Special Issue "100 Years of Chronogeometrodynamics: the Status of the Einstein's Theory of Gravitation in Its Centennial Year"

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (31 December 2016)

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Guest Editor
Prof. Dr. Stephon Alexander

Department of Physics & Astronomy, HB 6127, Wilder Lab, Dartmouth College, Hanover, New Hampshire 03755, USA
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Interests: theoretical cosmology; cosmological constant problem; baryogenesis; cosmic inflation; structure formation; singularity resolution; quantum fields in curved spaces; string cosmology; quantum gravity; string theory; modified theories of gravity; loop quantum gravity; spin foams; geometry and math of music; instrument acoustic modelling
Guest Editor
Prof. Dr. Jean-Michel Alimi

Laboratoire Univers et Théories, Observatoire de Paris, 92195 Meudon Cedex, France
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Fax: +33 1 45 07 71 23
Interests: cosmic structure formation; inhomogenous universes; dynamics of dark matter; dark energy; tensor scalar gravity theory; numerical simulation; HPC (high performance computing) in cosmology
Guest Editor
Prof. Dr. Elias C. Vagenas

Department of Physics, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
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Fax: +965 24819374
Interests: general relativity and gravitation; black hole physics; quantum gravity; quantum gravity phenomenology
Guest Editor
Prof. Dr. Lorenzo Iorio

Ministero dell' Istruzione, dell' Università e della Ricerca (M.I.U.R.)-Istruzione. Fellow of the Royal Astronomical Society (F.R.A.S.) Viale Unità di Italia 68, 70125, Bari (BA), Italy
Website | E-Mail
Interests: General relativity and gravitation; Classical general relativity; Post-Newtonian approximation, perturbation theory, related approximations; Gravitational waves; Observational cosmology; Mathematical and relativistic aspects of cosmology; Modified theories of gravity; Higher-dimensional gravity and other theories of gravity; Experimental studies of gravity; Experimental tests of gravitational theories; Geodesy and gravity; Harmonics of the gravity potential field; geopotential theory and determination; Satellite orbits; Orbit determination and improvement; Astrometry and reference systems; Ephemerides, almanacs, and calendars; Lunar, planetary, and deep-space probes

Special Issue Information

Dear Colleagues,

In 1692, Newton wrote: "That gravity should be innate inherent and essential to matter so that one body may act upon another at a distance through a vacuum without the mediation of anything else by and through which their action or force may be conveyed from one to another is to me so great an absurdity that I believe no man who has in philosophical matters any competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws, but whether this agent be material or immaterial is a question I have left to the consideration of my readers". One of them who, just over 200 years later, picked up the baton of Newton was Albert Einstein. His General Theory of Relativity, which marks the centenary this year, opened up new windows on our comprehension of Nature, disclosed new, previously unpredictable, phenomena occurring when relative velocities dramatically change in intense gravitational fields reaching values close to the speed of light and, for the first time after millennia of speculations, put Cosmology on the firm grounds of empirically testable science. This Special Issue is dedicated to such a grandest achievement of the human thought.

Prof. Lorenzo Iorio
Prof. Stephon Alexander
Prof. Jean-Michel Alimi
Prof. Elias C. Vagenas
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) is waived for well-prepared manuscripts submitted to this issue. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.


Published Papers (22 papers)

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Research

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Open AccessArticle Theoretical Tools for Relativistic Gravimetry, Gradiometry and Chronometric Geodesy and Application to a Parameterized Post-Newtonian Metric
Universe 2017, 3(1), 24; doi:10.3390/universe3010024
Received: 2 February 2017 / Revised: 8 March 2017 / Accepted: 8 March 2017 / Published: 13 March 2017
Cited by 2 | PDF Full-text (336 KB) | HTML Full-text | XML Full-text
Abstract
An extensive review of past work on relativistic gravimetry, gradiometry and chronometric geodesy is given. Then, general theoretical tools are presented and applied for the case of a stationary parameterized post-Newtonian metric. The special case of a stationary clock on the surface of
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An extensive review of past work on relativistic gravimetry, gradiometry and chronometric geodesy is given. Then, general theoretical tools are presented and applied for the case of a stationary parameterized post-Newtonian metric. The special case of a stationary clock on the surface of the Earth is studied. Full article
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Open AccessArticle Convexity and the Euclidean Metric of Space-Time
Universe 2017, 3(1), 8; doi:10.3390/universe3010008
Received: 1 November 2016 / Revised: 2 February 2017 / Accepted: 2 February 2017 / Published: 8 February 2017
Cited by 2 | PDF Full-text (328 KB) | HTML Full-text | XML Full-text
Abstract
We address the reasons why the “Wick-rotated”, positive-definite, space-time metric obeys the Pythagorean theorem. An answer is proposed based on the convexity and smoothness properties of the functional spaces purporting to provide the kinematic framework of approaches to quantum gravity. We employ moduli
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We address the reasons why the “Wick-rotated”, positive-definite, space-time metric obeys the Pythagorean theorem. An answer is proposed based on the convexity and smoothness properties of the functional spaces purporting to provide the kinematic framework of approaches to quantum gravity. We employ moduli of convexity and smoothness which are eventually extremized by Hilbert spaces. We point out the potential physical significance that functional analytical dualities play in this framework. Following the spirit of the variational principles employed in classical and quantum Physics, such Hilbert spaces dominate in a generalized functional integral approach. The metric of space-time is induced by the inner product of such Hilbert spaces. Full article
Open AccessArticle Strategies to Ascertain the Sign of the Spatial Curvature
Universe 2016, 2(4), 27; doi:10.3390/universe2040027
Received: 14 October 2016 / Revised: 14 November 2016 / Accepted: 15 November 2016 / Published: 24 November 2016
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Abstract
The second law of thermodynamics, in the presence of gravity, is known to hold at small scales, as in the case of black holes and self-gravitating radiation spheres. Using the Friedmann–Lemaître–Robertson–Walker metric and the history of the Hubble factor, we argue that this
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The second law of thermodynamics, in the presence of gravity, is known to hold at small scales, as in the case of black holes and self-gravitating radiation spheres. Using the Friedmann–Lemaître–Robertson–Walker metric and the history of the Hubble factor, we argue that this law also holds at cosmological scales. Based on this, we study the connection between the deceleration parameter and the spatial curvature of the metric, Ω k , and set limits on the latter, valid for any homogeneous and isotropic cosmological model. Likewise, we devise strategies to determine the sign of the spatial curvature index k. Finally, assuming the lambda cold dark matter model is correct, we find that the acceleration of the cosmic expansion is increasing today. Full article
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Open AccessFeature PaperArticle A Solution of the Mitra Paradox
Universe 2016, 2(4), 26; doi:10.3390/universe2040026
Received: 7 September 2016 / Revised: 27 October 2016 / Accepted: 1 November 2016 / Published: 4 November 2016
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Abstract
The “Mitra paradox” refers to the fact that while the de Sitter spacetime appears non-static in a freely falling reference frame, it looks static with reference to a fixed reference frame. The coordinate-independent nature of the paradox may be gauged from the fact
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The “Mitra paradox” refers to the fact that while the de Sitter spacetime appears non-static in a freely falling reference frame, it looks static with reference to a fixed reference frame. The coordinate-independent nature of the paradox may be gauged from the fact that the relevant expansion scalar, θ = 3 Λ , is finite if Λ > 0 . The trivial resolution of the paradox would obviously be to set Λ = 0 . However, here it is assumed that Λ > 0 , and the paradox is resolved by invoking the concept of “expansion of space”. This is a reference-dependent concept, and it is pointed out that the solution of the Mitra paradox is obtained by taking into account the properties of the reference frame in which the coordinates are co-moving. Full article
Open AccessArticle On the Effect of the Cosmological Expansion on the Gravitational Lensing by a Point Mass
Universe 2016, 2(4), 25; doi:10.3390/universe2040025
Received: 2 September 2016 / Revised: 6 October 2016 / Accepted: 9 October 2016 / Published: 18 October 2016
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Abstract
We analyse the effect of the cosmological expansion on the deflection of light caused by a point mass, adopting the McVittie metric as the geometrical description of a point-like lens embedded in an expanding universe. In the case of a generic, non-constant Hubble
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We analyse the effect of the cosmological expansion on the deflection of light caused by a point mass, adopting the McVittie metric as the geometrical description of a point-like lens embedded in an expanding universe. In the case of a generic, non-constant Hubble parameter, H, we derive and approximately solve the null geodesic equations, finding an expression for the bending angle δ, which we expand in powers of the mass-to-closest approach distance ratio and of the impact parameter-to-lens distance ratio. It turns out that the leading order of the aforementioned expansion is the same as the one calculated for the Schwarzschild metric and that cosmological corrections contribute to δ only at sub-dominant orders. We explicitly calculate these cosmological corrections for the case of the H constant and find that they provide a correction of order 10−11 on the lens mass estimate. Full article
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Open AccessFeature PaperArticle General Relativity and Cosmology: Unsolved Questions and Future Directions
Universe 2016, 2(4), 23; doi:10.3390/universe2040023
Received: 21 August 2016 / Revised: 13 September 2016 / Accepted: 14 September 2016 / Published: 28 September 2016
Cited by 18 | PDF Full-text (964 KB) | HTML Full-text | XML Full-text
Abstract
For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations,
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For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations, which have allowed GR to remain the champion of gravitational theories against several other classes of competing theories. We pay particular attention to the role of GR and gravity in cosmology, one of the areas in which one gravity dominates and new phenomena and effects challenge the orthodoxy. We also review other areas where there are likely conflicts pointing to the need to replace or revise GR to represent correctly observations and consistent theoretical framework. Observations have long been key both to the theoretical liveliness and viability of GR. We conclude with a discussion of the likely developments over the next 100 years. Full article
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Open AccessArticle Warm Inflation
Universe 2016, 2(3), 20; doi:10.3390/universe2030020
Received: 25 July 2016 / Revised: 24 August 2016 / Accepted: 27 August 2016 / Published: 6 September 2016
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Abstract
I show here that there are some interesting differences between the predictions of warm and cold inflation models focusing in particular upon the scalar spectral index ns and the tensor-to-scalar ratio r. The first thing to be noted is that the
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I show here that there are some interesting differences between the predictions of warm and cold inflation models focusing in particular upon the scalar spectral index n s and the tensor-to-scalar ratio r. The first thing to be noted is that the warm inflation models in general predict a vanishingly small value of r. Cold inflationary models with the potential V = M 4 ( ϕ / M P ) p and a number of e-folds N = 60 predict δ n s C 1 n s ( p + 2 ) / 120 , where n s is the scalar spectral index, while the corresponding warm inflation models with constant value of the dissipation parameter Γ predict δ n s W = [ ( 20 + p ) / ( 4 + p ) ] / 120 . For example, for p = 2 this gives δ n s W = 1.1 δ n s C . The warm polynomial model with Γ = V seems to be in conflict with the Planck data. However, the warm natural inflation model can be adjusted to be in agreement with the Planck data. It has, however, more adjustable parameters in the expressions for the spectral parameters than the corresponding cold inflation model, and is hence a weaker model with less predictive force. However, it should be noted that the warm inflation models take into account physical processes such as dissipation of inflaton energy to radiation energy, which is neglected in the cold inflationary models. Full article
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Open AccessArticle The Teleparallel Equivalent of General Relativity and the Gravitational Centre of Mass
Universe 2016, 2(3), 19; doi:10.3390/universe2030019
Received: 9 June 2016 / Revised: 11 August 2016 / Accepted: 22 August 2016 / Published: 31 August 2016
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Abstract
We present a brief review of the teleparallel equivalent of general relativity and analyse the expression for the centre of mass density of the gravitational field. This expression has not been sufficiently discussed in the literature. One motivation for the present analysis is
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We present a brief review of the teleparallel equivalent of general relativity and analyse the expression for the centre of mass density of the gravitational field. This expression has not been sufficiently discussed in the literature. One motivation for the present analysis is the investigation of the localization of dark energy in the three-dimensional space, induced by a cosmological constant in a simple Schwarzschild-de Sitter space-time. We also investigate the gravitational centre of mass density in a particular model of dark matter, in the space-time of a point massive particle and in an arbitrary space-time with axial symmetry. The results are plausible, and lead to the notion of gravitational centre of mass (COM) distribution function. Full article
Open AccessArticle Symplectic Structure of Intrinsic Time Gravity
Universe 2016, 2(3), 18; doi:10.3390/universe2030018
Received: 13 June 2016 / Revised: 4 August 2016 / Accepted: 5 August 2016 / Published: 30 August 2016
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Abstract
The Poisson structure of intrinsic time gravity is analysed. With the starting point comprising a unimodular three-metric with traceless momentum, a trace-induced anomaly results upon quantization. This leads to a revision of the choice of momentum variable to the (mixed index) traceless momentric.
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The Poisson structure of intrinsic time gravity is analysed. With the starting point comprising a unimodular three-metric with traceless momentum, a trace-induced anomaly results upon quantization. This leads to a revision of the choice of momentum variable to the (mixed index) traceless momentric. This latter choice unitarily implements the fundamental commutation relations, which now take on the form of an affine algebra with SU(3) Lie algebra amongst the momentric variables. The resulting relations unitarily implement tracelessness upon quantization. The associated Poisson brackets and Hamiltonian dynamics are studied. Full article
Open AccessArticle What Is the Validity Domain of Einstein’s Equations? Distributional Solutions over Singularities and Topological Links in Geometrodynamics
Universe 2016, 2(3), 17; doi:10.3390/universe2030017
Received: 13 July 2016 / Revised: 18 August 2016 / Accepted: 22 August 2016 / Published: 29 August 2016
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Abstract
The existence of singularities alerts that one of the highest priorities of a centennial perspective on general relativity should be a careful re-thinking of the validity domain of Einstein’s field equations. We address the problem of constructing distinguishable extensions of the smooth spacetime
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The existence of singularities alerts that one of the highest priorities of a centennial perspective on general relativity should be a careful re-thinking of the validity domain of Einstein’s field equations. We address the problem of constructing distinguishable extensions of the smooth spacetime manifold model, which can incorporate singularities, while retaining the form of the field equations. The sheaf-theoretic formulation of this problem is tantamount to extending the algebra sheaf of smooth functions to a distribution-like algebra sheaf in which the former may be embedded, satisfying the pertinent cohomological conditions required for the coordinatization of all of the tensorial physical quantities, such that the form of the field equations is preserved. We present in detail the construction of these distribution-like algebra sheaves in terms of residue classes of sequences of smooth functions modulo the information of singular loci encoded in suitable ideals. Finally, we consider the application of these distribution-like solution sheaves in geometrodynamics by modeling topologically-circular boundaries of singular loci in three-dimensional space in terms of topological links. It turns out that the Borromean link represents higher order wormhole solutions. Full article
Open AccessArticle Charged and Electromagnetic Fields from Relativistic Quantum Geometry
Universe 2016, 2(2), 13; doi:10.3390/universe2020013
Received: 3 May 2016 / Revised: 3 June 2016 / Accepted: 7 June 2016 / Published: 21 June 2016
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Abstract
In the recently introduced Relativistic Quantum Geometry (RQG) formalism, the possibility was explored that the variation of the tensor metric can be done in a Weylian integrable manifold using a geometric displacement, from a Riemannian to a Weylian integrable manifold, described by the
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In the recently introduced Relativistic Quantum Geometry (RQG) formalism, the possibility was explored that the variation of the tensor metric can be done in a Weylian integrable manifold using a geometric displacement, from a Riemannian to a Weylian integrable manifold, described by the dynamics of an auxiliary geometrical scalar field θ, in order that the Einstein tensor (and the Einstein equations) can be represented on a Weyl-like manifold. In this framework we study jointly the dynamics of electromagnetic fields produced by quantum complex vector fields, which describes charges without charges. We demonstrate that complex fields act as a source of tetra-vector fields which describe an extended Maxwell dynamics. Full article
Open AccessArticle Virial Theorem in Nonlocal Newtonian Gravity
Universe 2016, 2(2), 9; doi:10.3390/universe2020009
Received: 6 April 2016 / Revised: 3 May 2016 / Accepted: 11 May 2016 / Published: 30 May 2016
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Abstract
Nonlocal gravity is the recent classical nonlocal generalization of Einstein’s theory of gravitation in which the past history of the gravitational field is taken into account. In this theory, nonlocality appears to simulate dark matter. The virial theorem for the Newtonian regime of
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Nonlocal gravity is the recent classical nonlocal generalization of Einstein’s theory of gravitation in which the past history of the gravitational field is taken into account. In this theory, nonlocality appears to simulate dark matter. The virial theorem for the Newtonian regime of nonlocal gravity theory is derived and its consequences for “isolated” astronomical systems in virial equilibrium at the present epoch are investigated. In particular, for a sufficiently isolated nearby galaxy in virial equilibrium, the galaxy’s baryonic diameter D 0 —namely, the diameter of the smallest sphere that completely surrounds the baryonic system at the present time—is predicted to be larger than the effective dark matter fraction f D M times a universal length that is the basic nonlocality length scale λ 0 3 ± 2 kpc. Full article
Open AccessArticle Effect of the Cosmological Constant on Light Deflection: Time Transfer Function Approach
Universe 2016, 2(1), 5; doi:10.3390/universe2010005
Received: 9 February 2016 / Revised: 8 March 2016 / Accepted: 8 March 2016 / Published: 14 March 2016
Cited by 3 | PDF Full-text (285 KB) | HTML Full-text | XML Full-text
Abstract
We revisit the role of the cosmological constant Λ in the deflection of light by means of the Schwarzschild–de Sitter/Kottler metric. In order to obtain the total deflection angle α, the time transfer function approach is adopted, instead of the commonly used
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We revisit the role of the cosmological constant Λ in the deflection of light by means of the Schwarzschild–de Sitter/Kottler metric. In order to obtain the total deflection angle α, the time transfer function approach is adopted, instead of the commonly used approach of solving the geodesic equation of photon. We show that the cosmological constant does appear in expression of the deflection angle, and it diminishes light bending due to the mass of the central body M. However, in contrast to previous results, for instance, that by Rindler and Ishak (Phys. Rev. D. 2007), the leading order effect due to the cosmological constant does not couple with the mass of the central body M. Full article
Open AccessArticle Autoparallel vs. Geodesic Trajectories in a Model of Torsion Gravity
Universe 2015, 1(3), 422-445; doi:10.3390/universe1030422
Received: 14 October 2015 / Revised: 11 November 2015 / Accepted: 13 November 2015 / Published: 25 November 2015
Cited by 4 | PDF Full-text (340 KB) | HTML Full-text | XML Full-text
Abstract
We consider a parametrized torsion gravity model for Riemann–Cartan geometry around a rotating axisymmetric massive body. In this model, the source of torsion is given by a circulating vector potential following the celestial parallels around the rotating object. Ours is a variant of
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We consider a parametrized torsion gravity model for Riemann–Cartan geometry around a rotating axisymmetric massive body. In this model, the source of torsion is given by a circulating vector potential following the celestial parallels around the rotating object. Ours is a variant of the Mao, Tegmark, Guth and Cabi (MTGC model) in which the total angular momentum is proposed as a source of torsion. We study the motion of bodies around the rotating object in terms of autoparallel trajectories and determine the leading perturbations of the orbital elements by using standard celestial mechanics techniques. We find that this torsion model implies new gravitational physical consequences in the Solar system and, in particular, secular variations of the semi-major axis of the planetary orbits. Perturbations on the longitude of the ascending node and the perihelion of the planets are already under discussion in the astronomical community, and if confirmed as truly non-zero effects at a statistically significant level, we might be at the dawn of an era of torsion phenomenology in the Solar system. Full article
Open AccessArticle Editorial for the Special Issue 100 Years of Chronogeometrodynamics: The Status of the Einstein’s Theory of Gravitation in Its Centennial Year
Universe 2015, 1(1), 38-81; doi:10.3390/universe1010038
Received: 6 March 2015 / Revised: 10 April 2015 / Accepted: 17 April 2015 / Published: 27 April 2015
Cited by 25 | PDF Full-text (387 KB) | HTML Full-text | XML Full-text
Abstract
The present Editorial introduces the Special Issue dedicated by the journal Universe to the General Theory of Relativity, the beautiful theory of gravitation of Einstein, a century after its birth. It reviews some of its key features in a historical perspective, and, in
[...] Read more.
The present Editorial introduces the Special Issue dedicated by the journal Universe to the General Theory of Relativity, the beautiful theory of gravitation of Einstein, a century after its birth. It reviews some of its key features in a historical perspective, and, in welcoming distinguished researchers from all over the world to contribute it, some of the main topics at the forefront of the current research are outlined. Full article

Review

Jump to: Research

Open AccessReview The Singularity Problem in Brane Cosmology
Universe 2017, 3(1), 15; doi:10.3390/universe3010015
Received: 31 December 2016 / Revised: 7 February 2017 / Accepted: 7 February 2017 / Published: 16 February 2017
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Abstract
We review results about the development and asymptotic nature of singularities in “brane–bulk” systems. These arise for warped metrics obeying the five-dimensional Einstein equations with fluid-like sources, and including a brane four-metric that is either Minkowski, de Sitter, or Anti-de Sitter. We characterize
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We review results about the development and asymptotic nature of singularities in “brane–bulk” systems. These arise for warped metrics obeying the five-dimensional Einstein equations with fluid-like sources, and including a brane four-metric that is either Minkowski, de Sitter, or Anti-de Sitter. We characterize all singular Minkowski brane solutions, and look for regular solutions with nonzero curvature. We briefly comment on matching solutions, energy conditions, and finite Planck mass criteria for admissibility, and we briefly discuss the connection of these results to ambient theory. Full article
Open AccessReview Tests of Lorentz Symmetry in the Gravitational Sector
Universe 2016, 2(4), 30; doi:10.3390/universe2040030
Received: 15 October 2016 / Revised: 21 November 2016 / Accepted: 22 November 2016 / Published: 1 December 2016
Cited by 13 | PDF Full-text (564 KB) | HTML Full-text | XML Full-text
Abstract
Lorentz symmetry is one of the pillars of both General Relativity and the Standard Model of particle physics. Motivated by ideas about quantum gravity, unification theories and violations of CPT symmetry, a significant effort has been put the last decades into testing Lorentz
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Lorentz symmetry is one of the pillars of both General Relativity and the Standard Model of particle physics. Motivated by ideas about quantum gravity, unification theories and violations of CPT symmetry, a significant effort has been put the last decades into testing Lorentz symmetry. This review focuses on Lorentz symmetry tests performed in the gravitational sector. We briefly review the basics of the pure gravitational sector of the Standard-Model Extension (SME) framework, a formalism developed in order to systematically parametrize hypothetical violations of the Lorentz invariance. Furthermore, we discuss the latest constraints obtained within this formalism including analyses of the following measurements: atomic gravimetry, Lunar Laser Ranging, Very Long Baseline Interferometry, planetary ephemerides, Gravity Probe B, binary pulsars, high energy cosmic rays, … In addition, we propose a combined analysis of all these results. We also discuss possible improvements on current analyses and present some sensitivity analyses for future observations. Full article
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Open AccessFeature PaperReview A Brief History of Gravitational Waves
Universe 2016, 2(3), 22; doi:10.3390/universe2030022
Received: 21 July 2016 / Revised: 2 September 2016 / Accepted: 2 September 2016 / Published: 13 September 2016
Cited by 4 | PDF Full-text (5467 KB) | HTML Full-text | XML Full-text
Abstract
This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the
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This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the subsequent successful discovery. Full article
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Open AccessReview Testing General Relativity with the Radio Science Experiment of the BepiColombo mission to Mercury
Universe 2016, 2(3), 21; doi:10.3390/universe2030021
Received: 29 June 2016 / Revised: 31 August 2016 / Accepted: 1 September 2016 / Published: 12 September 2016
Cited by 2 | PDF Full-text (586 KB) | HTML Full-text | XML Full-text
Abstract
The relativity experiment is part of the Mercury Orbiter Radio science Experiment (MORE) on-board the ESA/JAXA BepiColombo mission to Mercury. Thanks to very precise radio tracking from the Earth and accelerometer, it will be possible to perform an accurate test of General Relativity,
[...] Read more.
The relativity experiment is part of the Mercury Orbiter Radio science Experiment (MORE) on-board the ESA/JAXA BepiColombo mission to Mercury. Thanks to very precise radio tracking from the Earth and accelerometer, it will be possible to perform an accurate test of General Relativity, by constraining a number of post-Newtonian and related parameters with an unprecedented level of accuracy. The Celestial Mechanics Group of the University of Pisa developed a new dedicated software, ORBIT14, to perform the simulations and to determine simultaneously all the parameters of interest within a global least squares fit. After highlighting some critical issues, we report on the results of a full set of simulations, carried out in the most up-to-date mission scenario. For each parameter we discuss the achievable accuracy, in terms of a formal analysis through the covariance matrix and, furthermore, by the introduction of an alternative, more representative, estimation of the errors. We show that, for example, an accuracy of some parts in 10 6 for the Eddington parameter β and of 10 5 for the Nordtvedt parameter η can be attained, while accuracies at the level of 5 × 10 7 and 1 × 10 7 can be achieved for the preferred frames parameters α 1 and α 2 , respectively. Full article
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Open AccessReview Einstein and Beyond: A Critical Perspective on General Relativity
Universe 2016, 2(2), 11; doi:10.3390/universe2020011
Received: 4 January 2016 / Revised: 4 May 2016 / Accepted: 11 May 2016 / Published: 30 May 2016
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Abstract
An alternative approach to Einstein’s theory of General Relativity (GR) is reviewed, which is motivated by a range of serious theoretical issues inflicting the theory, such as the cosmological constant problem, presence of non-Machian solutions, problems related with the energy-stress tensor Ti
[...] Read more.
An alternative approach to Einstein’s theory of General Relativity (GR) is reviewed, which is motivated by a range of serious theoretical issues inflicting the theory, such as the cosmological constant problem, presence of non-Machian solutions, problems related with the energy-stress tensor T i k and unphysical solutions. The new approach emanates from a critical analysis of these problems, providing a novel insight that the matter fields, together with the ensuing gravitational field, are already present inherently in the spacetime without taking recourse to T i k . Supported by lots of evidence, the new insight revolutionizes our views on the representation of the source of gravitation and establishes the spacetime itself as the source, which becomes crucial for understanding the unresolved issues in a unified manner. This leads to a new paradigm in GR by establishing equation R i k = 0 as the field equation of gravitation plus inertia in the very presence of matter. Full article
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Open AccessReview The Scales of Gravitational Lensing
Universe 2016, 2(1), 6; doi:10.3390/universe2010006
Received: 5 January 2016 / Revised: 19 February 2016 / Accepted: 7 March 2016 / Published: 14 March 2016
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Abstract
After exactly a century since the formulation of the general theory of relativity, the phenomenon of gravitational lensing is still an extremely powerful method for investigating in astrophysics and cosmology. Indeed, it is adopted to study the distribution of the stellar component in
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After exactly a century since the formulation of the general theory of relativity, the phenomenon of gravitational lensing is still an extremely powerful method for investigating in astrophysics and cosmology. Indeed, it is adopted to study the distribution of the stellar component in the Milky Way, to study dark matter and dark energy on very large scales and even to discover exoplanets. Moreover, thanks to technological developments, it will allow the measure of the physical parameters (mass, angular momentum and electric charge) of supermassive black holes in the center of ours and nearby galaxies. Full article
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Open AccessFeature PaperReview The Status of Cosmic Topology after Planck Data
Universe 2016, 2(1), 1; doi:10.3390/universe2010001
Received: 19 November 2015 / Revised: 4 January 2016 / Accepted: 7 January 2016 / Published: 15 January 2016
Cited by 4 | PDF Full-text (867 KB) | HTML Full-text | XML Full-text
Abstract
In the last decade, the study of the overall shape of the universe, called Cosmic Topology, has become testable by astronomical observations, especially the data from the Cosmic Microwave Background (hereafter CMB) obtained by WMAP and Planck telescopes. Cosmic Topology involves both global
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In the last decade, the study of the overall shape of the universe, called Cosmic Topology, has become testable by astronomical observations, especially the data from the Cosmic Microwave Background (hereafter CMB) obtained by WMAP and Planck telescopes. Cosmic Topology involves both global topological features and more local geometrical properties such as curvature. It deals with questions such as whether space is finite or infinite, simply-connected or multi-connected, and smaller or greater than its observable counterpart. A striking feature of some relativistic, multi-connected small universe models is to create multiples images of faraway cosmic sources. While the last CMB (Planck) data fit well the simplest model of a zero-curvature, infinite space model, they remain consistent with more complex shapes such as the spherical Poincaré Dodecahedral Space, the flat hypertorus or the hyperbolic Picard horn. We review the theoretical and observational status of the field. Full article
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