198,694 research outputs found
Dark energy as an elastic strain fluid
No full textThe origin of the accelerated expansion of the universe is still unclear and new physics is needed on cosmological scales. We propose and test a novel interpretation of dark energy as originated by an elastic strain due to a cosmic defect in an otherwise Euclidean space-time. The strain modifies the expansion history of the universe. This new effective contribution tracks radiation at early times and mimics a cosmological constant at late times. The theory is tested against observations, from nucleosynthesis to the cosmic microwave background and formation and evolution of large scale structure to supernovae. Data are very well reproduced with Lamé parameters of the order of 10^(−52) m^(−2
Laganosuchus Sereno & Larsson 2009, gen. n.
No full textLaganosuchus gen. n. urn:lsid:zoobank.org:act: E23D26F4-42BB-4D63-9E75-FF1F8A4E73D3 Etymology. Laganon, pancake (Greek); souchos, crocodile (Greek). Named for the shallow depth of its skull. Type species. Laganosuchus thaumastos. Diagnosis. Mid-sized (̴ 4–6 m) stomatosuchid with spaced teeth and an undulating, or festooned, alveolar margin; spike-shaped crowns that lack recurvature; crowns flattened buccolingually with sharp unornamented mesial and distal carinae; d1 enlarged (subequal to d4 caniniform); postcaniniform teeth (d5–24) gradually decrease in size; Meckel’s canal developed as a very narrow, sharply delimited groove on the anterior one-half of the dentary.Published as part of Sereno, Paul & Larsson, Hans, 2009, Cretaceous Crocodyliforms from the Sahara, pp. 1-143 in ZooKeys 28 (28) on page 78, DOI: 10.3897/zookeys.28.325, http://zenodo.org/record/57657
The mass-concentration relation in massive galaxy clusters at redshift ~1
No full textMass and concentration of clusters of galaxies are related and evolving with redshift. We study the properties of a sample of 31 massive galaxy clusters at high redshift, 0.8 < ∼ z < ∼ 1.5, using weak and strong lensing observations. Concentration is a steep function of mass, c200 ∝ M−0.83±0.39200, with higher-redshift clusters being less concentrated. Mass and concentration from the stacked analysis, M200 = (4.1 ± 0.4) × 1014M/h and c200 = 2.3 ± 0.2, are in line with theoretical results extrapolated from the local universe. Clusters with signs of dynamical activity preferentially feature high concentrations. We discuss the possibility that the whole sample is a mix of two different kinds of haloes. Over-concentrated clusters might be accreting haloes out of equilibrium in a transient phase of compression, whereas less concentrated ones might be more relaxed
Weakly perturbed Schwarzschild lens in the strong deflection limit
No full textWe investigate the strong deflection limit of gravitational lensing by a Schwarzschild black hole embedded in an external gravitational field. The study of this model, analogous to the Chang and Refsdal lens in the weak deflection limit, is important to evaluate the gravitational perturbations on the relativistic images that appear in proximity of supermassive black holes hosted in galactic centers. By a simple dimensional argument, we prove that the tidal effect on the light ray propagation mainly occurs in the weak field region far away from the black hole and that the external perturbation can be treated as a weak field quadrupole term. We provide a description of relativistic critical curves and caustics and discuss the inversion of the lens mapping. Relativistic caustics are shifted and acquire a finite diamond shape. Sources inside the caustics produce four sequences of relativistic images. On the other hand, retro-lensing caustics are only shifted while remaining pointlike to the lowest order
Spherical symmetry in a dark energy permeated space-time
No full textThe properties of a spherically symmetric static space-time permeated of dark energy are worked out. Dark energy is viewed as the strain energy of an elastically deformable four dimensional manifold. The metric is worked out in the vacuum region around a central spherical mass/defect in the linear approximation. We discuss analogies and differences with the analogue in the de Sitter space time and how these competing scenarios could be differentiated on an observational ground. The comparison with the tests at the solar system scale puts upper limits to the parameters of the theory, consistent with the values obtained applying the classical cosmological test
GSPN Semantics for Queueing Networks with Blocking
No full textQueueing network models with finite capacity queues and blocking are used to represent systems with finite capacity resource constraints, such as production, communication and computer systems. Various blocking mechanisms have been defined in the literature to represent the different behaviours of real systems with limited resources. In this paper we propose a technique that allows to represent this type of queueing networks by means of generalized stochastic Petri nets. The method allows to obtain several benefits both for the qualitative and the quantitative analysis of these queueing networks. In particular it offers the possibility of using results and tools developed within the framework of the Petri nets. In the paper some of these potentialities are presented
On the use of structural Petri net analysis for studying product form equilibrium distributions of queueing networks with blocking
No full textIn this paper we investigate some relations between the Petri net formalism and the queueing networks with blocking. This type of queueing network models are used to represent systems with finite capacity resource constraints, such as production, communication and computer systems. Various blocking mechanisms have been defined in the literature to represent the different behaviours of real systems with limited resources. We show that the representation of these queueing networks by means of Generalized Stochastic Petri Nets offers the possibility of using results developed within the Petri net framework. In particular, we investigate product form equilibrium distributions for queueing networks with blocking by means of structural Petri net results. More precisely, we use the notion of implicit places. With this concept we characterise a class of queueing networks with blocking having interesting properties. For each queueing network of this class there exists another model with the same performance measures and exhibiting product form equilibrium distribution
Simulation of Fluid Stochastic Petri Nets
No full textThis paper describes a method for the simulation of Fluid Stochastic Petri Nets (FSPNs). The FSPNs are a promising formalism for modeling hybrid dynamic systems, that is, systems having both discrete and continuous components that evolve over time. Unfortunately analytical evaluation of performance measures of such nets requires the solution of a complex system of integro-differential equations whose numerical analysis often becomes a formidable task. One of the possible ways for computing performance measures is the use of simulative approaches. Because of the mixed (discrete and continuous) state space, simulation of FSPNs models poses some interesting challenges, which are addressed in the paper. This paper provides a simulative approach for deriving performance measures for a class of FSPNs. The techniques described in the present paper are included in a simulation tool for this class of FSPNs
Modelling the Milky Way halo through adiabatic compression of cold dark matter halo
No full textWe use the adiabatic compression theory to build a physically well-motivated Milky Way mass model in agreement with observational data. The visible mass of the Galaxy is distributed in a spheroidal bulge and a multi-component disc parametrized by three galactic parameters, the Sun distance to the galactic centre, R0, the total bulge mass, , and the local disc surface density, . To model the dark matter component, we adiabatically compress a Navarro, Frenk and White (NFW) halo (with concentration c and total mass ) for fixed values of the spin parameter λ, the fraction of the mass in baryons mb, and the thin disc contribution to total angular momentum jd. An iterative selection procedure is used to explore in detail the wide space of parameters only selecting those combinations of that give rise to a Milky Way model in agreement with observational constraints. This analysis leads us to conclude that only models with kpc, and can be reconciled with the set of observational constraints. As regards the parameters entering the adiabatic compression, we find and , while final estimates of the parameters describing the initial halo profile turn out to be 5 \la c \la 12 and 7 \times 10^{11}~M_{\odot} \la M_{{\rm vir}} \la 17 \times 10^{11}~M_{\odot} (all at 95.7% CL)
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