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633 KiB

Open AccessArticle

The Flyby Anomaly in an Extended Whitehead’s Theory

by Luis Acedo

Galaxies 2015, 3(3), 113-128; https://doi.org/10.3390/galaxies3030113 - 30 Jul 2015

Cited by 11 | Viewed by 5210

In this paper, we consider an extended version of Whitehead’s theory of gravity in connection with the flyby anomaly. Whitehead’s theory is a linear approximation defined in a background Minkowski spacetime, which gives the same solutions as standard general relativity for the Schwarzschild [...] Read more.

In this paper, we consider an extended version of Whitehead’s theory of gravity in connection with the flyby anomaly. Whitehead’s theory is a linear approximation defined in a background Minkowski spacetime, which gives the same solutions as standard general relativity for the Schwarzschild and Kerr metrics cast in Kerr–Schild coordinates. For a long time and because it gives the same results for the three classical tests—perihelion advance, light bending and gravitational redshift—it was considered a viable alternative to general relativity, but as it is really a linear approximation, it fails in more stringent tests. The model considered in this paper is a formal generalization of Whitehead’s theory, including all possible bilinear forms. In the resulting theory, a circulating vector field of force in the low velocities’ approximation for a rotating planet is deduced, in addition to Newtonian gravity. This extra force gives rise to small variations in the asymptotic velocities of flybys around the Earth to be compared to the recently reported flyby anomaly. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

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252 KiB

Open AccessArticle

Quantum Corrected Non-Thermal Radiation Spectrum from the Tunnelling Mechanism

by Subenoy Chakraborty, Subhajit Saha and Christian Corda

Galaxies 2015, 3(2), 103-112; https://doi.org/10.3390/galaxies3020103 - 12 Jun 2015

Cited by 4 | Viewed by 3997

Abstract

The tunnelling mechanism is today considered a popular and widely used method in describing Hawking radiation. However, in relation to black hole (BH) emission, this mechanism is mostly used to obtain the Hawking temperature by comparing the probability of emission of an outgoing [...] Read more.

The tunnelling mechanism is today considered a popular and widely used method in describing Hawking radiation. However, in relation to black hole (BH) emission, this mechanism is mostly used to obtain the Hawking temperature by comparing the probability of emission of an outgoing particle with the Boltzmann factor. On the other hand, Banerjee and Majhi reformulated the tunnelling framework deriving a black body spectrum through the density matrix for the outgoing modes for both the Bose-Einstein distribution and the Fermi-Dirac distribution. In contrast, Parikh and Wilczek introduced a correction term performing an exact calculation of the action for a tunnelling spherically symmetric particle and, as a result, the probability of emission of an outgoing particle corresponds to a non-strictly thermal radiation spectrum. Recently, one of us (C. Corda) introduced a BH effective state and was able to obtain a non-strictly black body spectrum from the tunnelling mechanism corresponding to the probability of emission of an outgoing particle found by Parikh and Wilczek. The present work introduces the quantum corrected effective temperature and the corresponding quantum corrected effective metric is written using Hawking’s periodicity arguments. Thus, we obtain further corrections to the non-strictly thermal BH radiation spectrum as the final distributions take into account both the BH dynamical geometry during the emission of the particle and the quantum corrections to the semiclassical Hawking temperature. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

460 KiB

Open AccessArticle

Sagnac Effect, Ring Lasers and Terrestrial Tests of Gravity

by Matteo Luca Ruggiero

Galaxies 2015, 3(2), 84-102; https://doi.org/10.3390/galaxies3020084 - 21 May 2015

Cited by 15 | Viewed by 6161

Abstract

Light can be used as a probe to explore the structure of space-time: this is usual in astrophysical and cosmological tests; however, it has been recently suggested that this can be done also in terrestrial laboratories. Namely, the Gyroscopes In General Relativity (GINGER) [...] Read more.

Light can be used as a probe to explore the structure of space-time: this is usual in astrophysical and cosmological tests; however, it has been recently suggested that this can be done also in terrestrial laboratories. Namely, the Gyroscopes In General Relativity (GINGER) project aims at measuring post-Newtonian effects, such as the gravito-magnetic ones, in an Earth-based laboratory, by means of a ring laser array. Here, we first review the theoretical foundations of the Sagnac effect, on which ring lasers are based, and then, we study the Sagnac effect in a terrestrial laboratory, emphasizing the origin of the gravitational contributions that GINGER aims at measuring. Moreover, we show that accurate measurements allow one to set constraints on theories of gravity different from general relativity. Eventually, we describe the experimental setup of GINGER. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

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324 KiB

Open AccessArticle

Perspectives on Gravity-Induced Radiative Processes in Astrophysics

by Giorgio Papini

Galaxies 2015, 3(2), 72-83; https://doi.org/10.3390/galaxies3020072 - 16 Apr 2015

Cited by 5 | Viewed by 3894

Abstract

Single-vertex Feynman diagrams represent the dominant contribution to physical processes, but are frequently forbidden kinematically. This is changed when the particles involved propagate in a gravitational background and acquire an effective mass. Procedures are introduced that allow the calculation of lowest order diagrams, [...] Read more.

Single-vertex Feynman diagrams represent the dominant contribution to physical processes, but are frequently forbidden kinematically. This is changed when the particles involved propagate in a gravitational background and acquire an effective mass. Procedures are introduced that allow the calculation of lowest order diagrams, their corresponding transition probabilities, emission powers and spectra to all orders in the metric deviation, for particles of any spin propagating in gravitational fields described by any metric. Physical properties of the “space-time medium” are also discussed. It is shown in particular that a small dissipation term in the particle wave equations can trigger a strong back-reaction that introduces resonances in the radiative process and affects the resulting gravitational background. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

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282 KiB

Open AccessArticle

Thermodynamic Relations for the Entropy and Temperature of Multi-Horizon Black Holes

by Wei Xu, Jia Wang and Xin-he Meng

Galaxies 2015, 3(1), 53-71; https://doi.org/10.3390/galaxies3010053 - 2 Feb 2015

Cited by 19 | Viewed by 4338

Abstract

We present some entropy and temperature relations of multi-horizons, even including the “virtual” horizon. These relations are related to the product, division and sum of the entropy and temperature of multi-horizons. We obtain the additional thermodynamic relations of both static and rotating black [...] Read more.

We present some entropy and temperature relations of multi-horizons, even including the “virtual” horizon. These relations are related to the product, division and sum of the entropy and temperature of multi-horizons. We obtain the additional thermodynamic relations of both static and rotating black holes in three- and four-dimensional (A)dS spacetime. Especially, a new dimensionless, charge-independence and T+S+ = T_S_-like relation is presented. This relation does not depend on the mass, electric charge, angular momentum and cosmological constant, as it is always a constant. These relations lead us to obtaining some interesting thermodynamic bounds of entropy and temperature, including the Penrose inequality, which is the first geometrical inequality of black holes. Besides, based on these new relations, one can obtain the first law of thermodynamics and the Smarr relation for all horizons of a black hole. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

287 KiB

Open AccessArticle

Nonlocal General Relativity

by Bahram Mashhoon

Galaxies 2015, 3(1), 1-17; https://doi.org/10.3390/galaxies3010001 - 26 Dec 2014

Cited by 18 | Viewed by 4636

Abstract

A brief account of the present status of the recent nonlocal generalization of Einstein’s theory of gravitation is presented. The main physical assumptions that underlie this theory are described. We clarify the physical meaning and significance of Weitzenböck’s torsion and emphasize its intimate [...] Read more.

A brief account of the present status of the recent nonlocal generalization of Einstein’s theory of gravitation is presented. The main physical assumptions that underlie this theory are described. We clarify the physical meaning and significance of Weitzenböck’s torsion and emphasize its intimate relationship with the gravitational field, characterized by the Riemannian curvature of spacetime. In this theory, nonlocality can simulate dark matter; in fact, in the Newtonian regime, we recover the phenomenological Tohline–Kuhn approach to modified gravity. To account for the observational data regarding dark matter, nonlocality is associated with a characteristic length scale of order 1 kpc. The confrontation of nonlocal gravity with observation is briefly discussed. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

338 KiB

Open AccessArticle

Orbital Motions and the Conservation-Law/Preferred-Frame α₃ Parameter

by Lorenzo Iorio

Galaxies 2014, 2(4), 482-495; https://doi.org/10.3390/galaxies2040482 - 29 Sep 2014

Cited by 3 | Viewed by 4376

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

358 KiB

Open AccessArticle

Constraints on Non-Standard Gravitomagnetism by the Anomalous Perihelion Precession of the Planets

by Luis Acedo

Galaxies 2014, 2(4), 466-481; https://doi.org/10.3390/galaxies2040466 - 29 Sep 2014

Cited by 15 | Viewed by 5223

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Advances in Gravitational Research)

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