Results 1 - 10 of 4629
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[en] We investigate neutrino oscillation in the field of an axially symmetric space-time, employing the so-called q-metric, in the context of general relativity. Following the standard approach, we compute the phase shift invoking the weak and strong field limits and small deformation. To do so, we consider neutron stars, white dwarfs and supernovae as strong gravitational regimes whereas the solar system as weak field regime. We argue that the inclusion of the quadrupole parameter leads to the modification of the well-known results coming from the spherical solution due to the Schwarschild space-time. Hence, we show that in the solar system regime, considering the Earth and Sun, there is a weak probability to detect deviations from the flat case, differently from the case of neutron stars and white dwarfs in which this probability is larger. Thus, we heuristically discuss some implications on constraining the free parameters of the phase shift by means of astrophysical neutrinos. A few consequences in cosmology and possible applications for future space experiments are also discussed throughout the text.
[en] We study the motion of particles in the background of a scalar–tensor theory of gravity in which the scalar field is kinetically coupled to the Einstein tensor. We constrain the value of the derivative parameter z through solar system tests. By considering the perihelion precession we obtain the constraint /m>2.6×10 m, the gravitational redshift m>2.7×10 m, the deflection of light /m>1.6×10 m, and the gravitational time delay /m>7.9×10 m; thereby, our results show that it is possible to constrain the value of the z parameter in agreement with the observational tests that have been considered.
[en] We analyze the possible variability of the effective Newtonian gravitational constant G in space and time in the framework of the geometric scalar theory of gravity suggested by Novello et al. (JCAP 06:014, 2013). Spatial variations of G in the Solar system are shown to have orders of magnitude detectable by modern instruments. As to variations of G with cosmological time, it is shown (at least for the particular formulation of the theory discussed in the original paper and the corresponding cosmological models) that these variations are more rapid than is allowed by observations.
[en] We present and analyzed bi-circular restricted four-body problem model that accounts for dissipative forces. Specifically, the model for Sun–Earth–Moon-Spacecraft system is formulated with inclusion of Stokes drag and Poynting–Robertson (P–R) drag. The Lagrange points are seen to be dependent on the strength and the kind of the dissipative force involved, comparatively, the P–R drag is found to exert greater influence on the spacecraft than the Stokes drag. The linear stability analysis of the model shows that the motion of the Spacecraft around the system’s Lagrange points is stable only at L4,5. Moreover, an examination of the dynamical behaviour of the system reveals it to be chaotic as the trajectories of the motion are exponentially divergent. This model finds great applications in the study of astronomical system and mission planning in space travels and interplanetary probes. (author)
[en] Young Kepler’s daring ideas on the structure of the Solar system are applied to the analysis of planetary distances in the exoplanetary system HD 10180. Using Zhukovsky’s transformation, the essence of the spinor regularization of Kepler’s problem is explained as extracting the square root of an ellipse and using a Kepler eccentric anomaly instead of the usual time. The achievements of Kharkiv radio astronomers in the search for radio recombination lines of Rydberg carbon atoms at the UTR-2 radio telescope are considered. A generalized spinor regularization of the Kepler problem is used to analyze the energy spectra of Rydberg hydrogen atoms in a magnetic field. (author)
[en] The photovoltaic–thermal (PVT) systems have been established for providing both electricity and heat using the existing photovoltaic (PV) system set-up. The PVT systems capture panel heat for some useful purpose. It is based on deploying a polymer sheet at the back of the PV panel to accommodate cooling water between the PV panel and the sheet to maximize the contact area between cooling water and panel. The present work compares the performance of a normal PV panel to that of the novel PVT panel. The PVT system is fabricated and experiments are conducted to evaluate electrical and thermal efficiencies. An improvement of 2.17% is observed in the electrical efficiency of the PVT panel in comparison with the normal PV panel. A brief cost analysis along with payback period calculations of the PVT panel is also included. (author)
[en] It is now established that, contrary to common belief, (electro-)vacuum Brans–Dicke gravity does not reduce to general relativity (GR) for large values of the Brans–Dicke coupling ω. Since the essence of experimental tests of scalar–tensor gravity consists of providing lower bounds on ω, in light of the misguided assumption of the equivalence between the limit ω→∞ and the GR limit of Brans–Dicke gravity, the parametrized post-Newtonian (PPN) formalism on which these tests are based could be in jeopardy. We show that, in the linearized approximation used by the PPN formalism, the anomaly in the limit to general relativity disappears. However, it survives to second (and higher) order and in strong gravity. In other words, while the weak gravity regime cannot tell apart GR and ω→∞ Brans–Dicke gravity, when higher order terms in the PPN analysis of Brans–Dicke gravity are included, the latter never reduces to the one of GR in this limit. This fact is relevant for experiments aiming to test second order light deflection and Shapiro time delay.
[en] In the near-future, atmospheric characterization of Earth-like planets in the habitable zone will become possible via reflectance spectroscopy with future telescopes such as the proposed LUVOIR and HabEx missions. While previous studies have considered the effect of clouds on the reflectance spectra of Earth-like planets, the molecular detectability considering a wide range of cloud properties has not been previously explored in detail. In this study, we explore the effect of cloud altitude and coverage on the reflectance spectra of Earth-like planets at different geological epochs and examine the detectability of , and CH4 with test parameters for the future mission concept, LUVOIR, using a coronagraph noise simulator previously designed for WFIRST-AFTA. Considering an Earth-like planet located at 5 pc away, we have found that for the proposed LUVOIR telescope, the detection of the O2 A-band feature (0.76 μm) will take approximately 100, 30, and 10 hr for the majority of the cloud parameter space modeled for the atmospheres with 10%, 50%, and 100% of modern Earth O2 abundances, respectively. In particular, for the case of ≳50% of modern Earth O2 abundance, the feature will be detectable with an integration time ≲10 hr as long as there are lower-altitude (≲8 km) clouds with a global coverage of ≳20%. For the 1% of the modern Earth O2 abundance case, however, it will take more than 100 hr for all the cloud parameters we modeled.
[en] Over 100 rocky planets orbiting Sun-like stars in very short orbital periods (≲1 day) have been discovered by the Kepler mission. The origin of these planets, known as ultra-short-period (USP) planets, remains elusive. Here, we propose that most of these planets, originally at periods of ∼5–10 days, reach their current orbits via high-eccentricity migration. In a scaled-down version of the dynamics that may have been experienced by their high-mass analogs, the hot Jupiters, these planets reach high eccentricities via chaotic secular interactions with their companion planets and then undergo orbital circularization due to dissipation from tides raised on the planet. This proposal is motivated by the following observations: planetary systems observed by Kepler often contain several super-Earths with non-negligible eccentricities and inclinations, possibly extending beyond ∼au distances; by contrast, only a small fraction of USP planets have known transiting companions, which are generally not closely spaced, and we argue that most of them should have companions with periods ≳10 days. The proposed scenario naturally explains the observation that most USP planets have significantly more distant transiting companions compared to their counterparts at slightly longer periods (1–3 days). Our model predicts that USP planets should have: (i) spin–orbit angles, and inclinations relative to outer planets, in the range of ∼10–50°; (ii) several outer planetary companions extending beyond ∼1 au distances. Both of these predictions may be tested by TESS and its follow-up observations.