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Universe, Volume 1, Issue 2 (September 2015) – 8 articles , Pages 92-306

Cover Story (view full-size image): In extended theories of gravity, the universe can self-accelerate without introducing a cosmological constant or dark energy. In this context, also the dynamics of galaxies and larger structures might be explained without dark matter. A natural framework for such extensions is achieved by a new gravitational connection besides the standard metric connection. This leads to the hybrid metric-Palatini gravity, where a further f(R) gravity contribution is considered. The curvature of the new connection can bring about interesting astrophysical and cosmological phenomena, whilst the local tests of gravity are passed more smoothly than in models based on solely the metrical connection, and while avoiding the theoretical problems that might compromise the models without the metrical connection. In general, hybrid-gravity can account for dark side without introducing new unknown matter ingredients. View Paper [...] Read more.
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Article
Super Virasoro Algebras From Chiral Supergravity
Universe 2015, 1(2), 292-306; https://doi.org/10.3390/universe1020292 - 09 Sep 2015
Cited by 3 | Viewed by 1315
Abstract
In this note, we construct Noether charges for the chiral supergravity, which contains the Lorentz Chern–Simons term, by applying Wald’s prescription to the vielbein formalism. We investigate the AdS3/CFT2 correspondence by using the vielbein formalism. The asymptotic symmetry group is carefully examined by [...] Read more.
In this note, we construct Noether charges for the chiral supergravity, which contains the Lorentz Chern–Simons term, by applying Wald’s prescription to the vielbein formalism. We investigate the AdS3/CFT2 correspondence by using the vielbein formalism. The asymptotic symmetry group is carefully examined by taking into account the local Lorentz transformation, and we construct super Virasoro algebras with central extensions from the chiral supergravity. Full article
Article
Interior Dynamics of Neutral and Charged Black Holes in f(R) Gravity
Universe 2015, 1(2), 239-291; https://doi.org/10.3390/universe1020239 - 02 Sep 2015
Cited by 1 | Viewed by 2210
Abstract
In this paper, we explore the interior dynamics of neutral and charged black holes in f(R) gravity. We transform f(R) gravity from the Jordan frame into the Einstein frame and simulate scalar collapses in flat, Schwarzschild, and [...] Read more.
In this paper, we explore the interior dynamics of neutral and charged black holes in f(R) gravity. We transform f(R) gravity from the Jordan frame into the Einstein frame and simulate scalar collapses in flat, Schwarzschild, and Reissner-Nordström geometries. In simulating scalar collapses in Schwarzschild and Reissner-Nordström geometries, Kruskal and Kruskal-like coordinates are used, respectively, with the presence of f′ and a physical scalar field being taken into account. The dynamics in the vicinities of the central singularity of a Schwarzschild black hole and of the inner horizon of a Reissner-Nordström black hole is examined. Approximate analytic solutions for different types of collapses are partially obtained. The scalar degree of freedom Φ, transformed from f′, plays a similar role as a physical scalar field in general relativity. Regarding the physical scalar field in f(R) case, when /dt is negative (positive), the physical scalar field is suppressed (magnified) by Φ, where t is the coordinate time. For dark energy f(R) gravity, inside black holes, gravity can easily push f′ to 1. Consequently, the Ricci scalar R becomes singular, and the numerical simulation breaks down. This singularity problem can be avoided by adding an R2 term to the original f(R) function, in which case an infinite Ricci scalar is pushed to regions where f′ is also infinite. On the other hand, in collapse for this combined model, a black hole, including a central singularity, can be formed. Moreover, under certain initial conditions, f′ and R can be pushed to infinity as the central singularity is approached. Therefore, the classical singularity problem, which is present in general relativity, remains in collapse for this combined model. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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Review
Hybrid Metric-Palatini Gravity
Universe 2015, 1(2), 199-238; https://doi.org/10.3390/universe1020199 - 27 Aug 2015
Cited by 110 | Viewed by 2819
Abstract
Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several [...] Read more.
Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
Article
Generalized ƒ(R,Φ, X) Gravity and the Late-Time Cosmic Acceleration
Universe 2015, 1(2), 186-198; https://doi.org/10.3390/universe1020186 - 18 Aug 2015
Cited by 35 | Viewed by 2521
Abstract
High-precision observational data have confirmed with startling evidence that the Universe is currently undergoing a phase of accelerated expansion. This phase, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational equations. Historically, physics [...] Read more.
High-precision observational data have confirmed with startling evidence that the Universe is currently undergoing a phase of accelerated expansion. This phase, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational equations. Historically, physics has addressed such imbalances by either identifying sources that were previously unaccounted for or by altering the gravitational theory. Several candidates, responsible for this expansion, have been proposed in the literature, in particular dark energy models and modified gravity models, amongst others. Outstanding questions are related to the nature of this so-called “dark energy” that is driving this acceleration, and whether it is due to the vacuum energy or a dynamical field. On the other hand, the late-time cosmic acceleration may be due to modifications of general relativity. In this work, we explore a generalised modified gravity theory, namely ƒ(R,Φ, X) gravity, where R is the Ricci scalar, R is a scalar field and X is a kinetic term. This theory contains a wide range of dark energy and modified gravity models. We considered specific models and applications to the late-time cosmic acceleration. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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Article
Nonsingular Black Holes in ƒ (R) Theories
Universe 2015, 1(2), 173-185; https://doi.org/10.3390/universe1020173 - 04 Aug 2015
Cited by 73 | Viewed by 3257
Abstract
We study the structure of a family of static, spherically symmetric space-times generated by an anisotropic fluid and governed by a particular type of f(R) theory. We find that for a range of parameters with physical interest, such solutions represent black holes with [...] Read more.
We study the structure of a family of static, spherically symmetric space-times generated by an anisotropic fluid and governed by a particular type of f(R) theory. We find that for a range of parameters with physical interest, such solutions represent black holes with the central singularity replaced by a finite size wormhole. We show that time-like geodesics and null geodesics with nonzero angular momentum never reach the wormhole throat due to an infinite potential barrier. For null radial geodesics, it takes an infinite affine time to reach the wormhole. This means that the resulting space-time is geodesically complete and, therefore, nonsingular despite the generic existence of curvature divergences at the wormhole throat. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
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Article
Anti-Evaporation of Black Holes in Bigravity
Universe 2015, 1(2), 158-172; https://doi.org/10.3390/universe1020158 - 03 Aug 2015
Cited by 7 | Viewed by 2054
Abstract
We review properties of solutions in bigravity theory for a specific case where two metric tensors, \(g_{\mu \nu}\) and \(f_{\mu \nu}\), satisfy proportional relation \(f_{\mu \nu}=C^{2}g_{\mu \nu}\). For this condition, we find that the solutions describing the asymptotically de Sitter space-time can be [...] Read more.
We review properties of solutions in bigravity theory for a specific case where two metric tensors, \(g_{\mu \nu}\) and \(f_{\mu \nu}\), satisfy proportional relation \(f_{\mu \nu}=C^{2}g_{\mu \nu}\). For this condition, we find that the solutions describing the asymptotically de Sitter space-time can be obtained and investigate the perturbation around the Schwarzschild–de Sitter solutions and corresponding anti-evaporation. We discuss the stability under special perturbations related to the anti-evaporation and the importance of the non-diagonal components of the metric in bigravity. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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Review
Constraining ƒ(R) Gravity by the Large-Scale Structure
Universe 2015, 1(2), 123-157; https://doi.org/10.3390/universe1020123 - 30 Jul 2015
Cited by 52 | Viewed by 2754
Abstract
Over the past few decades, general relativity and the concordance ΛCDM model have been successfully tested using several different astrophysical and cosmological probes based on large datasets (precision cosmology). Despite their successes, some shortcomings emerge due to the fact that general relativity should [...] Read more.
Over the past few decades, general relativity and the concordance ΛCDM model have been successfully tested using several different astrophysical and cosmological probes based on large datasets (precision cosmology). Despite their successes, some shortcomings emerge due to the fact that general relativity should be revised at infrared and ultraviolet limits and to the fact that the fundamental nature of dark matter and dark energy is still a puzzle to be solved. In this perspective, ƒ(R) gravity has been extensively investigated, being the most straightforward way to modify general relativity and to overcame some of the above shortcomings. In this paper, we review various aspects of ƒ(R) gravity at extragalactic and cosmological levels. In particular, we consider a cluster of galaxies, cosmological perturbations and N-body simulations, focusing on those models that satisfy both cosmological and local gravity constraints. The perspective is that some classes of ƒ(R) models can be consistently constrained by the large-scale structure. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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Article
Higher Derivative Gravity and Conformal Gravity from Bimetric and Partially Massless Bimetric Theory
Universe 2015, 1(2), 92-122; https://doi.org/10.3390/universe1020092 - 20 Jul 2015
Cited by 71 | Viewed by 2801
Abstract
In this paper, we establish the correspondence between ghost-free bimetric theory and a class of higher derivative gravity actions, including conformal gravity and new massive gravity. We also characterize the relation between the respective equations of motion and classical solutions. We illustrate that, [...] Read more.
In this paper, we establish the correspondence between ghost-free bimetric theory and a class of higher derivative gravity actions, including conformal gravity and new massive gravity. We also characterize the relation between the respective equations of motion and classical solutions. We illustrate that, in this framework, the spin-2 ghost of higher derivative gravity at the linear level is an artifact of the truncation to a four-derivative theory. The analysis also gives a relation between the proposed partially massless (PM) bimetric theory and conformal gravity, showing, in particular, the equivalence of their equations of motion at the four-derivative level. For the PM bimetric theory, this provides further evidence for the existence of an extra gauge symmetry and the associated loss of a propagating mode away from de Sitter backgrounds. The new symmetry is an extension of Weyl invariance, which may suggest the candidate PM bimetric theory as a possible ghost-free completion of conformal gravity. Full article
(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)
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