Publicaciones durante 2012 | Menu principal
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Aquí podrá encontrar detalladas las publicaciones de los miembros del grupo durante el último año. Se incluyen tanto los artículos regulares como las contribuciones a conferencias. En algunos casos se ha incluído un enlace a la versión publicada del artículo. |
Publicaciones 2011
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Title: Introduction to modified gravity: from the cosmic speedup problem to quantum gravity phenomenology. Authors: Gonzalo J. Olmo Journal-ref: Chapter in the book Aspects of Today's Cosmology, Edited by Antonio Alfonso-Faus, ISBN 978-953-307-626-3. Abstract: These notes represent a summary of the introductory part of a course on modified gravity delivered at several Spanish Universities (Granada, Valencia, and Valladolid), at the University of Wisconsin-Milwaukee (WI, USA), and at the Karl-Franzens Universitaet (Graz, Austria) during the period 2008-2011. We begin with a discussion of the classical Newtonian framework and how special relativity boosted the interest on new theories of gravity. Then we focus on Nordstr\"om's scalar theories of gravity and their influence on Einstein's theory of general relativity. We comment on the meaning of the Einstein equivalence principle and its implications for the construction of alternative theories of gravity. We present the cosmic speedup problem and how $f(R)$ theories can be constrained attending to their weak-field behavior. We conclude by showing that Palatini $f(R)$ and $f(R,Q)$ theories can be used to address different aspects of quantum gravity phenomenology and singularity problems. |
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Title: The Hawking signal in density-density correlations in BECs. Authors: Alessandro Fabbri Journal-ref: J.Phys.Conf.Ser. 314:012029,2011. Abstract: We outline the derivation of the Hawking quanta-partner signal in correlations, and highlight the specific application to detect it in density-density correlations in BECs. |
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Title: Palatini Approach Beyond Einstein's Gravity. Authors: Gonzalo J. Olmo Journal-ref: Proceedings of FFP12. To appear. Abstract: I review recent results obtained for extensions of general relativity formulated within the Palatini formalism, an approach in which metric and connection are treated as independent geometrical entities. The peculiar dynamics of these theories, governed by second-order equations and having no new degrees of freedom, makes them specially suitable to address certain aspects of quantum gravity phenomenology, construct nonsingular bouncing cosmologies, and explore black hole interiors, which in the Reissner-Nordstr\"om case develop a compact core of finite density instead of a point-like singularity. |
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Title: Nonsingular Black Holes Authors: Gonzalo J. Olmo and Diego Rubiera-García Abstract: We find that if general relativity is modified at the Planck scale by a Ricci-squared term, electrically charged black holes may be nonsingular. These objects concentrate their mass in a microscopic sphere of radius $r_{core}\approx N_q^{1/2}l_P/3$, where $l_P$ is the Planck length and $N_q$ is the number of electric charges. The singularity is avoided if the mass of the object satisfies the condition $M_0^2\approx m_P^2 \alpha_{em}^{3/2} N_q^3/2$, where $m_P$ is the Planck mass and $\alpha_{em}$ is the fine-structure constant. For astrophysical black holes this amount of charge is so small that their external horizon almost coincides with their Schwarzschild radius. We work within a first-order (Palatini) approach. |
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Title: Palatini approach to bouncing cosmologies and DSR-like effects. Authors: Gonzalo J. Olmo Journal-ref: Proceedings of Loops'11, Madrid. To appear in Journal of Physics: Conference Series (JPCS) Abstract: It is shown that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, in this theory field excitations propagating with different energy-densities perceive different background metrics, a fundamental characteristic of the DSR and Rainbow Gravity approaches. This theory, however, avoids the so-called soccer ball problem. Also, the resulting isotropic and anisotropic cosmologies are free from the big bang singularity. This singularity avoidance occurs non-perturbatively and shares some similitudes with the effective dynamics of loop quantum cosmology. |
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Title: Metric-Palatini gravity unifying local constraints and late-time cosmic acceleration Authors: Tiberiu Harko, Tomi S. Koivisto, Francisco S.N. Lobo, Gonzalo J. Olmo Abstract: We present a novel approach to modified theories of gravity that consists of adding to the Einstein-Hilbert Lagrangian an f(R) term constructed a la Palatini. Using the respective dynamically equivalent scalar-tensor representation, we show that the theory can pass the Solar System observational constraints even if the scalar field is very light. This implies the existence of a long-range scalar field, which is able to modify the cosmological and galactic dynamics, but leaves the Solar System unaffected. We also verify the absence of instabilities in perturbations and provide explicit models which are consistent with local tests and lead to the late-time cosmic acceleration. |
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Title: Palatini f(R) Black Holes in Nonlinear Electrodynamics. Authors: Gonzalo J. Olmo, D. Rubiera-Garcia. Abstract: The electrically charged Born-Infeld black holes in the Palatini formalism for $f(R)$ theories are analyzed. Specifically we study those supported by a theory $f(R)=R\pm R^2/R_P$, where $R_P$ is Planck's curvature. These black holes only differ from their General Relativity counterparts very close to the center, but may give rise to different geometrical structures in terms of inner horizons. The nature and strength of the central singularities are also significantly affected. In particular, for the model $f(R)=R - R^2/R_P$ the singularity is shifted to a finite radius, $r_+$, and the Kretschmann scalar diverges only as $1/(r-r_+)^{2}$. |
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Title: Remarks on the renormalization of primordial cosmological perturbations. Authors: Ivan Agullo, Jose Navarro-Salas, Gonzalo J. Olmo, Leonard Parker Journal-ref: Phys. Rev. D 84, 107304 (2011) . Abstract: We briefly review the need to perform renormalization of inflationary perturbations to properly work out the physical power spectra. We also summarize the basis of (momentum-space) renormalization in curved spacetime and address several misconceptions found in recent literature on this subject. |
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Title: Non-singular Universes a la Palatini Authors: Gonzalo J. Olmo Journal-ref: J.Phys.Conf.Ser.314, 012054 (2011) . Abstract: It has recently been shown that f(R) theories formulated in the Palatini variational formalism are able to avoid the big bang singularity yielding instead a bouncing solution. The mechanism responsible for this behavior is similar to that observed in the effective dynamics of loop quantum cosmology and an f(R) theory exactly reproducing that dynamics has been found. I will show here that considering more general actions, with quadratic contributions of the Ricci tensor, results in a much richer phenomenology that yields bouncing solutions even in anisotropic (Bianchi I) scenarios. Some implications of these results are discussed. |
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Title: Palatini Approach to Modified Gravity: f(R) Theories and Beyond Authors: Gonzalo J. Olmo Journal-ref: Int.J.Mod.Phys.D20, 413-462 (2011) . Abstract: We review the recent literature on modified theories of gravity in the Palatini approach. After discussing the motivations that lead to consider alternatives to Einstein's theory and to treat the metric and the connection as independent objects, we review several topics that have been recently studied within this framework. In particular, we provide an in-depth analysis of the cosmic speedup problem, laboratory and solar systems tests, the structure of stellar objects, the Cauchy problem, and bouncing cosmologies. We also discuss the importance of going beyond the f(R) models to capture other phenomenological aspects related with dark matter/energy and quantum gravity. |
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Title: Hamiltonian Formulation of Palatini f(R) theories a la Brans-Dicke. Authors: Gonzalo J. Olmo and Hélios Sanchis-Alepuz Journal-ref: Phys.Rev.D83, 104036 (2011) . Abstract: We study the Hamiltonian formulation of f(R) theories of gravity both in metric and in Palatini formalism using their classical equivalence with Brans-Dicke theories with a non-trivial potential. The Palatini case, which corresponds to the w=-3/2 Brans-Dicke theory, requires special attention because of new constraints associated with the scalar field, which is non-dynamical. We derive, compare, and discuss the constraints and evolution equations for the ww=-3/2 and w\neq -3/2 cases. Based on the properties of the constraint and evolution equations, we find that, contrary to certain claims in the literature, the Cauchy problem for the w=-3/2 case is well-formulated and there is no reason to believe that it is not well-posed in general. |
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Title: Palatini Actions and Quantum Gravity Phenomenology. Authors: Gonzalo J. Olmo Journal-ref: JCAP 10 (2011) 018 . Abstract: We show that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, we show that in this theory field excitations propagating with different energy-densities perceive different background metrics, which is a fundamental characteristic of the DSR and Rainbow Gravity approaches. Also, the resulting isotropic and anisotropic cosmologies are free from the big bang singularity. This singularity avoidance occurs non-perturbatively and shares some similitudes with the effective dynamics of loop quantum cosmology. |