Next Issue
Volume 2, March
Previous Issue
Volume 1, September
 
 

Galaxies, Volume 1, Issue 3 (December 2013) – 6 articles , Pages 114-274

  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Reader to open them.
Order results
Result details
Select all
Export citation of selected articles as:
123 KiB  
Article
A No-Go Theorem for Rotating Stars of a Perfect Fluid without Radial Motion in Projectable Hořava–Lifshitz Gravity
by Naoki Tsukamoto and Tomohiro Harada
Galaxies 2013, 1(3), 261-274; https://doi.org/10.3390/galaxies1030261 - 16 Dec 2013
Cited by 3 | Viewed by 3817
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Aspects of Black Hole Physics)
595 KiB  
Article
Cosmographic Constraints and Cosmic Fluids
by Salvatore Capozziello, Mariafelicia De Laurentis, Orlando Luongo and Alan Cosimo Ruggeri
Galaxies 2013, 1(3), 216-260; https://doi.org/10.3390/galaxies1030216 - 04 Dec 2013
Cited by 94 | Viewed by 6174
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Cosmology with Fluid Components)
Show Figures

Figure 1

564 KiB  
Letter
Color Differences between Clockwise and Counterclockwise Spiral Galaxies
by Lior Shamir
Galaxies 2013, 1(3), 210-215; https://doi.org/10.3390/galaxies1030210 - 25 Oct 2013
Cited by 15 | Viewed by 6273
Abstract
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 [...] Read more.
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. Full article
Show Figures

Figure 1

285 KiB  
Article
A Closer Earth and the Faint Young Sun Paradox: Modification of the Laws of Gravitation or Sun/Earth Mass Losses?
by Lorenzo Iorio
Galaxies 2013, 1(3), 192-209; https://doi.org/10.3390/galaxies1030192 - 18 Oct 2013
Cited by 11 | Viewed by 5408
Abstract
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 = [...] Read more.
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 × 1011 a1 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 × 1011 a1, 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. Full article
Show Figures

Figure 1

667 KiB  
Essay
Explaining Holographic Dark Energy
by Shan Gao
Galaxies 2013, 1(3), 180-191; https://doi.org/10.3390/galaxies1030180 - 03 Oct 2013
Cited by 3 | Viewed by 7113
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Particle Physics and Quantum Gravity Implications for Cosmology)
590 KiB  
Review
Evolving Black Hole Horizons in General Relativity and Alternative Gravity
by Valerio Faraoni
Galaxies 2013, 1(3), 114-179; https://doi.org/10.3390/galaxies1030114 - 25 Sep 2013
Cited by 54 | Viewed by 6995
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Aspects of Black Hole Physics)
Show Figures

Figure 1

Previous Issue
Next Issue
Back to TopTop