Universe
http://www.mdpi.com/journal/universe
Latest open access articles published in Universe at http://www.mdpi.com/journal/universe<![CDATA[Universe, Vol. 3, Pages 30: Dark Energy Constraints from Espresso Tests of the Stability of Fundamental Couplings]]>
http://www.mdpi.com/2218-1997/3/2/30
ESPRESSO is a high-resolution-ultra-stable spectrograph for the Very Large Telescope (VLT), whose commissioning will start in 2017. One of its key science goals is to test the stability of nature’s fundamental couplings with unprecedented accuracy and control of possible systematics. A total of 27 nights of the ESPRESSO Consortium’s guaranteed time observations (GTO) will be spent on testing the stability of the fine-structure constant and other fundamental couplings. A set of 14 priority optimal targets have been selected for the GTO period. In this work, we discuss the criteria underlying this selection, describe the selected targets, and present some forecasts of the impact of these measurements on fundamental physics and cosmology, focusing on dark energy constraints and using future supernova type Ia surveys as a comparison point. This report is a summary of the results reported in Phys. Rev. D 2016, 94, 123512, to which we refer the reader for further details.Universe2017-03-2432Conference Report10.3390/universe3020030302218-19972017-03-24doi: 10.3390/universe3020030Ana LeiteCarlos MartinsPaolo Molaro<![CDATA[Universe, Vol. 3, Pages 29: Spin-Field Correspondence]]>
http://www.mdpi.com/2218-1997/3/2/29
In the recent article Phys. Lett. B 2016, 759, 424–429, a new class of field theories called Nonlinear Field Space Theory was proposed. In this approach, the standard field theories are considered as linear approximations to some more general theories characterized by nonlinear field phase spaces. The case of spherical geometry is especially interesting due to its relation with the spin physics. Here, we explore this possibility, showing that classical scalar field theory with such a field space can be viewed as a perturbation of a continuous spin system. In this picture, the spin precession and the scalar field excitations are dual descriptions of the same physics. The duality is studied in the example of the Heisenberg model. It is shown that the Heisenberg model coupled to a magnetic field leads to a non-relativistic scalar field theory, characterized by quadratic dispersion relation. Finally, on the basis of analysis of the relation between the spin phase space and the scalar field theory, we propose the Spin-Field correspondence between the known types of fields and the corresponding spin systems.Universe2017-03-2332Article10.3390/universe3020029292218-19972017-03-23doi: 10.3390/universe3020029Jakub Mielczarek<![CDATA[Universe, Vol. 3, Pages 28: Quantum Entanglement in the Multiverse]]>
http://www.mdpi.com/2218-1997/3/2/28
In this report, we consider cosmological implications of quantum entanglement between two causally disconnected universes in the multiverse. Supposing that our universe was initially entangled with a causally separated universe, we compute the spectrum of vacuum fluctuations of our universe. To clearly see the effect of entanglement, we compare it with the spectrum of an initially non-entangled state. It is found that, due to quantum interference, scale-dependent modulations may enter the spectrum for the case of an initially non-entangled state. We discuss that the existence of causally disconnected universes may be experimentally tested by analyzing correlators in detail.Universe2017-03-2332Conference Report10.3390/universe3020028282218-19972017-03-23doi: 10.3390/universe3020028Sugumi Kanno<![CDATA[Universe, Vol. 3, Pages 27: On the Causality and K-Causality between Measures]]>
http://www.mdpi.com/2218-1997/3/1/27
Drawing from the optimal transport theory adapted to the Lorentzian setting, we propose and study the extension of the Sorkin–Woolgar causal relation K + onto the space of Borel probability measures on a given spacetime. We show that it retains its fundamental properties of transitivity and closedness. Furthermore, we list and prove several characterizations of this relation, including the “measure-theoretic” analogue of the characterization of K + in terms of time functions.Universe2017-03-2031Article10.3390/universe3010027272218-19972017-03-20doi: 10.3390/universe3010027Tomasz Miller<![CDATA[Universe, Vol. 3, Pages 26: Cosmology with Varying Constants from a Thermodynamic Viewpoint]]>
http://www.mdpi.com/2218-1997/3/1/26
We study the variation of fundamental constants in cosmology while dealing with thermodynamic aspects of gravity. We focus on the variation of the speed of light, c, and Newton’s gravitational constant, G, with respect to cosmic time. We find the constraints on the possible variation of these constants by comparing varying constants of cosmological models with the latest observational data.Universe2017-03-1731Article10.3390/universe3010026262218-19972017-03-17doi: 10.3390/universe3010026Hussain Gohar<![CDATA[Universe, Vol. 3, Pages 25: The Geometry of Noncommutative Spacetimes]]>
http://www.mdpi.com/2218-1997/3/1/25
We review the concept of ‘noncommutative spacetime’ approached from an operational stand-point and explain how to endow it with suitable geometrical structures. The latter involves i.a. the causal structure, which we illustrate with a simple—‘almost-commutative’—example. Furthermore, we trace the footprints of noncommutive geometry in the foundations of quantum field theory.Universe2017-03-1631Review10.3390/universe3010025252218-19972017-03-16doi: 10.3390/universe3010025Michał Eckstein<![CDATA[Universe, Vol. 3, Pages 24: Theoretical Tools for Relativistic Gravimetry, Gradiometry and Chronometric Geodesy and Application to a Parameterized Post-Newtonian Metric]]>
http://www.mdpi.com/2218-1997/3/1/24
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.Universe2017-03-1331Article10.3390/universe3010024242218-19972017-03-13doi: 10.3390/universe3010024Pacôme DelvaJan Geršl<![CDATA[Universe, Vol. 3, Pages 23: Thermally Induced Effective Spacetimes in Self-Assembled Hyperbolic Metamaterials]]>
http://www.mdpi.com/2218-1997/3/1/23
Recent developments in gravitation theory indicate that the classic general relativity is an effective macroscopic theory which will be eventually replaced with a more fundamental theory based on thermodynamics of yet unknown microscopic degrees of freedom. Here we consider thermodynamics of an effective spacetime which may be formed under the influence of an external magnetic field in a cobalt ferrofluid. It appears that the extraordinary photons propagating inside the ferrofluid perceive thermal gradients in the ferrofluid as an effective gravitational field, which obeys the Newton law. Moreover, the effective de Sitter spacetime behaviour near the metric signature transition may mimic various cosmological inflation scenarios, which may be visualized directly using an optical microscope. Thus, some features of the hypothetic microscopic theory of gravity are illustrated in the ferrofluid-based analogue models of inflation.Universe2017-03-0831Article10.3390/universe3010023232218-19972017-03-08doi: 10.3390/universe3010023Igor Smolyaninov<![CDATA[Universe, Vol. 3, Pages 19: Janis–Newman Algorithm: Generating Rotating and NUT Charged Black Holes]]>
http://www.mdpi.com/2218-1997/3/1/19
In this review we present the most general form of the Janis–Newman algorithm. This extension allows generating configurations which contain all bosonic fields with spin less than or equal to two (real and complex scalar fields, gauge fields, metric field) and with five of the six parameters of the Plebański–Demiański metric (mass, electric charge, magnetic charge, NUT charge and angular momentum). Several examples are included to illustrate the algorithm. We also discuss the extension of the algorithm to other dimensions.Universe2017-03-0731Review10.3390/universe3010019192218-19972017-03-07doi: 10.3390/universe3010019Harold Erbin<![CDATA[Universe, Vol. 3, Pages 22: Cosmological Perturbations in Phantom Dark Energy Models]]>
http://www.mdpi.com/2218-1997/3/1/22
The ΛCDM paradigm, characterised by a constant equation of state w = − 1 for dark energy, is the model that better fits observations. However, the same observations strongly support the possibility of a dark energy content where the corresponding equation of state is close to but slightly smaller than − 1 . In this regard, we focus on three different models where the dark energy content is described by a perfect fluid with an equation of state w ≲ − 1 which can evolve or not. The three proposals show very similar behaviour at present, while the asymptotic evolution of each model drives the Universe to different abrupt events known as (i) Big Rip; (ii) Little Rip (LR); and (iii) Little Sibling of the Big Rip. With the aim of comparing these models and finding possible imprints in their predicted matter distribution, we compute the matter power spectrum and the growth rate f σ 8 . We conclude that the model which induces a LR seems to be favoured by observations.Universe2017-03-0631Article10.3390/universe3010022222218-19972017-03-06doi: 10.3390/universe3010022Imanol AlbarranMariam Bouhmadi-LópezJoão Morais<![CDATA[Universe, Vol. 3, Pages 21: 3-Form Cosmology: Phantom Behaviour, Singularities and Interactions]]>
http://www.mdpi.com/2218-1997/3/1/21
The latest cosmological observations by the Planck collaboration (and combined with others) are compatible with a phantom-like behaviour ( w &lt; − 1 ) for the dark energy equation of state that drives the current acceleration of the Universe. With this mindset, we look into models where dark energy is described by a 3-form field minimally coupled to gravity. When compared to a scalar field, these models have the advantage of more naturally accommodating a cosmological-constant and phantom-like behaviours. We show how the latter happens for a fairly general class of positive-valued potentials, and through a dynamical system approach, we find that in such cases the 3-form field leads the Universe into a Little Sibling of the Big Rip singular event into the future. In this work, we explore the possibility of avoiding such singularity via an interaction in the dark sector between cold dark matter and the 3-form field. For the kind of interactions considered, we deduce a condition for replacing the LSBR by a late time de Sitter phase. For specific examples of interactions that meet this condition, we look for distinctive imprints in the statefinder hierarchy { S 3 ( 1 ) ; S 4 ( 1 ) } , { S 3 ( 1 ) ; S 5 ( 1 ) } , and in the growth rate of matter, ϵ ( z ) , through the composite null diagnostic (CND).Universe2017-03-0331Article10.3390/universe3010021212218-19972017-03-03doi: 10.3390/universe3010021João MoraisMariam Bouhmadi-LópezJoão Marto<![CDATA[Universe, Vol. 3, Pages 18: The Baryon Phase-Transition Model and the too strange Standard Model of Cosmology]]>
http://www.mdpi.com/2218-1997/3/1/18
The Standard Model of Cosmology (SMC) has evolved in the decades since the 1965 Penzias and Wilson observations of the Cosmic Microwave Background (CMB). Over this 50-year period, the SMC has become increasingly strange due to a number of questionable assumptions. This paper examines some of these assumptions and compares them to our Baryon Phase-Transition cosmological model.Universe2017-03-0331Article10.3390/universe3010018182218-19972017-03-03doi: 10.3390/universe3010018Frederick Mayer<![CDATA[Universe, Vol. 3, Pages 20: Dark Energy and Spacetime Symmetry]]>
http://www.mdpi.com/2218-1997/3/1/20
The Petrov classification of stress-energy tensors provides a model-independent definition of a vacuum by the algebraic structure of its stress-energy tensor and implies the existence of vacua whose symmetry is reduced as compared with the maximally symmetric de Sitter vacuum associated with the Einstein cosmological term. This allows to describe a vacuum in general setting by dynamical vacuum dark fluid, presented by a variable cosmological term with the reduced symmetry which makes vacuum fluid essentially anisotropic and allows it to be evolving and clustering. The relevant solutions to the Einstein equations describe regular cosmological models with time-evolving and spatially inhomogeneous vacuum dark energy, and compact vacuum objects generically related to a dark energy: regular black holes, their remnants and self-gravitating vacuum solitons with de Sitter vacuum interiors—which can be responsible for observational effects typically related to a dark matter. The mass of objects with de Sitter interior is generically related to vacuum dark energy and to breaking of space-time symmetry. In the cosmological context spacetime symmetry provides a mechanism for relaxing cosmological constant to a needed non-zero value.Universe2017-03-0331Review10.3390/universe3010020202218-19972017-03-03doi: 10.3390/universe3010020Irina Dymnikova<![CDATA[Universe, Vol. 3, Pages 17: Non-Standard Hierarchies of the Runnings of the Spectral Index in Inﬂation]]>
http://www.mdpi.com/2218-1997/3/1/17
Recent analyses of cosmic microwave background surveys have revealed hints that there may be a non-trivial running of the running of the spectral index. If future experiments were to conﬁrm these hints, it would prove a powerful discriminator of inﬂationary models, ruling out simple single ﬁeld models. We discuss how isocurvature perturbations in multi-ﬁeld models can be invoked to generate large runnings in a non-standard hierarchy, and ﬁnd that a minimal model capable of practically realising this would be a two-ﬁeld model with a non-canonical kinetic structure. We also consider alternative scenarios such as variable speed-of-light models and canonical quantum gravity effects and their implications for runnings of the spectral index.Universe2017-03-0231Article10.3390/universe3010017172218-19972017-03-02doi: 10.3390/universe3010017Chris Longden<![CDATA[Universe, Vol. 3, Pages 16: Infinitesimal Structure of Singularities]]>
http://www.mdpi.com/2218-1997/3/1/16
Some important problems of general relativity, such as the quantisation of gravity or classical singularity problems, crucially depend on geometry on very small scales. The so-called synthetic differential geometry—a categorical counterpart of the standard differential geometry—provides a tool to penetrate infinitesimally small portions of space-time. We use this tool to show that on any “infinitesimal neighbourhood” the components of the curvature tensor are themselves infinitesimal, and construct a simplified model in which the curvature singularity disappears, owing to this effect. However, one pays a price for this result. Using topoi as a generalisation of spaces requires a weakening of arithmetic (the existence of infinitesimals) and of logic (to the intuitionistic logic). Is this too high a price to pay for acquiring a new method of solving unsolved problems in physics? Without trying, we shall never know the answer.Universe2017-02-2731Article10.3390/universe3010016162218-19972017-02-27doi: 10.3390/universe3010016Michael HellerJerzy Król<![CDATA[Universe, Vol. 3, Pages 13: Bell Violation in Primordial Cosmology]]>
http://www.mdpi.com/2218-1997/3/1/13
In this paper, we have worked on the possibility of setting up an Bell’s inequality violating experiment in the context of primordial cosmology following the fundamental principles of quantum mechanics. To set up this proposal, we have introduced a model-independent theoretical framework using which we have studied the creation of new massive particles for the scalar fluctuations in the presence of an additional time-dependent mass parameter. Next we explicitly computed the one-point and two-point correlation functions from this setup. Then, we comment on the measurement techniques of isospin breaking interactions of newly introduced massive particles and its further prospects. After that, we give an example of the string theory-originated axion monodromy model in this context. Finally, we provide a bound on the heavy particle mass parameter for any arbitrary spin field.Universe2017-02-1731Article10.3390/universe3010013132218-19972017-02-17doi: 10.3390/universe3010013Sayantan ChoudhurySudhakar PandaRajeev Singh<![CDATA[Universe, Vol. 3, Pages 14: A Zeroth Law Compatible Model to Kerr Black Hole Thermodynamics]]>
http://www.mdpi.com/2218-1997/3/1/14
We consider the thermodynamic and stability problem of Kerr black holes arising from the nonextensive/nonadditive nature of the Bekenstein–Hawking entropy formula. Nonadditive thermodynamics is often criticized by asserting that the zeroth law cannot be compatible with nonadditive composition rules, so in this work we follow the so-called formal logarithm method to derive an additive entropy function for Kerr black holes also satisfying the zeroth law’s requirement. Starting from the most general, equilibrium compatible, nonadditive entropy composition rule of Abe, we consider the simplest non-parametric approach that is generated by the explicit nonadditive form of the Bekenstein–Hawking formula. This analysis extends our previous results on the Schwarzschild case, and shows that the zeroth law-compatible temperature function in the model is independent of the mass–energy parameter of the black hole. By applying the Poincaré turning point method, we also study the thermodynamic stability problem in the system.Universe2017-02-1631Article10.3390/universe3010014142218-19972017-02-16doi: 10.3390/universe3010014Viktor CzinnerHideo Iguchi<![CDATA[Universe, Vol. 3, Pages 15: The Singularity Problem in Brane Cosmology]]>
http://www.mdpi.com/2218-1997/3/1/15
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.Universe2017-02-1631Review10.3390/universe3010015152218-19972017-02-16doi: 10.3390/universe3010015Ignatios AntoniadisSpiros Cotsakis<![CDATA[Universe, Vol. 3, Pages 11: Nonlinear Gravitational Waves as Dark Energy in Warped Spacetimes]]>
http://www.mdpi.com/2218-1997/3/1/11
We find an azimuthal-angle dependent approximate wave like solution to second order on a warped five-dimensional manifold with a self-gravitating U(1) scalar gauge field (cosmic string) on the brane using the multiple-scale method. The spectrum of the several orders of approximation show maxima of the energy distribution dependent on the azimuthal-angle and the winding numbers of the subsequent orders of the scalar field. This breakup of the quantized flux quanta does not lead to instability of the asymptotic wavelike solution due to the suppression of the n-dependency in the energy momentum tensor components by the warp factor. This effect is triggered by the contribution of the five dimensional Weyl tensor on the brane. This contribution can be understood as dark energy and can trigger the self-acceleration of the universe without the need of a cosmological constant. There is a striking relation between the symmetry breaking of the Higgs field described by the winding number and the SO(2) breaking of the axially symmetric configuration into a discrete subgroup of rotations of about 180 ∘ . The discrete sequence of non-axially symmetric deviations, cancelled by the emission of gravitational waves in order to restore the SO(2) symmetry, triggers the pressure T z z for discrete values of the azimuthal-angle. There could be a possible relation between the recently discovered angle-preferences of polarization axes of quasars on large scales and our theoretical predicted angle-dependency and this could be evidence for the existence of cosmic strings. Careful comparison of this spectrum of extremal values of the first and second order φ-dependency and the distribution of the alignment of the quasar polarizations is necessary. This can be accomplished when more observational data become available. It turns out that, for late time, the vacuum 5D spacetime is conformally invariant if the warp factor fulfils the equation of a vibrating “drum”, describing standing normal modes of the brane.Universe2017-02-1531Article10.3390/universe3010011112218-19972017-02-15doi: 10.3390/universe3010011Reinoud Slagter<![CDATA[Universe, Vol. 3, Pages 12: Peccei–Quinn Transformations and Black Holes: Orbit Transmutations and Entanglement Generation]]>
http://www.mdpi.com/2218-1997/3/1/12
In a recent paper (Mod. Phys. Lett. A 2015, 30, 1550104), the black-hole/qubit correspondence (BHQC) was exploited to deﬁne “black hole quantum circuits” allowing for a change of the supersymmetry-preserving features of electromagnetic charge conﬁgurations supporting the black hole solution. This resulted in switching from one U-duality orbit to another, or equivalently, from an element of the corresponding Freudenthal triple system with a deﬁnite rank to another one. On the supergravity side of BHQC, such quantum gates are related to particular symplectic transformations acting on the black hole charges; namely, such transformations cannot belong to the U-duality group, otherwise switching among orbits would be impossible. In this paper, we consider a particular class of such symplectic transformations, namely the ones belonging to the so-called Peccei–Quinn symplectic group, introduced some time ago within the study of very special Kähler geometries of the vector multiplets’ scalar manifolds in N = 2 supergravity in D =4 spacetime dimensions.Universe2017-02-1431Article10.3390/universe3010012122218-19972017-02-14doi: 10.3390/universe3010012Thiago PrudêncioAlessio MarraniDiego Cirilo-Lombardo<![CDATA[Universe, Vol. 3, Pages 10: Exact Solutions of the Field Equations for Empty Space in the Nash Gravitational Theory]]>
http://www.mdpi.com/2218-1997/3/1/10
John Nash has proposed a new theory of gravity. We define a Nash-tensor equal to the curvature tensor appearing in the Nash field equations for empty space, and calculate its components for two cases: 1. A static, spherically symmetric space; and 2. The expanding, homogeneous and isotropic space of the Friedmann-Lemaitre-Robertson-Walker (FLRW) universe models. We find the general, exact solution of Nash’s field equations for empty space in the static case. The line element turns out to represent the Schwarzschild-de Sitter spacetime. Also we find the simplest non-trivial solution of the field equations in the cosmological case, which gives the scale factor corresponding to the de Sitter spacetime. Hence empty space in the Nash theory corresponds to a space with Lorentz Invariant Vacuum Energy (LIVE) in the Einstein theory. This suggests that dark energy may be superfluous according to the Nash theory. We also consider a radiation filled universe model in an effort to find out how energy and matter may be incorporated into the Nash theory. A tentative interpretation of the Nash theory as a unified theory of gravity and electromagnetism leads to a very simple form of the field equations in the presence of matter. It should be noted, however, that the Nash theory is still unfinished. A satisfying way of including energy momentum into the theory has yet to be found.Universe2017-02-1431Article10.3390/universe3010010102218-19972017-02-14doi: 10.3390/universe3010010Matthew AadneØyvind Grøn<![CDATA[Universe, Vol. 3, Pages 9: Initial Energy Density of √s = 7 and 8 TeV p–p Collisions at the LHC]]>
http://www.mdpi.com/2218-1997/3/1/9
Results from the Relativistic Heavy Ion Colloder (RHIC) and the Large Hadron Collider (LHC) experiments show that in relativistic heavy ion collisions, a new state of matter, a strongly interacting perfect fluid, is created. Accelerating, exact and explicit solutions of relativistic hydrodynamics allow for a simple and natural description of this medium. A finite rapidity distribution arises from these solutions, leading to an advanced estimate of the initial energy density of high energy collisions. These solutions can be utilized to describe various aspects of proton–proton collisions, as originally suggested by Landau. We show that an advanced estimate based on hydrodynamics yields an initial energy density in s = 7 and 8 TeV proton–proton (p–p) collisions at the LHC on the same order as the critical energy density from lattice Quantum Chromodynamics (QCD). The advanced estimate yields a corresponding initial temperature that is around the critical temperature from QCD and the Hagedorn temperature. The multiplicity dependence of the estimated initial energy density suggests that in high multiplicity p–p collisions at the LHC, there is large enough initial energy density to create a non-hadronic perfect fluid.Universe2017-02-1131Article10.3390/universe301000992218-19972017-02-11doi: 10.3390/universe3010009Máté CsanádTamás CsörgőZe-Fang JiangChun-Bin Yang<![CDATA[Universe, Vol. 3, Pages 8: Convexity and the Euclidean Metric of Space-Time]]>
http://www.mdpi.com/2218-1997/3/1/8
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.Universe2017-02-0831Article10.3390/universe301000882218-19972017-02-08doi: 10.3390/universe3010008Nikolaos Kalogeropoulos<![CDATA[Universe, Vol. 3, Pages 7: Phenomenological Review on Quark–Gluon Plasma: Concepts vs. Observations]]>
http://www.mdpi.com/2218-1997/3/1/7
In this review, we present an up-to-date phenomenological summary of research developments in the physics of the Quark–Gluon Plasma (QGP). A short historical perspective and theoretical motivation for this rapidly developing field of contemporary particle physics is provided. In addition, we introduce and discuss the role of the quantum chromodynamics (QCD) ground state, non-perturbative and lattice QCD results on the QGP properties, as well as the transport models used to make a connection between theory and experiment. The experimental part presents the selected results on bulk observables, hard and penetrating probes obtained in the ultra-relativistic heavy-ion experiments carried out at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (BNL RHIC) and CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) accelerators. We also give a brief overview of new developments related to the ongoing searches of the QCD critical point and to the collectivity in small (p + p and p + A) systems.Universe2017-01-2731Review10.3390/universe301000772218-19972017-01-27doi: 10.3390/universe3010007Roman PasechnikMichal Šumbera<![CDATA[Universe, Vol. 3, Pages 5: Quark Deconfinement in Rotating Neutron Stars]]>
http://www.mdpi.com/2218-1997/3/1/5
In this paper, we use a three flavor non-local Nambu–Jona-Lasinio (NJL) model, an improved effective model of Quantum Chromodynamics (QCD) at low energies, to investigate the existence of deconfined quarks in the cores of neutron stars. Particular emphasis is put on the possible existence of quark matter in the cores of rotating neutron stars (pulsars). In contrast to non-rotating neutron stars, whose particle compositions do not change with time (are frozen in), the type and structure of the matter in the cores of rotating neutron stars depends on the spin frequencies of these stars, which opens up a possible new window on the nature of matter deep in the cores of neutron stars. Our study shows that, depending on mass and rotational frequency, up to around 8% of the mass of a massive neutron star may be in the mixed quark-hadron phase, if the phase transition is treated as a Gibbs transition. We also find that the gravitational mass at which quark deconfinement occurs in rotating neutron stars varies quadratically with spin frequency, which can be fitted by a simple formula.Universe2017-01-2431Article10.3390/universe301000552218-19972017-01-24doi: 10.3390/universe3010005Richard MellingerFridolin WeberWilliam SpinellaGustavo ContreraMilva Orsaria<![CDATA[Universe, Vol. 3, Pages 6: The Relation between Fundamental Constants and Particle Physics Parameters]]>
http://www.mdpi.com/2218-1997/3/1/6
The observed constraints on the variability of the proton to electron mass ratio μ and the fine structure constant α are used to establish constraints on the variability of the Quantum Chromodynamic Scale and a combination of the Higgs Vacuum Expectation Value and the Yukawa couplings. Further model dependent assumptions provide constraints on the Higgs VEV and the Yukawa couplings separately. A primary conclusion is that limits on the variability of dimensionless fundamental constants such as μ and α provide important constraints on the parameter space of new physics and cosmologies.Universe2017-01-2431Article10.3390/universe301000662218-19972017-01-24doi: 10.3390/universe3010006Rodger Thompson<![CDATA[Universe, Vol. 3, Pages 4: Acknowledgement to Reviewers of Universe in 2016]]>
http://www.mdpi.com/2218-1997/3/1/4
n/aUniverse2017-01-1031Editorial10.3390/universe301000442218-19972017-01-10doi: 10.3390/universe3010004 Universe Editorial Office<![CDATA[Universe, Vol. 3, Pages 3: Gravitational Lensing of Rays through the Levitating Atmospheres of Compact Objects]]>
http://www.mdpi.com/2218-1997/3/1/3
Electromagnetic rays travel on curved paths under the influence of gravity. When a dispersive optical medium is included, these trajectories are frequency-dependent. In this work we consider the behaviour of rays when a spherically symmetric, luminous compact object described by the Schwarzschild metric is surrounded by an optically thin shell of plasma supported by radiation pressure. Such levitating atmospheres occupy a position of stable radial equilibrium, where radiative flux and gravitational effects are balanced. Using general relativity and an inhomogeneous plasma we find the existence of a stable circular orbit within the atmospheric shell for low-frequency rays. We explore families of bound orbits that exist between the shell and the compact object, and identify sets of novel periodic orbits. Finally, we examine conditions necessary for the trapping and escape of low-frequency radiation.Universe2017-01-0131Article10.3390/universe301000332218-19972017-01-01doi: 10.3390/universe3010003Adam Rogers<![CDATA[Universe, Vol. 3, Pages 2: Quantum Correlations in de Sitter Space]]>
http://www.mdpi.com/2218-1997/3/1/2
We study quantum correlation of a massive scalar field in a maximally entangled state in de Sitter space. We prepare two observers, one in a global chart and the other in an open chart of de Sitter space. We find that the state becomes less entangled as the curvature of the open chart gets larger. In particular, for the cases of a massless and a conformally coupled scalar field, the quantum entanglement vanishes in the limit of infinite curvature. However, we find that the quantum discord never disappears, even in the limit that entanglement disappears.Universe2017-01-0131Conference Report10.3390/universe301000222218-19972017-01-01doi: 10.3390/universe3010002Jiro SodaSugumi KannoJonathan Shock<![CDATA[Universe, Vol. 3, Pages 1: Bouncing Cosmologies with Dark Matter and Dark Energy]]>
http://www.mdpi.com/2218-1997/3/1/1
We review matter bounce scenarios where the matter content is dark matter and dark energy. These cosmologies predict a nearly scale-invariant power spectrum with a slightly red tilt for scalar perturbations and a small tensor-to-scalar ratio. Importantly, these models predict a positive running of the scalar index, contrary to the predictions of the simplest inflationary and ekpyrotic models, and hence, could potentially be falsified by future observations. We also review how bouncing cosmological space-times can arise in theories where either the Einstein equations are modified or where matter fields that violate the null energy condition are included.Universe2016-12-2331Article10.3390/universe301000112218-19972016-12-23doi: 10.3390/universe3010001Yi-Fu CaiAntonino MarcianòDong-Gang WangEdward Wilson-Ewing<![CDATA[Universe, Vol. 2, Pages 34: New Constraints on Spatial Variations of the Fine Structure Constant from Clusters of Galaxies]]>
http://www.mdpi.com/2218-1997/2/4/34
We have constrained the spatial variation of the fine structure constant using multi-frequency measurements of the thermal Sunyaev-Zeldovich effect of 618 X-ray selected clusters. Although our results are not competitive with the ones from quasar absorption lines, we improved by a factor 10 and ∼2.5 previous results from Cosmic Microwave Background power spectrum and from galaxy clusters, respectively.Universe2016-12-2124Conference Report10.3390/universe2040034342218-19972016-12-21doi: 10.3390/universe2040034Ivan De MartinoCarlos MartinsHarald EbelingDale Kocevski<![CDATA[Universe, Vol. 2, Pages 32: Baryon Number Transfer Could Delay Quark–Hadron Transition in Cosmology]]>
http://www.mdpi.com/2218-1997/2/4/32
In the early Universe, strongly interacting matter was a quark–gluon plasma. Both lattice computations and heavy ion collision experiments, however, tell us that, in the absence of chemical potentials, no plasma survives at T &lt; ∼ 150 MeV. The cosmological Quark–Hadron transition, however, seems to have been a crossover; cosmological consequences envisaged when it was believed to be a phase transition no longer hold. In this paper, we discuss whether even a crossover transition can leave an imprint that cosmological observations can seek or, vice versa, if there are questions cosmology should address to QCD specialists. In particular, we argue that it is still unclear how baryons (not hadrons) could form at the cosmological transition. A critical role should be played by diquark states, whose abundance in the early plasma needs to be accurately evaluated. We estimate that, if the number of quarks belonging to a diquark state, at the beginning of the cosmological transition, is &lt; ∼ 1 : 10 6 , its dynamics could be modified by the process of B-transfer from plasma to hadrons. In turn, by assuming B-transfer to cause just mild perturbations and, in particular, no entropy input, we study the deviations from the tracking regime, in the frame of SCDEW models. We find that, in some cases, residual deviations could propagate down to primeval nuclesynthesis.Universe2016-12-1324Article10.3390/universe2040032322218-19972016-12-13doi: 10.3390/universe2040032Silvio BonomettoRoberto Mainini<![CDATA[Universe, Vol. 2, Pages 31: The Problem of Embedded Eigenvalues for the Dirac Equation in the Schwarzschild Black Hole Metric]]>
http://www.mdpi.com/2218-1997/2/4/31
We use the Dirac equation in a fixed black hole background and different independent techniques to demonstrate the absence of fermionic bound states around a Schwarzschild black hole. In particular, we show that no embedded eigenvalues exist which has been claimed for the case when the energy is less than the particle’s mass. We explicitly prove that the claims regarding the embedded eigenvalues can be traced back to an oversimplified approximation in the calculation. We conclude that no bound states exist regardless of the value of the mass.Universe2016-12-0224Article10.3390/universe2040031312218-19972016-12-02doi: 10.3390/universe2040031Davide BaticMarek NowakowskiKirk Morgan<![CDATA[Universe, Vol. 2, Pages 30: Tests of Lorentz Symmetry in the Gravitational Sector]]>
http://www.mdpi.com/2218-1997/2/4/30
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.Universe2016-12-0124Review10.3390/universe2040030302218-19972016-12-01doi: 10.3390/universe2040030Aurélien HeesQuentin BaileyAdrien BourgoinHélène Pihan-Le BarsChristine GuerlinChristophe Le Poncin-Lafitte<![CDATA[Universe, Vol. 2, Pages 29: Experimental Studies on the Lorentz Symmetry in Post-Newtonian Gravity with Pulsars]]>
http://www.mdpi.com/2218-1997/2/4/29
Local Lorentz invariance (LLI) is one of the most important fundamental symmetries in modern physics. While the possibility of LLI violation (LLIv) was studied extensively in flat spacetime, its counterpart in gravitational interaction also deserves significant examination from experiments. In this contribution, I review several recent studies of LLI in post-Newtonian gravity, using powerful tools of pulsar timing. It shows that precision pulsar timing experiments hold a unique position to probe LLIv in post-Newtonian gravity.Universe2016-12-0124Conference Report10.3390/universe2040029292218-19972016-12-01doi: 10.3390/universe2040029Lijing Shao<![CDATA[Universe, Vol. 2, Pages 28: World-Line Formalism: Non-Perturbative Applications]]>
http://www.mdpi.com/2218-1997/2/4/28
This review addresses the impact on various physical observables which is produced by confinement of virtual quarks and gluons at the level of the one-loop QCD diagrams. These observables include the quark condensate for various heavy flavors, the Yang-Mills running coupling with an infra-red stable fixed point, and the correlation lengths of the stochastic Yang-Mills fields. Other non-perturbative applications of the world-line formalism presented in the review are devoted to the determination of the electroweak phase-transition critical temperature, to the derivation of a semi-classical analogue of the relation between the chiral and the gluon QCD condensates, and to the calculation of the free energy of the gluon plasma in the high-temperature limit. As a complementary result, we demonstrate Casimir scaling of k-string tensions in the Gaussian ensemble of the stochastic Yang-Mills fields.Universe2016-11-2824Review10.3390/universe2040028282218-19972016-11-28doi: 10.3390/universe2040028Dmitry Antonov<![CDATA[Universe, Vol. 2, Pages 27: Strategies to Ascertain the Sign of the Spatial Curvature]]>
http://www.mdpi.com/2218-1997/2/4/27
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.Universe2016-11-2424Article10.3390/universe2040027272218-19972016-11-24doi: 10.3390/universe2040027Pedro FerreiraDiego Pavón<![CDATA[Universe, Vol. 2, Pages 26: A Solution of the Mitra Paradox]]>
http://www.mdpi.com/2218-1997/2/4/26
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 &gt; 0 . The trivial resolution of the paradox would obviously be to set Λ = 0 . However, here it is assumed that Λ &gt; 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.Universe2016-11-0424Article10.3390/universe2040026262218-19972016-11-04doi: 10.3390/universe2040026Øyvind Grøn<![CDATA[Universe, Vol. 2, Pages 25: On the Effect of the Cosmological Expansion on the Gravitational Lensing by a Point Mass]]>
http://www.mdpi.com/2218-1997/2/4/25
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.Universe2016-10-1824Article10.3390/universe2040025252218-19972016-10-18doi: 10.3390/universe2040025Oliver Piattella<![CDATA[Universe, Vol. 2, Pages 24: Which Quantum Theory Must be Reconciled with Gravity? (And What Does it Mean for Black Holes?)]]>
http://www.mdpi.com/2218-1997/2/4/24
We consider the nature of quantum properties in non-relativistic quantum mechanics (QM) and relativistic quantum field theories, and examine the connection between formal quantization schemes and intuitive notions of wave-particle duality. Based on the map between classical Poisson brackets and their associated commutators, such schemes give rise to quantum states obeying canonical dispersion relations, obtained by substituting the de Broglie relations into the relevant (classical) energy-momentum relation. In canonical QM, this yields a dispersion relation involving ℏ but not c, whereas the canonical relativistic dispersion relation involves both. Extending this logic to the canonical quantization of the gravitational field gives rise to loop quantum gravity, and a map between classical variables containing G and c, and associated commutators involving ℏ. This naturally defines a “wave-gravity duality”, suggesting that a quantum wave packet describing self-gravitating matter obeys a dispersion relation involving G, c and ℏ. We propose an Ansatz for this relation, which is valid in the semi-Newtonian regime of both QM and general relativity. In this limit, space and time are absolute, but imposing v max = c allows us to recover the standard expressions for the Compton wavelength λ C and the Schwarzschild radius r S within the same ontological framework. The new dispersion relation is based on “extended” de Broglie relations, which remain valid for slow-moving bodies of any mass m. These reduce to canonical form for m ≪ m P , yielding λ C from the standard uncertainty principle, whereas, for m ≫ m P , we obtain r S as the natural radius of a self-gravitating quantum object. Thus, the extended de Broglie theory naturally gives rise to a unified description of black holes and fundamental particles in the semi-Newtonian regime.Universe2016-10-1724Article10.3390/universe2040024242218-19972016-10-17doi: 10.3390/universe2040024Matthew Lake<![CDATA[Universe, Vol. 2, Pages 23: General Relativity and Cosmology: Unsolved Questions and Future Directions]]>
http://www.mdpi.com/2218-1997/2/4/23
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.Universe2016-09-2824Article10.3390/universe2040023232218-19972016-09-28doi: 10.3390/universe2040023Ivan DebonoGeorge Smoot<![CDATA[Universe, Vol. 2, Pages 22: A Brief History of Gravitational Waves]]>
http://www.mdpi.com/2218-1997/2/3/22
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.Universe2016-09-1323Review10.3390/universe2030022222218-19972016-09-13doi: 10.3390/universe2030022Jorge Cervantes-CotaSalvador Galindo-UribarriGeorge Smoot<![CDATA[Universe, Vol. 2, Pages 21: Testing General Relativity with the Radio Science Experiment of the BepiColombo mission to Mercury]]>
http://www.mdpi.com/2218-1997/2/3/21
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.Universe2016-09-1223Review10.3390/universe2030021212218-19972016-09-12doi: 10.3390/universe2030021Giulia SchettinoGiacomo Tommei<![CDATA[Universe, Vol. 2, Pages 20: Warm Inflation]]>
http://www.mdpi.com/2218-1997/2/3/20
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.Universe2016-09-0623Article10.3390/universe2030020202218-19972016-09-06doi: 10.3390/universe2030020Øyvind Grøn<![CDATA[Universe, Vol. 2, Pages 19: The Teleparallel Equivalent of General Relativity and the Gravitational Centre of Mass]]>
http://www.mdpi.com/2218-1997/2/3/19
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.Universe2016-08-3123Article10.3390/universe2030019192218-19972016-08-31doi: 10.3390/universe2030019José Maluf<![CDATA[Universe, Vol. 2, Pages 18: Symplectic Structure of Intrinsic Time Gravity]]>
http://www.mdpi.com/2218-1997/2/3/18
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.Universe2016-08-3023Article10.3390/universe2030018182218-19972016-08-30doi: 10.3390/universe2030018Eyo ItaAmos Kubeka<![CDATA[Universe, Vol. 2, Pages 17: What Is the Validity Domain of Einstein’s Equations? Distributional Solutions over Singularities and Topological Links in Geometrodynamics]]>
http://www.mdpi.com/2218-1997/2/3/17
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.Universe2016-08-2923Article10.3390/universe2030017172218-19972016-08-29doi: 10.3390/universe2030017Elias Zafiris<![CDATA[Universe, Vol. 2, Pages 16: Predictions for Bottomonia Suppression in 5.023 TeV Pb-Pb Collisions]]>
http://www.mdpi.com/2218-1997/2/3/16
We compute the suppression of the bottomonia states Υ ( 1 S ) , Υ ( 2 S ) , Υ ( 3 S ) , χ b ( 1 P ) , χ b ( 2 P ) , and χ b ( 3 P ) states in Large Hadron Collider (LHC) s N N = 5.023 TeV Pb-Pb collisions. For the background evolution we use 3+1d anisotropic hydrodynamics with conditions extrapolated from s N N = 2.76 TeV and we self-consistently compute bottomonia decay rates including non-equilibrium corrections to the interaction potential. For our final results, we make predictions for R A A as function of centrality, rapidity, and p T for the Υ ( 1 S ) and Υ ( 2 S ) states, including feed down effects. In order to assess the dependence on some of the model assumptions, we vary the shear viscosity-to-entropy density ratio, 4 π η / s ∈ { 1 , 2 , 3 } , and the initial momentum-space anisotropy parameter, ξ 0 ∈ { 0 , 10 , 50 } , while holding the total light hadron multiplicity fixed.Universe2016-08-2523Article10.3390/universe2030016162218-19972016-08-25doi: 10.3390/universe2030016Brandon KrouppaMichael Strickland<![CDATA[Universe, Vol. 2, Pages 15: Loop Quantum Cosmology, Modified Gravity and Extra Dimensions]]>
http://www.mdpi.com/2218-1997/2/3/15
Loop quantum cosmology (LQC) is a framework of quantum cosmology based on the quantization of symmetry reduced models following the quantization techniques of loop quantum gravity (LQG). This paper is devoted to reviewing LQC as well as its various extensions including modified gravity and higher dimensions. For simplicity considerations, we mainly focus on the effective theory, which captures main quantum corrections at the cosmological level. We set up the basic structure of Brans–Dicke (BD) and higher dimensional LQC. The effective dynamical equations of these theories are also obtained, which lay a foundation for the future phenomenological investigations to probe possible quantum gravity effects in cosmology. Some outlooks and future extensions are also discussed.Universe2016-08-1023Review10.3390/universe2030015152218-19972016-08-10doi: 10.3390/universe2030015Xiangdong Zhang<![CDATA[Universe, Vol. 2, Pages 14: Starobinsky-Like Inflation and Running Vacuum in the Context of Supergravity]]>
http://www.mdpi.com/2218-1997/2/3/14
We describe the primeval inflationary phase of the early Universe within a quantum field theoretical (QFT) framework that can be viewed as the effective action of vacuum decay in the early times. Interestingly enough, the model accounts for the “graceful exit” of the inflationary phase into the standard radiation regime. The underlying QFT framework considered here is supergravity (SUGRA), more specifically an existing formulation in which the Starobinsky-type inflation (de Sitter background) emerges from the quantum corrections to the effective action after integrating out the gravitino fields in their (dynamically induced) massive phase. We also demonstrate that the structure of the effective action in this model is consistent with the generic idea of re-normalization group (RG) running of the cosmological parameters; specifically, it follows from the corresponding RG equation for the vacuum energy density as a function of the Hubble rate, ρ Λ ( H ) . Overall, our combined approach amounts to a concrete-model realization of inflation triggered by vacuum decay in a fundamental physics context, which, as it turns out, can also be extended for the remaining epochs of the cosmological evolution until the current dark energy era.Universe2016-07-2623Article10.3390/universe2030014142218-19972016-07-26doi: 10.3390/universe2030014Spyros BasilakosNick MavromatosJoan Solà<![CDATA[Universe, Vol. 2, Pages 13: Charged and Electromagnetic Fields from Relativistic Quantum Geometry]]>
http://www.mdpi.com/2218-1997/2/2/13
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.Universe2016-06-2122Article10.3390/universe2020013132218-19972016-06-21doi: 10.3390/universe2020013Marcos ArcodíaMauricio Bellini<![CDATA[Universe, Vol. 2, Pages 12: Brief Review on Black Hole Loop Quantization]]>
http://www.mdpi.com/2218-1997/2/2/12
Here, we present a review about the quantization of spherically-symmetric spacetimes adopting loop quantum gravity techniques. Several models that have been studied so far share similar properties: the resolution of the classical singularity and some of them an intrinsic discretization of the geometry. We also explain the extension to Reissner–Nordström black holes. Besides, we review how quantum test fields on these quantum geometries allow us to study phenomena, like the Casimir effect or Hawking radiation. Finally, we briefly describe a recent proposal that incorporates spherically-symmetric matter, discussing its relevance for the understanding of black hole evolution.Universe2016-06-1522Review10.3390/universe2020012122218-19972016-06-15doi: 10.3390/universe2020012Javier Olmedo<![CDATA[Universe, Vol. 2, Pages 11: Einstein and Beyond: A Critical Perspective on General Relativity]]>
http://www.mdpi.com/2218-1997/2/2/11
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.Universe2016-05-3022Review10.3390/universe2020011112218-19972016-05-30doi: 10.3390/universe2020011Ram Vishwakarma<![CDATA[Universe, Vol. 2, Pages 10: Reissner–Nordström Anti-de Sitter Black Holes in Mimetic F(R) Gravity]]>
http://www.mdpi.com/2218-1997/2/2/10
In this paper, we study under which conditions the Reissner–Nordström anti-de Sitter black hole can be a solution of the vacuum mimetic F ( R ) gravity with Lagrange multiplier and mimetic scalar potential. As the author demonstrates, the resulting picture in the mimetic F ( R ) gravity case is a trivial extension of the standard F ( R ) approach, and in effect, the metric perturbations in the mimetic F ( R ) gravity case, for the Reissner–Nordström anti-de Sitter black hole metric, at the first order of the perturbed variables are the same at the leading order.Universe2016-05-3022Article10.3390/universe2020010102218-19972016-05-30doi: 10.3390/universe2020010V. K. Oikonomou<![CDATA[Universe, Vol. 2, Pages 9: Virial Theorem in Nonlocal Newtonian Gravity]]>
http://www.mdpi.com/2218-1997/2/2/9
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.Universe2016-05-3022Article10.3390/universe202000992218-19972016-05-30doi: 10.3390/universe2020009Bahram Mashhoon<![CDATA[Universe, Vol. 2, Pages 8: Black Holes and Exotic Spinors]]>
http://www.mdpi.com/2218-1997/2/2/8
Exotic spin structures are non-trivial liftings, of the orthogonal bundle to the spin bundle, on orientable manifolds that admit spin structures according to the celebrated Geroch theorem. Exotic spin structures play a role of paramount importance in different areas of physics, from quantum field theory, in particular at Planck length scales, to gravity, and in cosmological scales. Here, we introduce an in-depth panorama in this field, providing black hole physics as the fount of spacetime exoticness. Black holes are then studied as the generators of a non-trivial topology that also can correspond to some inequivalent spin structure. Moreover, we investigate exotic spinor fields in this context and the way exotic spinor fields branch new physics. We also calculate the tunneling probability of exotic fermions across a Kerr-Sen black hole, showing that the exotic term does affect the tunneling probability, altering the black hole evaporation rate. Finally we show that it complies with the Hawking temperature universal law.Universe2016-05-2622Article10.3390/universe202000882218-19972016-05-26doi: 10.3390/universe2020008J. da SilvaC. VillalobosRoldao da Rocha<![CDATA[Universe, Vol. 2, Pages 7: Where Does the Physics of Extreme Gravitational Collapse Reside?]]>
http://www.mdpi.com/2218-1997/2/2/7
The gravitational collapse of massive stars serves to manifest the most severe deviations of general relativity with respect to Newtonian gravity: the formation of horizons and spacetime singularities. Both features have proven to be catalysts of deep physical developments, especially when combined with the principles of quantum mechanics. Nonetheless, it is seldom remarked that it is hardly possible to combine all these developments into a unified theoretical model, while maintaining reasonable prospects for the independent experimental corroboration of its different parts. In this paper we review the current theoretical understanding of the physics of gravitational collapse in order to highlight this tension, stating the position that the standard view on evaporating black holes stands for. This serves as the motivation for the discussion of a recent proposal that offers the opposite perspective, represented by a set of geometries that regularize the classical singular behavior and present modifications of the near-horizon Schwarzschild geometry as the result of the propagation of non-perturbative ultraviolet effects originated in regions of high curvature. We present an extensive exploration of the necessary steps on the explicit construction of these geometries, and discuss how this proposal could change our present understanding of astrophysical black holes and even offer the possibility of detecting genuine ultraviolet effects in gravitational-wave experiments.Universe2016-05-1322Review10.3390/universe202000772218-19972016-05-13doi: 10.3390/universe2020007Carlos BarcelóRaúl Carballo-RubioLuis Garay<![CDATA[Universe, Vol. 2, Pages 6: The Scales of Gravitational Lensing]]>
http://www.mdpi.com/2218-1997/2/1/6
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.Universe2016-03-1421Review10.3390/universe201000662218-19972016-03-14doi: 10.3390/universe2010006Francesco De PaolisMosè GiordanoGabriele IngrossoLuigi ManniAchille NucitaFrancesco Strafella<![CDATA[Universe, Vol. 2, Pages 5: Effect of the Cosmological Constant on Light Deflection: Time Transfer Function Approach]]>
http://www.mdpi.com/2218-1997/2/1/5
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.Universe2016-03-1421Article10.3390/universe201000552218-19972016-03-14doi: 10.3390/universe2010005Hideyoshi Arakida<![CDATA[Universe, Vol. 2, Pages 4: Quantum Yang–Mills Dark Energy]]>
http://www.mdpi.com/2218-1997/2/1/4
In this short review, I discuss basic qualitative characteristics of quantum non-Abelian gauge dynamics in the non-stationary background of the expanding Universe in the framework of the standard Einstein–Yang–Mills formulation. A brief outlook of existing studies of cosmological Yang–Mills fields and their properties will be given. Quantum effects have a profound impact on the gauge field-driven cosmological evolution. In particular, a dynamical formation of the spatially-homogeneous and isotropic gauge field condensate may be responsible for both early and late-time acceleration, as well as for dynamical compensation of non-perturbative quantum vacua contributions to the ground state of the Universe. The main properties of such a condensate in the effective QCD theory at the flat Friedmann–Lemaítre–Robertson–Walker (FLRW) background will be discussed within and beyond perturbation theory. Finally, a phenomenologically consistent dark energy can be induced dynamically as a remnant of the QCD vacua compensation arising from leading-order graviton-mediated corrections to the QCD ground state.Universe2016-02-2321Review10.3390/universe201000442218-19972016-02-23doi: 10.3390/universe2010004Roman Pasechnik<![CDATA[Universe, Vol. 2, Pages 3: String-Inspired Gravity through Symmetries]]>
http://www.mdpi.com/2218-1997/2/1/3
We study a string-inspired cosmological model from the symmetries point of view. We start by deducing the form that each physical quantity must take so that the field equations, in the string frame, admit self-similar solutions. In the same way, we formalize the use of power-law solutions (less restrictive than the self-similar ones) by studying the wave equation for the dilaton through the Lie group method. Furthermore, we show how to generate more solutions by using this approach. As examples, we calculate exact solutions to several cosmological models in the four-dimensional NS-NS (Neveu-Schwarz-Neveu-Schwarz) sector of low-energy effective string theory coupled to a dilaton and an axion-like H-field within the string frame background, with FRW and the Bianchi Type II metrics. We also study the existence of Noether symmetries, which allow us to determine the form of the physical quantities in the framework of FRW geometry and to find exact cosmological solutions.Universe2016-02-0521Article10.3390/universe201000332218-19972016-02-05doi: 10.3390/universe2010003José Belinchón<![CDATA[Universe, Vol. 2, Pages 2: Acknowledgement to Reviewers of Universe in 2015]]>
http://www.mdpi.com/2218-1997/2/1/2
The editors of Universe would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2015. [...]Universe2016-01-2221Editorial10.3390/universe201000222218-19972016-01-22doi: 10.3390/universe2010002 Universe Editorial Office<![CDATA[Universe, Vol. 2, Pages 1: The Status of Cosmic Topology after Planck Data]]>
http://www.mdpi.com/2218-1997/2/1/1
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.Universe2016-01-1521Review10.3390/universe201000112218-19972016-01-15doi: 10.3390/universe2010001Jean-Pierre Luminet<![CDATA[Universe, Vol. 1, Pages 446-475: Chameleonic Theories: A Short Review]]>
http://www.mdpi.com/2218-1997/1/3/446
In the chameleon mechanism, a field (typically scalar) has a mass that depends on the matter density of the environment: the larger is the matter density, the larger is the mass of the chameleon. We briefly review some aspects of chameleonic theories. In particular, in a typical class of these theories, we discuss the lagrangian, the role of conformal transformations, the equation of motion and the thin-shell effect. We also discuss f ( R ) theories and chameleonic quantum gravity.Universe2015-12-0113Article10.3390/universe10304464464752218-19972015-12-01doi: 10.3390/universe1030446Andrea Zanzi<![CDATA[Universe, Vol. 1, Pages 422-445: Autoparallel vs. Geodesic Trajectories in a Model of Torsion Gravity]]>
http://www.mdpi.com/2218-1997/1/3/422
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.Universe2015-11-2513Article10.3390/universe10304224224452218-19972015-11-25doi: 10.3390/universe1030422Luis Acedo<![CDATA[Universe, Vol. 1, Pages 412-421: Thermodynamic Analysis of Non-Linear Reissner-Nordström Black Holes]]>
http://www.mdpi.com/2218-1997/1/3/412
In the present article we study the Inverse Electrodynamics Model. This model is a gauge and parity invariant non-linear Electrodynamics theory, which respects the conformal invariance of standard Electrodynamics. This modified Electrodynamics model, when minimally coupled to General Relativity, is compatible with static and spherically symmetric Reissner-Nordström-like black-hole solutions. However, these black-hole solutions present more complex thermodynamic properties than their Reissner-Nordström black-hole solutions counterparts in standard Electrodynamics. In particular, in the Inverse Model a new stability region, with both the heat capacity and the free energy negative, arises. Moreover, unlike the scenario in standard Electrodynamics, a sole transition phase is possible for a suitable choice in the set of parameters of these solutions.Universe2015-11-1613Article10.3390/universe10304124124212218-19972015-11-16doi: 10.3390/universe1030412Jose CembranosÁlvaro Cruz-DombrizJavier Jarillo<![CDATA[Universe, Vol. 1, Pages 357-411: A Cosmological Model Describing the Early Inflation, the Intermediate Decelerating Expansion, and the Late Accelerating Expansion of the Universe by a Quadratic Equation of State]]>
http://www.mdpi.com/2218-1997/1/3/357
We develop a cosmological model based on a quadratic equation of state \mbox{\(p/c^2=-(\alpha+1){\rho^2}/{\rho_P}+\alpha\rho-(\alpha+1)\rho_ {\Lambda}\)}, where \(\rho_P\) is the Planck density and \(\rho_{\Lambda}\) the cosmological density, ``unifying'' vacuum energy and dark energy in the spirit of a generalized Chaplygin gas model. For \(\rho\rightarrow \rho_P\), it reduces to \(p=-\rho_P c^2\) leading to a phase of early accelerating expansion (early inflation) with a constant density equal to the Planck density \(\rho_P=5.16 \times 10^{99}\, {\rm g}/{\rm m}^3\) (vacuum energy). For \(\rho_{\Lambda}\ll\rho\ll \rho_P\), we recover the standard linear equation of state \(p=\alpha \rho c^2\) describing radiation (\(\alpha=1/3\)) or pressureless matter (\(\alpha=0\)) and leading to an intermediate phase of decelerating expansion. For \(\rho\rightarrow \rho_{\Lambda}\), we get \(p=-\rho_{\Lambda} c^2\) leading to a phase of late accelerating expansion (late inflation) with a constant density equal to the cosmological density \(\rho_{\Lambda}=7.02\times 10^{-24}\, {\rm g}/{\rm m}^3\) (dark energy). The pressure is successively negative (vacuum energy), positive (radiation and matter), and negative again (dark energy). We show a nice ``symmetry'' between the early universe (vacuum energy \(+\) \(\alpha\)-fluid) and the late universe (\(\alpha\)-fluid \(+\) dark energy). In our model, they are described by two polytropic equations of state with index \(n=+1\) and \(n=-1\) respectively. Furthermore, the Planck density \(\rho_P\) in the early universe plays a role similar to the cosmological density \(\rho_{\Lambda}\) in the late universe. They represent fundamental upper and lower density bounds differing by \(122\) orders of magnitude. The cosmological constant ``problem'' may be a false problem. We study the evolution of the scale factor, density, and pressure. Interestingly, our quadratic equation of state leads to a fully analytical model describing the evolution of the universe from the early inflation (Planck era) to the late accelerating expansion (de Sitter era). These two phases are bridged by a decelerating algebraic expansion (\(\alpha\)-era). Our model does not present any singularity at \(t=0\) and exists eternally in the past (although it may be incorrect to extrapolate the solution to the infinite past). On the other hand, it admits a scalar field interpretation based on an inflaton, quintessence, or tachyonic field. Our model generalizes the standard \(\Lambda\)CDM model by incorporating naturally a phase of early inflation that avoids the primordial singularity. Furthermore, it describes the early inflation, the intermediate decelerating expansion, and the late accelerating expansion of the universe simultaneously in terms of a single equation of state. We determine the corresponding scalar field potential that unifies the inflaton and quintessence potentials.Universe2015-11-0613Article10.3390/universe10303573574112218-19972015-11-06doi: 10.3390/universe1030357Pierre-Henri Chavanis<![CDATA[Universe, Vol. 1, Pages 307-356: Expanding Space, Quasars and St. Augustine’s Fireworks]]>
http://www.mdpi.com/2218-1997/1/3/307
An attempt is made to explain time non-dilation allegedly observed in quasar light curves. The explanation is based on the assumption that quasar black holes are, in some sense, foreign for our Friedmann-Robertson-Walker universe and do not participate in the Hubble flow. Although at first sight such a weird explanation requires unreasonably fine-tuned Big Bang initial conditions, we find a natural justification for it using the Milne cosmological model as an inspiration.Universe2015-10-0113Article10.3390/universe10303073073562218-19972015-10-01doi: 10.3390/universe1030307Olga ChashchinaZurab Silagadze<![CDATA[Universe, Vol. 1, Pages 292-306: Super Virasoro Algebras From Chiral Supergravity]]>
http://www.mdpi.com/2218-1997/1/2/292
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.Universe2015-09-0912Article10.3390/universe10202922923062218-19972015-09-09doi: 10.3390/universe1020292Yoshifumi Hyakutake<![CDATA[Universe, Vol. 1, Pages 239-291: Interior Dynamics of Neutral and Charged Black Holes in f(R) Gravity]]>
http://www.mdpi.com/2218-1997/1/2/239
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 dΦ/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.Universe2015-09-0212Article10.3390/universe10202392392912218-19972015-09-02doi: 10.3390/universe1020239Jun-Qi GuoPankaj Joshi<![CDATA[Universe, Vol. 1, Pages 199-238: Hybrid Metric-Palatini Gravity]]>
http://www.mdpi.com/2218-1997/1/2/199
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.Universe2015-08-2712Review10.3390/universe10201991992382218-19972015-08-27doi: 10.3390/universe1020199Salvatore CapozzielloTiberiu HarkoTomi KoivistoFrancisco LoboGonzalo Olmo<![CDATA[Universe, Vol. 1, Pages 186-198: Generalized ƒ(R,Φ, X) Gravity and the Late-Time Cosmic Acceleration]]>
http://www.mdpi.com/2218-1997/1/2/186
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.Universe2015-08-1812Article10.3390/universe10201861861982218-19972015-08-18doi: 10.3390/universe1020186Sebastian BahamondeChristian BöhmerFrancisco LoboDiego Sáez-Gómez<![CDATA[Universe, Vol. 1, Pages 173-185: Nonsingular Black Holes in ƒ (R) Theories]]>
http://www.mdpi.com/2218-1997/1/2/173
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.Universe2015-08-0412Article10.3390/universe10201731731852218-19972015-08-04doi: 10.3390/universe1020173Gonzalo OlmoDiego Rubiera-Garcia<![CDATA[Universe, Vol. 1, Pages 158-172: Anti-Evaporation of Black Holes in Bigravity]]>
http://www.mdpi.com/2218-1997/1/2/158
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.Universe2015-08-0312Article10.3390/universe10201581581722218-19972015-08-03doi: 10.3390/universe1020158Taishi Katsuragawa<![CDATA[Universe, Vol. 1, Pages 123-157: Constraining ƒ(R) Gravity by the Large-Scale Structure]]>
http://www.mdpi.com/2218-1997/1/2/123
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.Universe2015-07-3012Review10.3390/universe10201231231572218-19972015-07-30doi: 10.3390/universe1020123Ivan de MartinoMariafelicia De LaurentisSalvatore Capozziello<![CDATA[Universe, Vol. 1, Pages 92-122: Higher Derivative Gravity and Conformal Gravity from Bimetric and Partially Massless Bimetric Theory]]>
http://www.mdpi.com/2218-1997/1/2/92
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.Universe2015-07-2012Article10.3390/universe1020092921222218-19972015-07-20doi: 10.3390/universe1020092Sayed HassanAngnis Schmidt-MayMikael von Strauss<![CDATA[Universe, Vol. 1, Pages 82-91: Arctan-Gravity Model]]>
http://www.mdpi.com/2218-1997/1/1/82
A new gravity model with the function F(R) = (1/β) arctan (βR – β2R2) instead of the Ricci scalar in the Einstein–Hilbert action, describing inflation of the Universe, is suggested and analyzed. We obtain constant curvature solutions of the model in the Jordan frame. Performing the conformal transformation of the metric, the potential and the mass of a scalar degree of freedom in the Einstein frame are found. The slow-roll and cosmological parameters of the model are evaluated. It was demonstrated that the index of the scalar spectrum power law, ns, is in agreement with the PLANCK data.Universe2015-05-2211Article10.3390/universe101008282912218-19972015-05-22doi: 10.3390/universe1010082Sergey Kruglov<![CDATA[Universe, Vol. 1, Pages 38-81: Editorial for the Special Issue 100 Years of Chronogeometrodynamics: The Status of the Einstein’s Theory of Gravitation in Its Centennial Year]]>
http://www.mdpi.com/2218-1997/1/1/38
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.Universe2015-04-2711Article10.3390/universe101003838812218-19972015-04-27doi: 10.3390/universe1010038Lorenzo Iorio<![CDATA[Universe, Vol. 1, Pages 24-37: Inhomogeneous Dark Fluid and Dark Matter, Leading to a Bounce Cosmology]]>
http://www.mdpi.com/2218-1997/1/1/24
The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for dark energy, when dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in terms of the EoS parameters for an exponential, a power-law, or a double-exponential law for the scale factor a. Stability issues are discussed by considering small perturbations around the critical points for the bounce, in the early as well as in the late, universe. The latter part of the paper is concerned with dark energy coupled with dark matter in viscous fluid cosmology. We allow the bulk viscosity ζ = ζ(H, t) to be a function of the Hubble parameter and the time, and consider the Little Rip, the Pseudo Rip, and the bounce universe. Analytic expressions for characteristic properties of these cosmological models are obtained.Universe2015-03-1611Review10.3390/universe101002424372218-19972015-03-16doi: 10.3390/universe1010024Iver BrevikAlexander Timoshkin<![CDATA[Universe, Vol. 1, Pages 17-23: Cosmological Linear Perturbations in the Models of Dark Energy and Modified Gravity]]>
http://www.mdpi.com/2218-1997/1/1/17
The quasi-static solutions of the matter density perturbation in various dark energy models and modified gravity models have been investigated in numerous papers. However, the oscillating solutions in those models have not been investigated enough so far. In this paper, we review the behavior of the oscillating solutions, which have a possibility to unveil the difference between the models of the late-time accelerated expansion of the Universe, by using appropriate approximations.Universe2015-02-1311Article10.3390/universe101001717232218-19972015-02-13doi: 10.3390/universe1010017Jiro Matsumoto<![CDATA[Universe, Vol. 1, Pages 6-16: Observational Constraints on Varying-Alpha Domain Walls]]>
http://www.mdpi.com/2218-1997/1/1/6
We consider the possibility that current hints of spatial variations of the fine structure constant at high redshift, based on VLT/UVES and Keck/HIRES observations, could be caused by a biased domain wall network described by a scalar field non-minimally coupled to the electromagnetic field. We show that in order to be responsible for the reported spatial variations of the fine structure constant, the fractional contribution of the domain wall network to the energy density of the Universe should be tightly constrained within the range \(10^{-10} &lt; \Omega_{w0} &lt; 10^{-5}\). We also show that the domain wall dynamics should be essentially frictionless, so that its characteristic scale is in the order of the Hubble radius at the present time.Universe2015-02-0511Article10.3390/universe10100066162218-19972015-02-05doi: 10.3390/universe1010006Pedro AvelinoLara Sousa<![CDATA[Universe, Vol. 1, Pages 1-5: Universe: An International Multidisciplinary Open Access Journal]]>
http://www.mdpi.com/2218-1997/1/1/1
There is an unescapable connection between mathematics and the Universe [1–4]. It has been so since the dawn of the rationalist view of approaching to the natural world superseding the mythological eras when its various aspects were nothing but manifestations of this or that godhead. This thread never got broken up over the centuries, surviving to vast and profound changes in the nature of mathematics itself and our concept of Universe, and in the relation of mathematics with the natural world as well. At the times of the Pythagoreans, the Universe was essentially a mathematical construct in the sense that the integer numbers were thought as building blocks of the natural world whose constituents could be put in numerical relationships in terms of ratios of integer numbers. The Pythagoreans were serious with mathematics; when they discovered that the square root of 2 could not be expressed as a rational number, they decided not to divulge such a terrifying discovery, and a strict secret was posed on it. Later, at the time of Ptolemy, the Universe was essentially limited to a part of our Solar System. [...]Universe2014-11-2711Editorial10.3390/universe1010001152218-19972014-11-27doi: 10.3390/universe1010001Lorenzo Iorio