2,569 research outputs found
Balmer line shifts in quasars
We offer a broad review of Balmer line phenomenology in type 1 active
galactic nuclei, briefly sum- marising luminosity and radio loudness effects,
and discussing interpretation in terms of nebular physics along the 4D
eigenvector 1 sequence of quasars. We stress that relatively rare, peculiar
Balmer line profiles (i.e., with large shifts with respect to the rest frame or
double and multiple peaked) that start attracted attentions since the 1970s are
still passable of multiple dynamical interpretation. More mainstream objects
are still not fully understood as well, since competing dynamical models and
geometries are possible. Further progress may come from inter-line comparison
across the 4D Eigenvector 1 sequence.Comment: Accepted for publication in Astrophysics and Space Science, Special
Issue on Line Shifts in Astrophysics and Laboratory Plasm
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
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
Wormholes supported by hybrid metric-Palatini gravity
Recently, a modified theory of gravity was presented, which consists of the
superposition of the metric Einstein-Hilbert Lagrangian with an
term constructed \`{a} la Palatini. The theory possesses extremely interesting
features such as predicting the existence of a long-range scalar field, that
explains the late-time cosmic acceleration and passes the local tests, even in
the presence of a light scalar field. In this brief report, we consider the
possibility that wormholes are supported by this hybrid metric-Palatini
gravitational theory. We present here the general conditions for wormhole
solutions according to the null energy conditions at the throat and find
specific examples. In the first solution, we specify the redshift function, the
scalar field and choose the potential that simplifies the modified Klein-Gordon
equation. This solution is not asymptotically flat and needs to be matched to a
vacuum solution. In the second example, by adequately specifying the metric
functions and choosing the scalar field, we find an asymptotically flat
spacetime.Comment: 4 pages. V2: 5 pages, discussion added; matches published versio
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