Galaxies
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Latest open access articles published in Galaxies at http://www.mdpi.com/journal/galaxies<![CDATA[Galaxies, Vol. 2, Pages 496-519: Bianchi Type I Cosmological Models in Eddington-inspired Born–Infeld Gravity]]>
http://www.mdpi.com/2075-4434/2/4/496
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.Galaxies2014-10-2824Article10.3390/galaxies20404964965192075-44342014-10-28doi: 10.3390/galaxies2040496Tiberiu HarkoFrancisco LoboMan Mak<![CDATA[Galaxies, Vol. 2, Pages 482-495: Orbital Motions and the Conservation-Law/Preferred-Frame α3 Parameter]]>
http://www.mdpi.com/2075-4434/2/4/482
We analytically calculate some orbital effects induced by the Lorentz-invariance momentum-conservation parameterized post-Newtonian (PPN) parameter \(\alpha_3\) in a gravitationally bound binary system made of a primary orbited by a test particle. We neither restrict ourselves to any particular orbital configuration nor to specific orientations of the primary's spin axis \( {\hat{\psi}}\). We use our results to put preliminary upper bounds on \(\alpha_3\) in the weak-field regime by using the latest data from Solar System's planetary dynamics. By linearly combining the supplementary perihelion precessions \(\Delta\dot\varpi\) of the Earth, Mars and Saturn, determined by astronomers with the Ephemerides of Planets and the Moon (EPM) 2011 ephemerides for the general relativistic values of the PPN parameters \(\beta=\gamma=1\), we infer \(|\alpha_3|\lesssim 6\times 10^{-10}\). Our result is about three orders of magnitude better than the previous weak-field constraints existing in the literature and of the same order of magnitude of the constraint expected from the future BepiColombo mission to Mercury. It is, by construction, independent of the other preferred-frame PPN parameters \(\alpha_1,\alpha_2\), both preliminarily constrained down to a \(\approx 10^{-6}\) level. Future analyses should be performed by explicitly including \(\alpha_3\) and a selection of other PPN parameters in the models fitted by the astronomers to the observations and estimating them in dedicated covariance analyses.Galaxies2014-09-2924Article10.3390/galaxies20404824824952075-44342014-09-29doi: 10.3390/galaxies2040482Lorenzo Iorio<![CDATA[Galaxies, Vol. 2, Pages 466-481: Constraints on Non-Standard Gravitomagnetism by the Anomalous Perihelion Precession of the Planets]]>
http://www.mdpi.com/2075-4434/2/4/466
In 2008, a team of astronomers reported an anomalous retrograde precession of the perihelion of Saturn amounting to \(\Delta \dot{\omega}_{\mathrm{SATURN}}=-0.006(2)\) arcsec per century (arcsec cy\(^{-1}\)). This unexplained precession was obtained after taking into account all classical and relativistic effects in the context of the highly refined EPM2008 ephemerides. More recent analyzes have not confirmed this effect, but they have found similar discrepancies in other planets. Our objective in this paper is to discuss a non-standard model involving transversal gravitomagnetism generated by the Sun as a possible source of these potential anomalies, to be confirmed by further data analyses. In order to compute the Lense–Thirring perturbations induced by the suggested interaction, we should consider the orientation of the Sun's rotational axis in Carrington elements and the inclination of the planetary orbits with respect to the ecliptic plane. We find that an extra component of the gravitomagnetic field not predicted by General Relativity could explain the reported anomalies without conflicting with the Gravity Probe B experiment and the orbits of the geodynamics satellites.Galaxies2014-09-2924Article10.3390/galaxies20404664664812075-44342014-09-29doi: 10.3390/galaxies2040466Luis Acedo<![CDATA[Galaxies, Vol. 2, Pages 410-465: Generalized Curvature-Matter Couplings in Modified Gravity]]>
http://www.mdpi.com/2075-4434/2/3/410
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.Galaxies2014-07-2823Review10.3390/galaxies20304104104652075-44342014-07-28doi: 10.3390/galaxies2030410Tiberiu HarkoFrancisco Lobo<![CDATA[Galaxies, Vol. 2, Pages 382-409: Compton Composites Late in the Early Universe]]>
http://www.mdpi.com/2075-4434/2/3/382
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.Galaxies2014-07-1523Concept Paper10.3390/galaxies20303823824092075-44342014-07-15doi: 10.3390/galaxies2030382Frederick MayerJohn Reitz<![CDATA[Galaxies, Vol. 2, Pages 300-381: Monolithic View of Galaxy Formation and Evolution]]>
http://www.mdpi.com/2075-4434/2/3/300
We review and critically discuss the current understanding of galaxy formation and evolution limited to Early Type Galaxies (ETGs) as inferred from the observational data and briefly contrast the hierarchical and quasi-monolithic paradigms of formation and evolution. Since in Cold Dark Matter (CDM) cosmogony small scale structures typically collapse early and form low-mass haloes that subsequently can merge to assembly larger haloes, galaxies formed in the gravitational potential well of a halo are also expected to merge thus assembling their mass hierarchically. Mergers should occur all over the Hubble time and large mass galaxies should be in place only recently. However, recent observations of high redshift galaxies tell a different story: massive ETGs are already in place at high redshift. To this aim, we propose here a revision of the quasi-monolithic scenario as an alternative to the hierarchical one, in which mass assembling should occur in early stages of a galaxy lifetime and present recent models of ETGs made of Dark and Baryonic Matter in a Λ-CDM Universe that obey the latter scheme. The galaxies are followed from the detachment from the linear regime and Hubble flow at z ≥ 20 down to the stage of nearly complete assembly of the stellar content (z ∼ 2 − 1) and beyond. It is found that the total mass (Mh = MDM + MBM ) and/or initial over-density of the proto-galaxy drive the subsequent star formation histories (SFH). Massive galaxies (Mh ~ _1012M⊙) experience a single, intense burst of star formation (with rates ≥ 103M⊙/yr) at early epochs, consistently with observations, with a weak dependence on the initial over-density; intermediate mass haloes (Mh~_ 1010 − 1011M⊙) have star formation histories that strongly depend on their initial over-density; finally, low mass haloes (Mh ~_ 109M⊙) always have erratic, burst-like star forming histories. The present-day properties (morphology, structure, chemistry and photometry) of the model galaxies closely resemble those of the real galaxies. In this context, we also try to cast light on the physical causes of the Stellar Mass-Radius Relation (MRR) of galaxies. The MRR is the result of two complementary mechanisms: i.e., local physical processes that fix the stellar mass and the radius of each galaxy and cosmological global, statistical principles, which shape the distribution of galaxies in the MR-plane. Finally, we also briefly comment on the spectro-photometric properties of the model galaxies and how nicely they match the observational data. The picture emerging from this analysis is that the initial physical conditions of a proto-galaxy, i.e., nature, seem to play the dominant role in building up the ETGs we see today, whereas nurture by recurrent captures of small objects is a secondary actor of the fascinating and intriguing story of galaxy formation and evolution.Galaxies2014-07-1423Review10.3390/galaxies20303003003812075-44342014-07-14doi: 10.3390/galaxies2030300Cesare ChiosiEmiliano MerlinLorenzo PiovanRosaria Tantalo<![CDATA[Galaxies, Vol. 2, Pages 292-299: The Inner Regions of Disk Galaxies: A Constant Baryonic Fraction?]]>
http://www.mdpi.com/2075-4434/2/3/292
For disk galaxies (spirals and irregulars), the inner circular-velocity gradient dRV0 (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 ƒbar,0 is fixed to 1 (no dark matter) and the observed scatter is due to differences in the baryonic mass-to-light ratio Mbar / LR (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 Mbar/LR and Δz. Regardless of the actual value of ƒbar,0, the fact that different types of galaxies do not show strong variations in ƒbar,0 is surprising, and may represent a challenge for models of galaxy formation in a Λ Cold Dark Matter (ΛCDM) cosmology.Galaxies2014-07-1023Article10.3390/galaxies20302922922992075-44342014-07-10doi: 10.3390/galaxies2030292Federico Lelli<![CDATA[Galaxies, Vol. 2, Pages 275-291: Quantum Gravity and Cosmological Density Perturbations]]>
http://www.mdpi.com/2075-4434/2/2/275
We explore the possible cosmological consequences of a running Newton’s constant, G(⎕), as suggested by the non-trivial ultraviolet fixed point scenario for Einstein gravity with a cosmological constant term. Here, we examine what possible effects a scale-dependent coupling might have on large-scale cosmological density perturbations. Starting from a set of manifestly covariant effective field equations, we develop the linear theory of density perturbations for a non-relativistic perfect fluid. The result is a modified equation for the matter density contrast, which can be solved and thus provides an estimate for the corrections to the growth index parameter, ɤ.Galaxies2014-06-1122Article10.3390/galaxies20202752752912075-44342014-06-11doi: 10.3390/galaxies2020275Herbert HamberReiko Toriumi<![CDATA[Galaxies, Vol. 2, Pages 263-274: Satellite Galaxy Velocity Dispersions in the SDSS and Modified Gravity Models]]>
http://www.mdpi.com/2075-4434/2/2/263
The Sloan Digital Sky Survey (SDSS) provides data on several hundred thousand galaxies. The precise location of these galaxies in the sky, along with information about their luminosities and line-of-sight (Doppler) velocities, allows one to construct a three-dimensional map of their location and estimate their line-of-sight velocity dispersion. This information, in principle, allows one to test dynamical gravity models, specifically models of satellite galaxy velocity dispersions near massive hosts. A key difficulty is the separation of true satellites from interlopers. We sidestep this problem by not attempting to derive satellite galaxy velocity dispersions from the data, but instead incorporate an interloper background into the mathematical models and compare the result to the actual data. We find that due to the presence of interlopers, it is not possible to exclude several gravitational theories on the basis of the SDSS data.Galaxies2014-05-0522Article10.3390/galaxies20202632632742075-44342014-05-05doi: 10.3390/galaxies2020263John MoffatViktor Toth<![CDATA[Galaxies, Vol. 2, Pages 259-262: The Lingering Anomalous Secular Increase of the Eccentricity of the Orbit of the Moon: Further Attempts of Explanations of Cosmological Origin]]>
http://www.mdpi.com/2075-4434/2/2/259
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 e ˙ of the eccentricity e of the orbit of the Moon, which is still in place with a magnitude of e ˙ =(5±2)×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 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.Galaxies2014-05-0222Letter10.3390/galaxies20202592592622075-44342014-05-02doi: 10.3390/galaxies2020259Lorenzo Iorio<![CDATA[Galaxies, Vol. 2, Pages 223-258: String Phenomenology: Past, Present and Future Perspectives]]>
http://www.mdpi.com/2075-4434/2/2/223
The observation of a scalar resonance at the Large Hadron Collider (LHC), compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model (SM) parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains viable up to the Planck scale, where gravitational effects are of comparable strength. String theory provides a perturbatively consistent scheme to explore how the parameters of the Standard Model may be determined from a theory of quantum gravity. The free fermionic heterotic string models provide concrete examples of exact string solutions that reproduce the spectrum of the Minimal Supersymmetric Standard Model. Contemporary studies entail the development of methods to classify large classes of models. This led to the discovery of exophobic heterotic-string vacua and the observation of spinor-vector duality, which provides an insight to the global structure of the space of (2,0) heterotic-string vacua. Future directions entail the study of the role of the massive string states in these models and their incorporation in cosmological scenarios. A complementary direction is the formulation of quantum gravity from the principle of manifest phase space duality and the equivalence postulate of quantum mechanics, which suggest that space is compact. The compactness of space, which implies intrinsic regularisation, may be tightly related to the intrinsic finite length scale, implied by string phenomenology.Galaxies2014-04-2222Review10.3390/galaxies20202232232582075-44342014-04-22doi: 10.3390/galaxies2020223Alon Faraggi<![CDATA[Galaxies, Vol. 2, Pages 199-222: Mass Distribution in Rotating Thin-Disk Galaxies According to Newtonian Dynamics]]>
http://www.mdpi.com/2075-4434/2/2/199
An accurate computational method is presented for determining the mass distribution in a mature spiral galaxy from a given rotation curve by applying Newtonian dynamics for an axisymmetrically rotating thin disk of finite size with or without a central spherical bulge. The governing integral equation for mass distribution is transformed via a boundary-element method into a linear algebra matrix equation that can be solved numerically for rotation curves with a wide range of shapes. To illustrate the effectiveness of this computational method, mass distributions in several mature spiral galaxies are determined from their measured rotation curves. All the surface mass density profiles predicted by our model exhibit approximately a common exponential law of decay, quantitatively consistent with the observed surface brightness distributions. When a central spherical bulge is present, the mass distribution in the galaxy is altered in such a way that the periphery mass density is reduced, while more mass appears toward the galactic center. By extending the computational domain beyond the galactic edge, we can determine the rotation velocity outside the cut-off radius, which appears to continuously decrease and to gradually approach the Keplerian rotation velocity out over twice the cut-off radius. An examination of circular orbit stability suggests that galaxies with flat or rising rotation velocities are more stable than those with declining rotation velocities especially in the region near the galactic edge. Our results demonstrate the fact that Newtonian dynamics can be adequate for describing the observed rotation behavior of mature spiral galaxies.Galaxies2014-04-1422Article10.3390/galaxies20201991992222075-44342014-04-14doi: 10.3390/galaxies2020199James FengC. Gallo<![CDATA[Galaxies, Vol. 2, Pages 189-198: Musings on Firewalls and the Information Paradox]]>
http://www.mdpi.com/2075-4434/2/2/189
The past year has seen an explosion of new and old ideas about black hole physics. Prior to the firewall paper, the dominant picture was the thermofield model apparently implied by anti-de Sitter conformal field theory duality. While some seek a narrow responce to Almheiri, Marolf, Polchinski, and Sully (AMPS) , there are a number of competing models. One problem in the field is the ambiguity of the competing proposals. Some are equivalent while others incompatible. This paper will attempt to define and classify a few models representative of the current discussions.Galaxies2014-04-1422Short Communication10.3390/galaxies20201891891982075-44342014-04-14doi: 10.3390/galaxies2020189Michael Devin<![CDATA[Galaxies, Vol. 2, Pages 160-188: ƒ(R) Gravity, Relic Coherent Gravitons and Optical Chaos]]>
http://www.mdpi.com/2075-4434/2/1/160
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.Galaxies2014-03-0421Article10.3390/galaxies20101601601882075-44342014-03-04doi: 10.3390/galaxies2010160Lawrence CrowellChristian Corda<![CDATA[Galaxies, Vol. 2, Pages 89-159: Thermodynamics of Rotating Black Holes and Black Rings: Phase Transitions and Thermodynamic Volume]]>
http://www.mdpi.com/2075-4434/2/1/89
In this review we summarize, expand, and set in context recent developments on the thermodynamics of black holes in extended phase space, where the cosmological constant is interpreted as thermodynamic pressure and treated as a thermodynamic variable in its own right. We specifically consider the thermodynamics of higher-dimensional rotating asymptotically flat and AdS black holes and black rings in a canonical (fixed angular momentum) ensemble. We plot the associated thermodynamic potential—the Gibbs free energy—and study its behavior to uncover possible thermodynamic phase transitions in these black hole spacetimes. We show that the multiply-rotating Kerr-AdS black holes exhibit a rich set of interesting thermodynamic phenomena analogous to the “every day thermodynamics” of simple substances, such as reentrant phase transitions of multicomponent liquids, multiple first-order solid/liquid/gas phase transitions, and liquid/gas phase transitions of the van derWaals type. Furthermore, the reentrant phase transitions also occur for multiply-spinning asymptotically flat Myers–Perry black holes. These phenomena do not require a variable cosmological constant, though they are more naturally understood in the context of the extended phase space. The thermodynamic volume, a quantity conjugate to the thermodynamic pressure, is studied for AdS black rings and demonstrated to satisfy the reverse isoperimetric inequality; this provides a first example of calculation confirming the validity of isoperimetric inequality conjecture for a black hole with non-spherical horizon topology. The equation of state P = P(V,T) is studied for various black holes both numerically and analytically—in the ultraspinning and slow rotation regimes.Galaxies2014-03-0321Article10.3390/galaxies2010089891592075-44342014-03-03doi: 10.3390/galaxies2010089Natacha AltamiranoDavid KubizňákRobert MannZeinab Sherkatghanad<![CDATA[Galaxies, Vol. 2, Pages 81-88: A Toy Cosmology Using a Hubble-Scale Casimir Effect]]>
http://www.mdpi.com/2075-4434/2/1/81
The visible mass of the observable universe agrees with that needed for a flat cosmos, and the reason for this is not known. It is shown that this can be explained by modelling the Hubble volume as a black hole that emits Hawking radiation inwards, disallowing wavelengths that do not fit exactly into the Hubble diameter, since partial waves would allow an inference of what lies outside the horizon. This model of “horizon wave censorship” is equivalent to a Hubble-scale Casimir effect. This incomplete toy model is presented to stimulate discussion. It predicts a minimum mass and acceleration for the observable universe which are in agreement with the observed mass and acceleration, and predicts that the observable universe gains mass as it expands and was hotter in the past. It also predicts a suppression of variation on the largest cosmic scales that agrees with the low-l cosmic microwave background anomaly seen by the Planck satellite.Galaxies2014-02-1921Article10.3390/galaxies201008181882075-44342014-02-19doi: 10.3390/galaxies2010081Michael McCulloch<![CDATA[Galaxies, Vol. 2, Pages 72-80: Metamaterial Model of Tachyonic Dark Energy]]>
http://www.mdpi.com/2075-4434/2/1/72
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.Galaxies2014-02-1721Article10.3390/galaxies201007272802075-44342014-02-17doi: 10.3390/galaxies2010072Igor Smolyaninov<![CDATA[Galaxies, Vol. 2, Pages 62-71: Revisiting Vaidya Horizons]]>
http://www.mdpi.com/2075-4434/2/1/62
In this study, we located and compared different types of horizons in the spherically symmetric Vaidya solution. The horizons we found were trapping horizons, which can be null, timelike, or spacelike, null surfaces with constant area change and also conformal Killing horizons. The conformal Killing horizons only exist for certain choices of the mass function. Under a conformal transformation, the conformal Killing horizons can be mapped into true Killing horizons. This allows conclusions drawn in the dynamical Vaidya spacetime to be related to known properties of static spacetimes. We found the conformal factor that performs this transformation and wrote the new metric in explicitly static coordinates. Using this construction we found that the tunneling argument for Hawking radiation does not umabiguously support Hawking radiation being associated with the trapping horizon. We also used this transformation to derive the form of the surface gravity for a class of physical observers in Vaidya spacetimes.Galaxies2014-02-1021Article10.3390/galaxies201006262712075-44342014-02-10doi: 10.3390/galaxies2010062Alex Nielsen<![CDATA[Galaxies, Vol. 2, Pages 22-61: Large Scale Cosmological Anomalies and Inhomogeneous Dark Energy]]>
http://www.mdpi.com/2075-4434/2/1/22
A wide range of large scale observations hint towards possible modifications on the standard cosmological model which is based on a homogeneous and isotropic universe with a small cosmological constant and matter. These observations, also known as “cosmic anomalies” include unexpected Cosmic Microwave Background perturbations on large angular scales, large dipolar peculiar velocity flows of galaxies (“bulk flows”), the measurement of inhomogenous values of the fine structure constant on cosmological scales (“alpha dipole”) and other effects. The presence of the observational anomalies could either be a large statistical fluctuation in the context of ΛCDM or it could indicate a non-trivial departure from the cosmological principle on Hubble scales. Such a departure is very much constrained by cosmological observations for matter. For dark energy however there are no significant observational constraints for Hubble scale inhomogeneities. In this brief review I discuss some of the theoretical models that can naturally lead to inhomogeneous dark energy, their observational constraints and their potential to explain the large scale cosmic anomalies.Galaxies2014-01-1721Review10.3390/galaxies201002222612075-44342014-01-17doi: 10.3390/galaxies2010022Leandros Perivolaropoulos<![CDATA[Galaxies, Vol. 2, Pages 13-21: Two-Body Orbit Expansion Due to Time-Dependent Relative Acceleration Rate of the Cosmological Scale Factor]]>
http://www.mdpi.com/2075-4434/2/1/13
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 -1 around 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 Pb ≈ 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-3 level from latest Solar System’s planetary observations. The radial velocities of the double lined spectroscopic binary ALPHA Cen AB yield κ1 ≲ 10-8 year-3.Galaxies2014-01-0321Article10.3390/galaxies201001313212075-44342014-01-03doi: 10.3390/galaxies2010013Lorenzo Iorio<![CDATA[Galaxies, Vol. 2, Pages 1-12: Galaxy Rotation Curves in Covariant Hořava-Lifshitz Gravity]]>
http://www.mdpi.com/2075-4434/2/1/1
Using the multiplicity of solutions for the projectable case of the covariant extension of Hořava-Lifshitz gravity, we show that an appropriate choice for the auxiliary field allows for an effective description of galaxy rotation curves. This description is based on static and spherically symmetric solutions of covariant Hořava-Lifshitz gravity and does not require Dark Matter.Galaxies2013-12-2321Article10.3390/galaxies20100011122075-44342013-12-23doi: 10.3390/galaxies2010001Jean AlexandreMartyna Kostacinska<![CDATA[Galaxies, Vol. 1, Pages 261-274: A No-Go Theorem for Rotating Stars of a Perfect Fluid without Radial Motion in Projectable Hořava–Lifshitz Gravity]]>
http://www.mdpi.com/2075-4434/1/3/261
Hořava–Lifshitz gravity has covariance only under the foliation-preserving diffeomorphism. This implies that the quantities on the constant-time hypersurfaces should be regular. In the original theory, the projectability condition, which strongly restricts the lapse function, is proposed. We assume that a star is filled with a perfect fluid with no-radial motion and that it has reflection symmetry about the equatorial plane. As a result, we find a no-go theorem for stationary and axisymmetric star solutions in projectable Hořava–Lifshitz gravity under the physically reasonable assumptions in the matter sector. Since we do not use the gravitational action to prove it, our result also works out in other projectable theories and applies to not only strong gravitational fields, but also weak gravitational ones.Galaxies2013-12-1613Article10.3390/galaxies10302612612742075-44342013-12-16doi: 10.3390/galaxies1030261Naoki TsukamotoTomohiro Harada<![CDATA[Galaxies, Vol. 1, Pages 216-260: Cosmographic Constraints and Cosmic Fluids]]>
http://www.mdpi.com/2075-4434/1/3/216
The problem of reproducing dark energy effects is reviewed here with particular interest devoted to cosmography. We summarize some of the most relevant cosmological models, based on the assumption that the corresponding barotropic equations of state evolve as the universe expands, giving rise to the accelerated expansion. We describe in detail the ΛCDM (Λ-Cold Dark Matter) and ωCDM models, considering also some specific examples, e.g., Chevallier–Polarsky–Linder, the Chaplygin gas and the Dvali–Gabadadze–Porrati cosmological model. Finally, we consider the cosmological consequences of f(R) and f(T) gravities and their impact on the framework of cosmography. Keeping these considerations in mind, we point out the model-independent procedure related to cosmography, showing how to match the series of cosmological observables to the free parameters of each model. We critically discuss the role played by cosmography, as a selection criterion to check whether a particular model passes or does not present cosmological constraints. In so doing, we find out cosmological bounds by fitting the luminosity distance expansion of the redshift, z, adopting the recent Union 2.1 dataset of supernovae, combined with the baryonic acoustic oscillation and the cosmic microwave background measurements. We perform cosmographic analyses, imposing different priors on the Hubble rate present value. In addition, we compare our results with recent PLANCK limits, showing that the ΛCDM and ωCDM models seem to be the favorite with respect to other dark energy models. However, we show that cosmographic constraints on f(R) and f(T) cannot discriminate between extensions of General Relativity and dark energy models, leading to a disadvantageous degeneracy problem.Galaxies2013-12-0413Article10.3390/galaxies10302162162602075-44342013-12-04doi: 10.3390/galaxies1030216Salvatore CapozzielloMariafelicia De LaurentisOrlando LuongoAlan Ruggeri<![CDATA[Galaxies, Vol. 1, Pages 210-215: Color Differences between Clockwise and Counterclockwise Spiral Galaxies]]>
http://www.mdpi.com/2075-4434/1/3/210
While spiral galaxies observed from Earth clearly seem to spin in different directions, little is yet known about other differences between galaxies that spin clockwise and galaxies that spin counterclockwise. Here we compared the color of 64,399 spiral galaxies that spin clockwise to 63,215 spiral galaxies that spin counterclockwise. The results show that clockwise galaxies tend to be bluer than galaxies that spin counterclockwise. The probability that the color differences can be attributed to chance is ~0.019. g-r, r-i and i-z did not show significant differences between clockwise and counterclockwise galaxies.Galaxies2013-10-2513Letter10.3390/galaxies10302102102152075-44342013-10-25doi: 10.3390/galaxies1030210Lior Shamir<![CDATA[Galaxies, Vol. 1, Pages 192-209: A Closer Earth and the Faint Young Sun Paradox: Modification of the Laws of Gravitation or Sun/Earth Mass Losses?]]>
http://www.mdpi.com/2075-4434/1/3/192
Given a solar luminosity LAr = 0.75L0 at the beginning of the Archean 3.8 Ga ago, where L0 is the present-day one, if the heliocentric distance, r, of the Earth was rAr = 0.956r0, the solar irradiance would have been as large as IAr = 0.82I0. It would have allowed for a liquid ocean on the terrestrial surface, which, otherwise, would have been frozen, contrary to the empirical evidence. By further assuming that some physical mechanism subsequently displaced the Earth towards its current distance in such a way that the irradiance stayed substantially constant over the entire Archean from 3.8 to 2.5 Ga ago, a relative recession per year as large as r˙/r ≈3.4 × 10−11 a−1 would have been required. Although such a figure is roughly of the same order of magnitude of the value of the Hubble parameter 3.8 Ga ago HAr = 1.192H0 = 8.2 × 10−11 a−1, standard general relativity rules out cosmological explanations for the hypothesized Earth’s recession rate. Instead, a class of modified theories of gravitation with nonminimal coupling between the matter and the metric naturally predicts a secular variation of the relative distance of a localized two-body system, thus yielding a potentially viable candidate to explain the putative recession of the Earth’s orbit. Another competing mechanism of classical origin that could, in principle, allow for the desired effect is the mass loss, which either the Sun or the Earth itself may have experienced during the Archean. On the one hand, this implies that our planet should have lost 2% of its present mass in the form of eroded/evaporated hydrosphere. On the other hand, it is widely believed that the Sun could have lost mass at an enhanced rate, due to a stronger solar wind in the past for not more than ≈ 0.2–0.3 Ga.Galaxies2013-10-1813Article10.3390/galaxies10301921922092075-44342013-10-18doi: 10.3390/galaxies1030192Lorenzo Iorio<![CDATA[Galaxies, Vol. 1, Pages 180-191: Explaining Holographic Dark Energy]]>
http://www.mdpi.com/2075-4434/1/3/180
The possible holographic origin of dark energy is investigated. The main existing explanations, namely the UV/IR connection argument of Cohen et al., Thomas’ bulk holography argument, and Ng’s spacetime foam argument, are shown to be not wholly satisfactory. A new explanation is then proposed based on the ideas of Thomas and Ng. It is suggested that dark energy originates from the quantum fluctuations of spacetime limited by the event horizon of the universe. Several potential problems of the explanation are also discussed.Galaxies2013-10-0313Essay10.3390/galaxies10301801801912075-44342013-10-03doi: 10.3390/galaxies1030180Shan Gao<![CDATA[Galaxies, Vol. 1, Pages 114-179: Evolving Black Hole Horizons in General Relativity and Alternative Gravity]]>
http://www.mdpi.com/2075-4434/1/3/114
From the microscopic point of view, realistic black holes are time-dependent and the teleological concept of the event horizon fails. At present, the apparent or trapping horizon seem to be its best replacements in various areas of black hole physics. We discuss the known phenomenology of apparent and trapping horizons for analytical solutions of General Relativity and alternative theories of gravity. These specific examples (we focus on spherically symmetric inhomogeneities in a background cosmological spacetime) are useful as toy models for research on various aspects of black hole physics.Galaxies2013-09-2513Review10.3390/galaxies10301141141792075-44342013-09-25doi: 10.3390/galaxies1030114Valerio Faraoni<![CDATA[Galaxies, Vol. 1, Pages 107-113: Rip Cosmology via Inhomogeneous Fluid]]>
http://www.mdpi.com/2075-4434/1/2/107
The conditions for the appearance of the Little Rip, Pseudo Rip and Quasi Rip universes in the terms of the parameters in the equation of state of some dark fluid are investigated. Several examples of the Rip cosmologies are investigated.Galaxies2013-08-1512Review10.3390/galaxies10201071071132075-44342013-08-15doi: 10.3390/galaxies1020107Valerii ObukhovAlexander TimoshkinEvgenii Savushkin<![CDATA[Galaxies, Vol. 1, Pages 96-106: Conformally Coupled Inflation]]>
http://www.mdpi.com/2075-4434/1/2/96
A massive scalar field in a curved spacetime can propagate along the light cone, a causal pathology, which can, in principle, be eliminated only if the scalar couples conformally to the Ricci curvature of spacetime. This property mandates conformal coupling for the field driving inflation in the early universe. During slow-roll inflation, this coupling can cause super-acceleration and, as a signature, a blue spectrum of primordial gravitational waves.Galaxies2013-07-3012Article10.3390/galaxies1020096961062075-44342013-07-30doi: 10.3390/galaxies1020096Valerio Faraoni<![CDATA[Galaxies, Vol. 1, Pages 83-95: Inhomogeneous Viscous Fluids in a Friedmann-Robertson-Walker (FRW) Universe]]>
http://www.mdpi.com/2075-4434/1/2/83
We give a brief review of some aspects of inhomogeneous viscous fluids in a flat Friedmann-Robertson-Walker Universe. In general, it is pointed out that several fluid models may bring the future Universe evolution to become singular, with the appearance of the so-called Big Rip scenario. We investigate the effects of fluids coupled with dark matter in a de Sitter Universe, by considering several cases. Due to this coupling, the coincidence problem may be solved, and if the de Sitter solution is stable, the model is also protected against the Big Rip singularity.Galaxies2013-07-0912Article10.3390/galaxies102008383952075-44342013-07-09doi: 10.3390/galaxies1020083Ratbay MyrzakulovLorenzo SebastianiSergio Zerbini<![CDATA[Galaxies, Vol. 1, Pages 65-82: Cosmological Observations in a Modified Theory of Gravity (MOG)]]>
http://www.mdpi.com/2075-4434/1/1/65
Our Modified Gravity Theory (MOG) is a gravitational theory without exotic dark matter, based on an action principle. MOG has been used successfully tomodel astrophysical phenomena, such as galaxy rotation curves, galaxy cluster masses and lensing. MOG may also be able to account for cosmological observations. We assume that the MOG point source solution can be used to describe extended distributions of matter via an appropriately modified Poisson equation. We use this result to model perturbation growth in MOG and find that it agrees well with the observed matter power spectrum at present. As the resolution of the power spectrum improves with increasing survey size, however, significant differences emerge between the predictions of MOG and the standard Λ-cold dark matter (Λ-CDM) model, as in the absence of exotic darkmatter, oscillations of the power spectrum in MOG are not suppressed. We can also use MOG to model the acoustic power spectrum of the cosmic microwave background. A suitably adapted semi-analytical model offers a first indication that MOG may pass this test and correctly model the peak of the acoustic spectrum.Galaxies2013-06-2011Article10.3390/galaxies101006565822075-44342013-06-20doi: 10.3390/galaxies1010065John. MoffatViktor Toth<![CDATA[Galaxies, Vol. 1, Pages 44-64: Attitudes towards Autonomous Data Collection and Analysis in the Planetary Science Community]]>
http://www.mdpi.com/2075-4434/1/1/44
As missions are planned to targets further away from Earth, it becomes all but required to increase the role of autonomy in the mission. An investigation of what aspects of mission operations and decision making autonomy will be accepted in by the planetary science community is thus required to aid in development planning. This paper presents a data set collected regarding attitudes towards autonomous data collection and analysis in the planetary science community and initial analysis of this data. A survey, conducted at the 2013 Lunar and Planetary Science Conference, asked respondents to identify whether or not they would accept conclusions drawn by autonomous data collection techniques and what factors would impact this acceptance. It also looked at the acceptance of computers and computer software in the data collection and analysis process.Galaxies2013-06-1811Article10.3390/galaxies101004444642075-44342013-06-18doi: 10.3390/galaxies1010044Jeremy Straub<![CDATA[Galaxies, Vol. 1, Pages 31-43: Halo Models of Large Scale Structure and Reliability of Cosmological N-Body Simulations]]>
http://www.mdpi.com/2075-4434/1/1/31
Halo models of the large scale structure of the Universe are critically examined, focusing on the definition of halos as smooth distributions of cold dark matter. This definition is essentially based on the results of cosmological N-body simulations. By a careful analysis of the standard assumptions of halo models and N-body simulations and by taking into account previous studies of self-similarity of the cosmic web structure, we conclude that N-body cosmological simulations are not fully reliable in the range of scales where halos appear. Therefore, to have a consistent definition of halos is necessary either to define them as entities of arbitrary size with a grainy rather than smooth structure or to define their size in terms of small-scale baryonic physics.Galaxies2013-05-2911Article10.3390/galaxies101003131432075-44342013-05-29doi: 10.3390/galaxies1010031José Gaite<![CDATA[Galaxies, Vol. 1, Pages 6-30: Exact Expressions for the Pericenter Precession Caused by Some Dark Matter Distributions and Constraints on Them from Orbital Motions in the Solar System, in the Double Pulsar and in the Galactic Center]]>
http://www.mdpi.com/2075-4434/1/1/6
We analytically calculate the secular precession of the pericenter of a test particle orbiting a central body surrounded by a continuous distribution of Dark Matter (DM) by using some commonly adopted spherically symmetric density profiles for it. We obtain exact expressions without resorting to a-priori simplifying assumptions on the orbital geometry of the test particle. Our formulas allow us to put constraints on the parameters of the DM distributions considered in several local astronomical and astrophysical scenarios, such as the Sun's planetary system, the double pulsar, and the stellar system around the supermassive black hole in Sgr A∗, all characterized by a wide variety of orbital configuratio ns. As far as our Solar System is concerned, latest determinations of the supplementary perihelion precessions ̟˙ with the EPM2011 ephemerides and the common power-law DM density profile ρDM(r) = ρ0r−γ λγ yield 5 × 103 GeV cm−3 (γ = 0) ≤ ρ0 ≤ 8 × 103 GeV cm−3 (γ = 4), corresponding to 8.9 × 10−21 g cm−3 ≤ ρ0 ≤ 1.4 × 10−20 g cm−3, at the Saturn's distance. From the periastron of the pulsar PSR J0737-3039A and the same power-low DM density, one has 1.7 × 1016 GeV cm−3 (γ = 0) ≤ ρ0 ≤ 2 × 1016 (γ = 4) GeV cm−3, corresponding to 3.0 × 10−8 g cm−3 ≤ ρ0 ≤ 3.6 × 10−8 g cm−3. The perinigricon of the S0-2 star in Sgr A∗ and the power-law DM model give 1.2 × 1013 GeV cm−3 (γ = 0) ≤ ρ0 ≤ 1 × 1016 (γ = 4, λ = rmin) GeV cm−3, corresponding to 2.1 × 10−11 g cm−3 ≤ ρ0 ≤ 1.8 × 10−8 g cm−3.Galaxies2013-05-2811Article10.3390/galaxies10100066302075-44342013-05-28doi: 10.3390/galaxies1010006Lorenzo Iorio<![CDATA[Galaxies, Vol. 1, Pages 1-5: Galaxies: An International Multidisciplinary Open Access Journal]]>
http://www.mdpi.com/2075-4434/1/1/1
The knowledge of the universe as a whole, its origin, size and shape, its evolution and future, has always intrigued the human mind. Galileo wrote: “Nature’s great book is written in mathematical language.” This new journal will be devoted to both aspects of knowledge: the direct investigation of our universe and its deeper understanding, from fundamental laws of nature which are translated into mathematical equations, as Galileo and Newton—to name just two representatives of a plethora of past and present researchers—already showed us how to do. Those physical laws, when brought to their most extreme consequences—to their limits in their respective domains of applicability—are even able to give us a plausible idea of how the origin of our universe came about and also of how we can expect its future to evolve and, eventually, how its end will take place. These laws also condense the important interplay between mathematics and physics as just one first example of the interdisciplinarity that will be promoted in the Galaxies Journal.Galaxies2012-11-0811Editorial10.3390/galaxies1010001152075-44342012-11-08doi: 10.3390/galaxies1010001Emilio Elizalde