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

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Research

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Open AccessArticle A Closer Earth and the Faint Young Sun Paradox: Modification of the Laws of Gravitation or Sun/Earth Mass Losses?
Galaxies 2013, 1(3), 192-209; doi:10.3390/galaxies1030192
Received: 11 June 2013 / Revised: 21 August 2013 / Accepted: 11 October 2013 / Published: 18 October 2013
Cited by 7 | PDF Full-text (285 KB) | HTML Full-text | XML Full-text
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
Open AccessArticle Cosmographic Constraints and Cosmic Fluids
Galaxies 2013, 1(3), 216-260; doi:10.3390/galaxies1030216
Received: 10 September 2013 / Revised: 6 November 2013 / Accepted: 12 November 2013 / Published: 4 December 2013
Cited by 19 | PDF Full-text (595 KB) | HTML Full-text | XML Full-text
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 [...] 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)
Open AccessArticle A No-Go Theorem for Rotating Stars of a Perfect Fluid without Radial Motion in Projectable Hořava–Lifshitz Gravity
Galaxies 2013, 1(3), 261-274; doi:10.3390/galaxies1030261
Received: 16 October 2013 / Revised: 27 November 2013 / Accepted: 10 December 2013 / Published: 16 December 2013
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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 [...] 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)

Review

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Open AccessReview Evolving Black Hole Horizons in General Relativity and Alternative Gravity
Galaxies 2013, 1(3), 114-179; doi:10.3390/galaxies1030114
Received: 9 August 2013 / Revised: 17 September 2013 / Accepted: 17 September 2013 / Published: 25 September 2013
Cited by 19 | PDF Full-text (590 KB) | HTML Full-text | XML Full-text
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 [...] 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)

Other

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Open AccessEssay Explaining Holographic Dark Energy
Galaxies 2013, 1(3), 180-191; doi:10.3390/galaxies1030180
Received: 24 August 2013 / Revised: 29 September 2013 / Accepted: 30 September 2013 / Published: 3 October 2013
Cited by 1 | PDF Full-text (667 KB) | HTML Full-text | XML Full-text
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 [...] 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)
Open AccessLetter Color Differences between Clockwise and Counterclockwise Spiral Galaxies
Galaxies 2013, 1(3), 210-215; doi:10.3390/galaxies1030210
Received: 22 August 2013 / Revised: 23 October 2013 / Accepted: 24 October 2013 / Published: 25 October 2013
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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 [...] 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

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