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A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (31 October 2014) | Viewed by 66245

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Prof. Dr. Antonaldo Diaferio

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Department of Physics, University of Torino, 10125 Torino, Italy
Interests: cosmology; theory of gravitation; large-scale structure of the universe
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The literature on extended and modified theories of gravity has grown enormously over the last decade: although the currently standard cosmological model accurately describes the power spectrum of the cosmic microwave background and the properties of cosmic structures at the scales of galaxies and beyond, our understanding of the nature of dark matter and dark energy is still unsatisfactory. In addition, some tensions on the galaxy scales and on the largest scales probed by the Planck mission are not yet easily accommodated into the general framework.

The current and upcoming wealth of data from ground and from space on the growth of cosmic structures, where the degeneracy among different cosmological models may be lifted, urges a comprehensive and deep understanding of the most promising modified theories of gravity and their accompanying cosmological models.

This special issue of Galaxies is intended to highlight the strengths and weaknesses of these models when compared to current and future data. We would like to call for papers on models beyond standard gravity and cosmology that explicitly address quantitative comparisons with observations. Review papers are also most welcome.

Prof. Dr. Antonaldo Diaferio
Guest Editor

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Keywords

gravitation
modified theories of gravity
cosmology
dark matter and dark energy
formation and evolution of galaxies and cosmic structures

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Published Papers (12 papers)

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Research

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2096 KiB

Open AccessArticle

The 5D Fully-Covariant Theory of Gravitation and Its Astrophysical Applications

by Tianxi Zhang

Galaxies 2015, 3(1), 18-51; https://doi.org/10.3390/galaxies3010018 - 26 Dec 2014

Cited by 2 | Viewed by 5398

Abstract

In this paper, we comprehensively review the five-dimensional (5D) fully-covariant theory of gravitation developed by Zhang two decades ago and its recent applications in astrophysics and cosmology. This 5D gravity describes not only the fields, but also the matter and its motion in a 5D spacetime. The greatest advantage of this theory is that there does not exist any unknown parameter, so that we can apply it to explain astrophysical and cosmological issues by quantitatively comparing the results obtained from it with observations and to predict new effects that could not be derived from any other gravitational theories. First, the 5D covariant description of matter and its motion enabled Zhang to analytically derive the fifteenth component of the 5D energy-momentum tensor of matter (

{\bar{T}}^{44}

), which significantly distinguishes this 5D gravity from other 5D gravitational theories that usually assumed a

{\bar{T}}^{44}

with an unknown parameter, called the scalar charge s, and, thus, to split the 5D covariant field equation into (4 + 1) splitting form as the gravitational, electromagnetic, and scalar field equations. The gravitational field equation turns into the 4D Einstein’s field equation of general relativity if the scalar field is equal to unity. Then, Zhang solved the field equations and obtained an exact static spherically-symmetric external solution of the gravitational, electromagnetic and scalar fields, in which all integral constants were completely determined with a perfect set of simple numbers and parameters that only depend on the mass and electric charge of the matter, by comparing with the obtained weak internal solution of the fields at a large radial distance. In the Einstein frame, the exact field solution obtained from the 5D fully-covariant theory of gravitation reduces to the Schwarzschild solution when the matter is electrically neutral and the fields are weak in strength. This guarantees that the four fundamental tests (light deflection, gravitational redshift, perihelion advance and radar echo delay) of the 4D Einstein’s general relativity in the case of weak fields are also the tests of the 5D fully-covariant theory of gravitation. In the case of strong fields, especially when the matter is highly charged, however, the results from the 5D fully-covariant theory of gravitation are significantly different from the 4D Einstein’s general relativity. Applying this 5D gravity and its exact field solution, Zhang has recently developed a new redshift mechanism, called electric redshift, a new supernova explosion mechanism with gravitational field shielding, a new gravitationless black hole model, a modified neutron star mass-radius relation, a modified Friedmann equation for the accelerating universe, and so on. This paper provides an overview of this 5D fully-covariant theory of gravitation, including also its solution properties and astrophysical applications. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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896 KiB

Open AccessArticle

Astrophysical Tests of Kinematical Conformal Cosmology in Fourth-Order Conformal Weyl Gravity

by Gabriele U. Varieschi

Galaxies 2014, 2(4), 577-600; https://doi.org/10.3390/galaxies2040577 - 12 Dec 2014

Cited by 7 | Viewed by 4671

Abstract

In this work we analyze kinematical conformal cosmology (KCC), an alternative cosmological model based on conformal Weyl gravity (CG), and test it against current type Ia supernova (SNIa) luminosity data and other astrophysical observations. Expanding upon previous work on the subject, we revise the analysis of SNIa data, confirming that KCC can explain the evidence for an accelerating expansion of the Universe without using dark energy or other exotic components. We obtain an independent evaluation of the Hubble constant, H₀= 67:53 kms^-1 Mpc^-1, very close to the current best estimates. The main KCC and CG parameters are re-evaluated and their revised values are found to be close to previous estimates. We also show that available data for the Hubble parameter as a function of redshift can be fitted using KCC and that this model does not suffer from any apparent age problem. Overall, Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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1102 KiB

Open AccessArticle

Bianchi Type I Cosmological Models in Eddington-inspired Born–Infeld Gravity

by Tiberiu Harko, Francisco S.N. Lobo and Man Kwong Mak

Galaxies 2014, 2(4), 496-519; https://doi.org/10.3390/galaxies2040496 - 28 Oct 2014

Cited by 26 | Viewed by 5286

Abstract

We consider the dynamics of a barotropic cosmological fluid in an anisotropic, Bianchi type I space-time in Eddington-inspired Born–Infeld (EiBI) gravity. By assuming isotropic pressure distribution, we obtain the general solution of the field equations in an exact parametric form. The behavior of the geometric and thermodynamic parameters of the Bianchi type I Universe is studied, by using both analytical and numerical methods, for some classes of high density matter, described by the stiff causal, radiation, and pressureless fluid equations of state. In all cases the study of the models with different equations of state can be reduced to the integration of a highly nonlinear second order ordinary differential equation for the energy density. The time evolution of the anisotropic Bianchi type I Universe strongly depends on the initial values of the energy density and of the Hubble function. An important observational parameter, the mean anisotropy parameter, is also studied in detail, and we show that for the dust filled Universe the cosmological evolution always ends into isotropic phase, while for high density matter filled universes the isotropization of Bianchi type I universes is essentially determined by the initial conditions of the energy density. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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224 KiB

Open AccessArticle

The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction?

by Federico Lelli

Galaxies 2014, 2(3), 292-299; https://doi.org/10.3390/galaxies2030292 - 10 Jul 2014

Cited by 11 | Viewed by 4774

Abstract

For disk galaxies (spirals and irregulars), the inner circular-velocity gradient d_RV₀ (inner steepness of the rotation curve) correlates with the central surface brightness ∑_*,0 with a slope of ~0.5. This implies that the central dynamical mass density scales almost linearly with the central baryonic density. Here I show that this empirical relation is consistent with a simple model where the central baryonic fraction ƒ_bar,0 is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio M_bar / L_R (ranging from 1 to 3 in the R-band) and in the characteristic thickness of the central stellar component Δz (ranging from 100 to 500 pc). Models with lower baryonic fractions are possible, although they require some fine-tuning in the values of M_bar/L_Rand Δz. Regardless of the actual value of ƒ_bar,0, the fact that different types of galaxies do not show strong variations in ƒ_bar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter (ΛCDM) cosmology. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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333 KiB

Open AccessArticle

ƒ(R) Gravity, Relic Coherent Gravitons and Optical Chaos

by Lawrence B. Crowell and Christian Corda

Galaxies 2014, 2(1), 160-188; https://doi.org/10.3390/galaxies2010160 - 4 Mar 2014

Cited by 2 | Viewed by 4705

Abstract

We discuss the production of massive relic coherent gravitons in a particular class of ƒ(R) gravity, which arises from string theory, and their possible imprint in the Cosmic Microwave Background. In fact, in the very early Universe, these relic gravitons could have acted as slow gravity waves. They may have then acted to focus the geodesics of radiation and matter. Therefore, their imprint on the later evolution of the Universe could appear as filaments and a domain wall in the Universe today. In that case, the effect on the Cosmic Microwave Background should be analogous to the effect of water waves, which, in focusing light, create optical caustics, which are commonly seen on the bottom of swimming pools. We analyze this important issue by showing how relic massive gravity waves (GWs) perturb the trajectories of the Cosmic Microwave Background photons (gravitational lensing by relic GWs). The consequence of the type of physics discussed is outlined by illustrating an amplification of what might be called optical chaos. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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706 KiB

Open AccessArticle

Metamaterial Model of Tachyonic Dark Energy

by Igor I. Smolyaninov

Galaxies 2014, 2(1), 72-80; https://doi.org/10.3390/galaxies2010072 - 17 Feb 2014

Cited by 6 | Viewed by 6245

Abstract

Dark energy with negative pressure and positive energy density is believed to be responsible for the accelerated expansion of the universe. Quite a few theoretical models of dark energy are based on tachyonic fields interacting with itself and normal (bradyonic) matter. Here, we propose an experimental model of tachyonic dark energy based on hyperbolic metamaterials. Wave equation describing propagation of extraordinary light inside hyperbolic metamaterials exhibits 2 + 1 dimensional Lorentz symmetry. The role of time in the corresponding effective 3D Minkowski spacetime is played by the spatial coordinate aligned with the optical axis of the metamaterial. Nonlinear optical Kerr effect bends this spacetime resulting in effective gravitational force between extraordinary photons. We demonstrate that this model has a self-interacting tachyonic sector having negative effective pressure and positive effective energy density. Moreover, a composite multilayer SiC-Si hyperbolic metamaterial exhibits closely separated tachyonic and bradyonic sectors in the long wavelength infrared range. This system may be used as a laboratory model of inflation and late time acceleration of the universe. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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86 KiB

Open AccessArticle

Two-Body Orbit Expansion Due to Time-Dependent Relative Acceleration Rate of the Cosmological Scale Factor

by Lorenzo Iorio

Galaxies 2014, 2(1), 13-21; https://doi.org/10.3390/galaxies2010013 - 3 Jan 2014

Cited by 7 | Viewed by 4692

Abstract

By phenomenologically assuming a slow temporal variation of the percent acceleration rate S̈S ^-1 of the cosmic scale factor S(t), it is shown that the orbit of a local binary undergoes a secular expansion. To first order in the power expansion of S̈S ^-1around the present epoch t₀, a non-vanishing shift per orbit (Δr) of the two-body relative distance r occurs for eccentric trajectories. A general relativistic expression, which turns out to be cubic in the Hubble parameter H₀ at the present epoch, is explicitly calculated for it in the case of matter-dominated epochs with Dark Energy. For a highly eccentric Oort comet orbit with period P_b ≈ 31 Myr, the general relativistic distance shift per orbit turns out to be of the order of (Δr) ≈ 70 km. For the Large Magellanic Cloud, assumed on a bound elliptic orbit around the Milky Way, the shift per orbit is of the order of (Δr) ≈ 2–4 pc. Our result has a general validity since it holds in any cosmological model admitting the Hubble law and a slowly varying S̈S^-1(t). More generally, it is valid for an arbitrary Hooke-like extra-acceleration whose “elastic” parameter κ is slowly time-dependent, irrespectively of the physical mechanism which may lead to it. The coefficient κ₁ of the first-order term of the power expansion of κ(t) can be preliminarily constrained in a model-independent way down to a κ₁ ≲ 2 x 10^-13 year^-3level from latest Solar System’s planetary observations. The radial velocities of the double lined spectroscopic binary ALPHA Cen AB yield κ1 ≲ 10^-8year^-3. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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899 KiB

Open AccessReview

The Third Law of Galactic Rotation

by Stacy S. McGaugh

Galaxies 2014, 2(4), 601-622; https://doi.org/10.3390/galaxies2040601 - 17 Dec 2014

Cited by 45 | Viewed by 7424

Abstract

I review the connection between dynamics and the baryonic mass distribution in rotationally-supported galaxies. The enclosed dynamical mass-to-light ratio increases with decreasing galaxy luminosity and surface brightness. The correlation with surface brightness appears to be the more fundamental, with the dependence on luminosity following simply from the weaker correlation between luminosity and surface brightness. In addition to this global relation, there is also a local relation between the amplitude of the mass discrepancy and the acceleration predicted by the observed distribution of baryons. I provide an empirical calibration of this mass discrepancy-acceleration relation. The data are consistent with the operation of a singe effective force law in disk galaxies, making this relation tantamount to a natural law. I further provide formulae by which the radial dark matter distribution can be estimated from surface photometry. The form of the dark matter halo depends uniquely on the distribution of baryons in each galaxy and, in general, is neither a cusp nor a core. It remains difficult to see how galaxy formation models can reproduce the observed behavior, which is uniquely predicted by MOND. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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1919 KiB

Open AccessReview

Self-Gravitating Systems in Extended Gravity

by Arturo Stabile and Salvatore Capozziello

Galaxies 2014, 2(4), 520-576; https://doi.org/10.3390/galaxies2040520 - 4 Dec 2014

Cited by 6 | Viewed by 5099

Abstract

Starting from the weak field limit, we discuss astrophysical applications of Extended Theories of Gravity where higher order curvature invariants and scalar fields are considered by generalizing the Hilbert-Einstein action linear in the Ricci curvature scalar R. Results are compared to General Relativity in the hypothesis that Dark Matter contributions to the dynamics can be neglected thanks to modified gravity. In particular, we consider stellar hydrostatic equilibrium, galactic rotation curves, and gravitational lensing. Finally, we discuss the weak field limit in the Jordan and Einstein frames pointing out how effective quantities, as gravitational potentials, transform from one frame to the other and the interpretation of results can completely change accordingly. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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450 KiB

Open AccessReview

Generalized Curvature-Matter Couplings in Modified Gravity

by Tiberiu Harko and Francisco S.N. Lobo

Galaxies 2014, 2(3), 410-465; https://doi.org/10.3390/galaxies2030410 - 28 Jul 2014

Cited by 203 | Viewed by 6816

Abstract

In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature R and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and, consequently, leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra terms in the gravitational field equations modify the equations of motion of test particles and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions and for explaining the late-time cosmic acceleration. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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1241 KiB

Open AccessConcept Paper

Compton Composites Late in the Early Universe

by Frederick Mayer and John Reitz

Galaxies 2014, 2(3), 382-409; https://doi.org/10.3390/galaxies2030382 - 15 Jul 2014

Cited by 4 | Viewed by 5530

Abstract

Beginning roughly two hundred years after the big-bang, a tresino phase transition generated Compton-scale composite particles and converted most of the ordinary plasma baryons into new forms of dark matter. Our model consists of ordinary electrons and protons that have been bound into mostly undetectable forms. This picture provides an explanation of the composition and history of ordinary to dark matter conversion starting with, and maintaining, a critical density Universe. The tresino phase transition started the conversion of ordinary matter plasma into tresino-proton pairs prior to the the recombination era. We derive the appropriate Saha–Boltzmann equilibrium to determine the plasma composition throughout the phase transition and later. The baryon population is shown to be quickly modified from ordinary matter plasma prior to the transition to a small amount of ordinary matter and a much larger amount of dark matter after the transition. We describe the tresino phase transition and the origin, quantity and evolution of the dark matter as it takes place from late in the early Universe until the present. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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75 KiB

Open AccessLetter

The Lingering Anomalous Secular Increase of the Eccentricity of the Orbit of the Moon: Further Attempts of Explanations of Cosmological Origin

by Lorenzo Iorio

Galaxies 2014, 2(2), 259-262; https://doi.org/10.3390/galaxies2020259 - 2 May 2014

Cited by 11 | Viewed by 4823

Abstract

New analyses of extended data records collected with the Lunar Laser Ranging (LLR) technique performed with improved tidal models were not able to resolve the issue of the anomalous rate

\dot{e}

of the eccentricity e of the orbit of the Moon, which [...] Read more.

New analyses of extended data records collected with the Lunar Laser Ranging (LLR) technique performed with improved tidal models were not able to resolve the issue of the anomalous rate

\dot{e}

of the eccentricity e of the orbit of the Moon, which is still in place with a magnitude of

\dot{e} = (5 \pm 2) \times 10^{- 12} y r^{- 1}

. Some possible cosmological explanations are offered in terms of the post-Newtonian effects of the cosmological expansion and of the slow temporal variation of the relative acceleration rate

\ddot{S} S^{^{- 1}}

of the cosmic scale factor S. None of them is successful since their predicted secular rates of the lunar eccentricity are too small by several orders of magnitude. Full article

(This article belongs to the Special Issue Beyond Standard Gravity and Cosmology)

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