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Search Results: 1 - 10 of 190903 matches for " Christos G. Athanassiou "
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The effect of diatomaceous earth of different origin, temperature and relative humidity against adults of rice weevil (Sitophilus oryzae [L.], Coleoptera, Curculionidae) in stored wheat
Helena Rojht, Christos G. Athanassiou, Bill J. Vayias, Nickolas Kavallieratos, eljko Tomanovi , Matej Vidrih, Katarina Kos, Stanislav Trdan
Acta agriculturae Slovenica , 2010, DOI: 10.2478/v10014-010-0002-4
Abstract: Laboratory experiments were carried out to evaluate the impact of diatomaceous earth (DE) samples of different origin with their insecticidal properties to control one of the most important primary pest in stored grain. We tested the efficacy of three local DE, from Serbia, Greece and Slovenia, and commercial formulation SilicoSec against the rice weevil, Sitophilus oryzae, adults in stored wheat. The experiments were carried out at three temperatures (20, 25 and 30 °C) and two relative humidity (RH) levels (55 and 75 %). Mortality of pest was counted 7, 14 and 21 days after exposure (DAT) at the following DE dose rates: 100, 300, 500 and 900 ppm. The mortality of adults normally increased with increasing dose rates and DAT. In all samples the mortality of rice weevil adults (dose rate 900 ppm, 21 DAT) was above 90 %, except at Slovenian DE (at 20 °C and 55 % RH) and Greek DE (at 25 °C and 75 % RH), when the mortality was 85.3 and 67.6 %, respectively. With 100 % mortality (14 DAT and at 900 ppm) the most effective was SilicoSec. Slovenian DE was more effective at 55 % RH than at 75 % RH (7 DAT at all temperatures).
On a Deinotherium (Proboscidea) finding in the Neogene of Crete
Athanassiou Athanassios
Carnets de Géologie , 2004,
Abstract: This paper reports the discovery of an incomplete proboscidean mandible near the village of Maroniá in eastern Crete. The fragment described here includes the first molar (m1) of a deinothere, that because of its large size is identified as Deinotherium giganteum. The specimen was found in shallow-water marine sediments. The presence of Deinotherium on the island, together with other Miocene vertebrate faunas, suggests that during that epoch Crete was connected to the mainland by a wide land bridge.
Rearrangement Invariant, Coherent Risk Measures on L0  [PDF]
Christos E. Kountzakis, Dimitrios G. Konstantinides
Journal of Financial Risk Management (JFRM) , 2015, DOI: 10.4236/jfrm.2015.41003
Abstract: By this paper, we give an answer to the problem of definition of coherent risk measures on rearrangement invariant, solid subspaces of L0 with respect to some atom less probability space . This problem was posed by F. Delbaen, while in this paper we proposed a solution via ideals of L0 and the class of the dominated variation distributions, as well.
Large-scale magnetic fields in cosmology
Christos G. Tsagas
Physics , 2009, DOI: 10.1088/0741-3335/51/12/124013
Abstract: Despite the widespread presence of magnetic fields, their origin, evolution and role are still not well understood. Primordial magnetism sounds appealing but is not problem free. The magnetic implications for the large-scale structure of the universe still remain an open issue. This paper outlines the advantages and shortcomings of early-time magnetogenesis and the typical role of B-fields in linear structure-formation scenarios.
Large-scale peculiar motions and cosmic acceleration
Christos G Tsagas
Physics , 2009, DOI: 10.1111/j.1365-2966.2010.16460.x
Abstract: Recent surveys seem to support bulk peculiar velocities well in excess of those anticipated by the standard cosmological model. In view of these results, we consider here some of the theoretical implications of large-scale drift motions. We find that observers with small, but finite, peculiar velocities have generally different expansion rates than the smooth Hubble flow. In particular, it is possible for observers with larger than the average volume expansion at their location, to experience apparently accelerated expansion when the universe is actually decelerating. Analogous results have been reported in studies of inhomogeneous (nonlinear) cosmologies and within the context of the Lemaitre-Tolman-Bondi models. Here, they are obtained within the linear regime of a perturbed, dust-dominated Friedmann-Robertson-Walker cosmology.
Gravito-magnetic amplification in cosmology
Christos G. Tsagas
Physics , 2009, DOI: 10.1103/PhysRevD.81.043501
Abstract: Magnetic fields interact with gravitational waves in various ways. We consider the coupling between the Weyl and the Maxwell fields in cosmology and study the effects of the former on the latter. The approach is fully analytical and the results are gauge-invariant. We show that the nature and the outcome of the gravito-magnetic interaction depends on the electric properties of the cosmic medium. When the conductivity is high, gravitational waves reduce the standard (adiabatic) decay rate of the B-field, leading to its superadiabatic amplification. In poorly conductive environments, on the other hand, Weyl-curvature distortions can result into the resonant amplification of large-scale cosmological magnetic fields. Driven by the gravitational waves, these B-fields oscillate with an amplitude that is found to diverge when the wavelengths of the two sources coincide. We present technical and physical aspects of the gravito-magnetic interaction and discuss its potential implications.
Cosmological Perturbations
Christos G. Tsagas
Physics , 2002,
Abstract: The aim of these lecture notes is to familiarize graduate students and beginning postgraduates with the basic ideas of linear cosmological perturbation theory and of structure formation scenarios. We present both the Newtonian and the general relativistic approaches, derive the key equations and then apply them to a number of characteristic cases. The gauge problem in cosmology and ways to circumvent it are also discussed. We outline the basic framework of the baryonic and the non-baryonic structure formation scenarios and point out their strengths and shortcomings. Fundamental concepts, such as the Jeans length, Silk damping and collisionless dissipation, are highlighted and the underlying mathematics are presented in a simple and straightforward manner.
Resonant amplification of magnetic seed fields by gravitational waves in the early universe
Christos G. Tsagas
Physics , 2005, DOI: 10.1103/PhysRevD.72.123509
Abstract: Inflation is known to produce both gravitational waves and seed magnetic fields on scales well beyond the size of the horizon. The general relativistic interaction between these two sources at the end of inflation showed a significant amplification of the initial magnetic seed, which brought the latter within the currently accepted dynamo limits. In the present article we revisit this gravito-magnetic interaction and argue that the observed strong growth of the field is the result of resonance. More specifically, we show that the maximum magnetic boost always occurs when the wavelength of the inducing gravitational radiation and the scale of the original seed field coincide. We also look closer at the physics of the proposed Maxwell-Weyl coupling, consider the implications of finite electrical conductivity for the efficiency of the amplification mechanism and clarify further the mathematics of the analysis.
Peculiar motions, accelerated expansion and the cosmological axis
Christos G. Tsagas
Physics , 2011, DOI: 10.1103/PhysRevD.84.063503
Abstract: Peculiar velocities change the expansion rate of any observer moving relative to the smooth Hubble flow. As a result, observers in a galaxy like our Milky Way can experience accelerated expansion within a globally decelerating universe, even when the drift velocities are small. The effect is local, but the affected scales can be large enough to give the false impression that the whole cosmos has recently entered an accelerating phase. Generally, peculiar velocities are also associated with dipole-like anisotropies, triggered by the fact that they introduce a preferred spatial direction. This implies that observers experiencing locally accelerated expansion, as a result of their own drift motion, may also find that the acceleration is maximised in one direction and minimised in the opposite. We argue that, typically, such a dipole anisotropy should be relatively small and the axis should probably lie fairly close to the one seen in the spectrum of the Cosmic Microwave Background.
Magnetic tension and gravitational collapse
Christos G. Tsagas
Physics , 2003, DOI: 10.1088/0264-9381/23/13/002
Abstract: The gravitational collapse of a magnetised medium is investigated by studying qualitatively the convergence of a timelike family of non-geodesic worldlines in the presence of a magnetic field. Focusing on the field's tension we illustrate how the winding of the magnetic forcelines due to the fluid's rotation assists the collapse, while shear-like distortions in the distribution of the field's gradients resist contraction. We also show that the relativistic coupling between magnetism and geometry, together with the tension properties of the field, lead to a magneto-curvature stress that opposes the collapse. This tension stress grows stronger with increasing curvature distortion, which means that it could potentially dominate over the gravitational pull of the matter. If this happens, a converging family of non-geodesic lines can be prevented from focusing without violating the standard energy conditions.
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