om solution of GR. However, the
innermost region undergoes significant changes, as the GR singularity is
generically replaced by a wormhole structure. Such a structure becomes
completely regular for a certain charge-to-mass ratio. Moreover, the nontrivial
topology of the wormhole allows to define a charge in terms of lines of force
trapped in the topology such that the density of lines flowing across the
wormhole throat becomes a universal constant. To the light of our results we
comment on the physical significance of curvature divergences in this theory
and the topology change issue, which support the view that space-time could
have a foam-like microstructure pervaded by wormholes generated by quantum
gravitational effects.Comment: 14 pages, 3 figures, revtex4-1 style. New content added on section
VI. Other minor corrections introduced. Final version to appear in Phys. Rev.
Cosmology of hybrid metric-Palatini f(X)-gravity
A new class of modified theories of gravity, consisting of the superposition
of the metric Einstein-Hilbert Lagrangian with an term constructed
\`{a} la Palatini was proposed recently. The dynamically equivalent
scalar-tensor representation of the model was also formulated, and it was shown
that even if the scalar field is very light, the theory passes the Solar System
observational constraints. Therefore the model predicts the existence of a
long-range scalar field, modifying the cosmological and galactic dynamics. An
explicit model that passes the local tests and leads to cosmic acceleration was
also obtained. In the present work, it is shown that the theory can be also
formulated in terms of the quantity , where T and R are
the traces of the stress-energy and Ricci tensors, respectively. The variable X
represents the deviation with respect to the field equation trace of general
relativity. The cosmological applications of this hybrid metric-Palatini
gravitational theory are also explored, and cosmological solutions coming from
the scalar-tensor representation of f(X)-gravity are presented. Criteria to
obtain cosmic acceleration are discussed and the field equations are analyzed
as a dynamical system. Several classes of dynamical cosmological solutions,
depending on the functional form of the effective scalar field potential,
describing both accelerating and decelerating Universes are explicitly
obtained. Furthermore, the cosmological perturbation equations are derived and
applied to uncover the nature of the propagating scalar degree of freedom and
the signatures these models predict in the large-scale structure.Comment: 17 pages. V2: 18 pages; minor revision and references added; to
appear in JCA
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