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Articles 31 - 60 of 66
Full-Text Articles in Physical Sciences and Mathematics
Why Is An Einstein Ring Blue?, Jonathan Blackledge
Why Is An Einstein Ring Blue?, Jonathan Blackledge
Articles
Albert Einstein predicted the existence of `Einstein rings' as a consequence of his general theory of relativity. The phenomenon is a direct result of the idea that if a mass warps space-time then light (and other electromagnetic waves) will be `lensed' by the strong gravitational field produced by a large cosmological body such as a galaxy. Since 1998, when the first complete Einstein ring was observed, many more complete or partially complete Einstein rings have been observed in the radio and infrared spectra, for example, and by the Hubble Space Telescope in the optical spectrum. However, in the latter case, …
Extended Green-Liouville Asymptotics And Vacuum Polarization For Lukewarm Black Holes, Cormac Breen, Adrian Ottewill
Extended Green-Liouville Asymptotics And Vacuum Polarization For Lukewarm Black Holes, Cormac Breen, Adrian Ottewill
Articles
We consider a quantum field on a lukewarm black hole spacetime. We introduce a new uniform approximation to the radial equation, constructed using an extension of Green-Liouville asymptotics. We then use this new approximation to construct the renormalized vacuum polarization in the Hartle-Hawking vacuum. Previous calculations of the vacuum polarization rely on the WKB approximation to the solutions of the radial equation, however the nonuniformity of the WKB approximations obscures the results of these calculations near both horizons. The use of our new approximation eliminates these obscurities, enabling us to obtain explicitly finite and easily calculable values of the vacuum …
Relativistic Particle Acceleration In Tangled Magnetic Fields, Stephen O'Sullivan, Brian Reville, Andrew Taylor
Relativistic Particle Acceleration In Tangled Magnetic Fields, Stephen O'Sullivan, Brian Reville, Andrew Taylor
Conference papers
We present simulations of the transport of fast particles through three-dimensional turbulent magnetic field configurations. A time dependency is imposed on the plane wave modes used in constructing these fields such than acceleration via the second-order Fermi process is possible. We consider simulations of models with low and high turbulence levels for non-relativistic waves. The predictions of quasi-linear theory are discussed with respect to the simulation data. We conclude that for pure stochastic acceleration via Alfvén waves to be plausible as the generator of UHECR in Cen A, the baryon number density would need to be several orders of magnitude …
A Model For Predicting Extragalactic Jet Lifetimes, Daniel S. Spicer, Robert Bingham, Stephen O'Sullivan
A Model For Predicting Extragalactic Jet Lifetimes, Daniel S. Spicer, Robert Bingham, Stephen O'Sullivan
Articles
Abstract. In this letter, we propose a model to explain the disintegration of ex- tragalactic jets and to predict the associated timescale. The model assumes that a jet is current and charge neutral as well as collimated at its source; however, the forward electron current gradually decays producing a magnetic field transverse to the direction of jet propagation. This growing transverse magnetic field eventually causes the jet to disintegrate.
Off To A Good Start: Using Clustering To Select The Initial Training Set In Active Learning, Rong Hu, Brian Mac Namee, Sarah Jane Delany
Off To A Good Start: Using Clustering To Select The Initial Training Set In Active Learning, Rong Hu, Brian Mac Namee, Sarah Jane Delany
Conference papers
Active learning (AL) is used in textual classification to alleviate the cost of labelling documents for training. An important issue in AL is the selection of a representative sample of documents to label for the initial training set that seeds the process, and clustering techniques have been successfully used in this regard. However, the clustering techniques used are nondeterministic which causes inconsistent behaviour in the AL process. In this paper we first illustrate the problems associated with using non-deterministic clustering for initial training set selection in AL. We then examine the performance of three deterministic clustering techniques for this task …
Stochastic Particle Acceleration In The Lobes Of Giant Radio Galaxies, Stephen O'Sullivan, Brian Reville, Andrew Taylor
Stochastic Particle Acceleration In The Lobes Of Giant Radio Galaxies, Stephen O'Sullivan, Brian Reville, Andrew Taylor
Articles
We investigate the acceleration of particles by Alfv ́en waves via the second-order Fermi process in the lobes of giant radio galaxies. Such sites are candidates for the accelerators of ultra-high-energy cosmic rays (UHECR). We focus on the nearby Fanaroff–Riley type I radio galaxy Centaurus A. This is motivated by the coincidence of its position with the arrival direction of several of the highest energy Auger events. The conditions necessary for consistency with the acceleration time-scales predicted by quasi-linear theory are reviewed. Test particle calculations are performed in fields which guarantee electric fields with no component parallel to the local …
Nonideal Magnetohydrodynamic Turbulent Decay In Molecular Clouds, Turlough Downes, Stephen O'Sullivan
Nonideal Magnetohydrodynamic Turbulent Decay In Molecular Clouds, Turlough Downes, Stephen O'Sullivan
Articles
It is well known that non-ideal magnetohydrodynamic effects are important in the dynamics of molecular clouds: both ambipolar diffusion and possibly the Hall effect have been identified as significant. We present the results of a suite of simulations with a resolution of 512-cubed of turbulent decay in molecular clouds incorporating a simplified form of both ambipolar diffusion and the Hall effect simultaneously. The initial velocity field in the turbulence is varied from being super-Alfvénic and hypersonic, through to trans-Alfvénic but still supersonic.
The Very Basics Of Sustainability - An Alternative Viewpoint (Slides With Audio) (Large File! To Speed Up Download, Right-Click On "Download" Link To Save To Own Pc.), Jim Mcgovern
Other resources
This presentation sets out the very basics of ‘sustainability’, although a definition of sustainability is not attempted. Some of the very basics are the context in which the Earth and humankind exist in space and time, the Earth’s climate, the Earth’s population and humankind’s options and choices. The author advocates keeping an open mind on all available options, including the use of oil, gas, coal, tar sands, carbon capture and sequestration, nuclear power etc., as well as the technologies that are more widely considered ‘green’. The author also argues that, in addressing the challenges that humankind faces, globally concerted effort …
Scattering From A Tenuous Random Medium With Applications In Optics, Jonathan Blackledge
Scattering From A Tenuous Random Medium With Applications In Optics, Jonathan Blackledge
Articles
Mathematical models for the scattering of light (and other radiation) tend to fall into two categories based on a weak field condition (single scattering processes) or a strong field condition (multiple scattering processes). In the latter case, the complexity of deterministic models coupled with available solutions often fail to provide results that are of value to engineering systems (e.g. imaging systems). For this reason, multiple scattering problems are often approached using stochastic modelling methods whose foundations lie in random walk theory where the amplitude and phase of the scattered field are taken to conform to an appropriate statistical process and …
Daemon Decay And Inflation, Emil Prodanov
Daemon Decay And Inflation, Emil Prodanov
Articles
In 1971, Hawking suggested [1] that there may be a very large number of gravitationally collapsed charged objects of very low masses, formed as a result of fluctuations in the early Universe. A mass of of these objects could be accumulated at the centre of a star like the Sun. The masses of these collapsed objects are from and above and their charges are up to ±30 electron units [1].
Some Rather Mechanical Reflections On Symmetry: In Art, Science, Engineering, Mathematics, Etc., Jim Mcgovern
Some Rather Mechanical Reflections On Symmetry: In Art, Science, Engineering, Mathematics, Etc., Jim Mcgovern
Articles
This Inaugural Lecture consists of some of my rather mechanical, being an engineer, reflections on symmetry in diverse areas such as art, science, engineering, mathematics, etc. I explain what symmetry is to me, giving examples with lots of images and mentioning or at least barely referencing art, science, architecture, engineering, heritage, cosmology, bicycles, flight, invention, ingenuity, history, wallpaper, mathematics, typography, structures, regular shapes, coordinate systems, spacetime, thermodynamics and suchlike.
The Transport Of Cosmic Rays In Self‐Excited Magnetic Turbulence, Brian Reville, Stephen O'Sullivan, Peter Duffy, John Kirk
The Transport Of Cosmic Rays In Self‐Excited Magnetic Turbulence, Brian Reville, Stephen O'Sullivan, Peter Duffy, John Kirk
Articles
The process of diffusive shock acceleration relies on the efficacy with which hydromagnetic waves can scatter charged particles in the precursor of a shock. The growth of self-generated waves is driven by both resonant and non-resonant processes. We perform high-resolution magnetohydrodynamic simulations of the non-resonant cosmic ray driven instability, in which the unstable waves are excited beyond the linear regime. In a snapshot of the resultant field, particle transport simulations are carried out. The use of a static snapshot of the field is reasonable given that the Larmor period for particles is typically very short relative to the instability growth …
Implications Of The Hall Effect For Turbulent Molecular Clouds, Turlough Downes, Stephen O'Sullivan
Implications Of The Hall Effect For Turbulent Molecular Clouds, Turlough Downes, Stephen O'Sullivan
Conference papers
No abstract provided.
Numerical Modeling Of Weakly Ionized Plasmas, Stephen O'Sullivan, Turlough Downes
Numerical Modeling Of Weakly Ionized Plasmas, Stephen O'Sullivan, Turlough Downes
Conference papers
No abstract provided.
Interacting Jets From Binary Protostars, Gareth Murphy, Thibaut Lery, Stephen O'Sullivan, Daniel S. Spicer, Francesca Bacciotti, Alex Rosen
Interacting Jets From Binary Protostars, Gareth Murphy, Thibaut Lery, Stephen O'Sullivan, Daniel S. Spicer, Francesca Bacciotti, Alex Rosen
Articles
Aims. We investigate potential models that could explain why multiple proto-stellar systems predominantly show single jets. During their formation, stars most frequently produce energetic outflows and jets. However, binary jets have only been observed in a very small number of systems. Methods. We model numerically 3D binary jets for various outflow parameters. We also model the propagation of jets from a specific source, namely L1551 IRS 5, known to have two jets, using recent observations as constraints for simulations with a new MHD code. We examine their morphology and dynamics, and produce synthetic emission maps. Results. We find that the …
Equation Of State For A Van Der Waals Universe During Reissner–Nordstrom Expansion, Emil Prodanov, Rossen Ivanov, Vesselin Gueorguiev
Equation Of State For A Van Der Waals Universe During Reissner–Nordstrom Expansion, Emil Prodanov, Rossen Ivanov, Vesselin Gueorguiev
Articles
In a previous work [E.M. Prodanov, R.I. Ivanov and V.G. Gueorguiev, Reissner-Nordström expansion, Astropart. Phys. 27 (2007) 150], we proposed a classical model for the expansion of the Universe during the radiation-dominated epoch based on the gravitational repulsion of the Reissner-Nordström geometry — naked singularity description of particles that "grow" with the drop of the temperature. In this work we model the Universe during the Reissner-Nordström expansion as a van der Waals gas and determine the equation of state.
A Cosmic Ray Current-Driven Instability In Partially Ionised Media, Brian Reville, John Kirk, Peter Duffy, Stephen O'Sullivan
A Cosmic Ray Current-Driven Instability In Partially Ionised Media, Brian Reville, John Kirk, Peter Duffy, Stephen O'Sullivan
Articles
We investigate the growth of hydromagnetic waves driven by streaming cosmic rays in the precursor environment of a supernova remnant shock. It is known that transverse waves propagating parallel to the mean magnetic field are unstable to anisotropies in the cosmic ray distribution, and may provide a mechanism to substantially amplify the ambient magnetic field. We quantify the extent to which temperature and ionisation fractions modify this picture. Using a kinetic description of the plasma we derive the dispersion relation for a collisionless thermal plasma with a streaming cosmic ray current. Fluid equations are then used to discuss the effects …
A Three‐Dimensional Numerical Method For Modelling Weakly Ionized Plasmas, Stephen O'Sullivan, Turlough Downes
A Three‐Dimensional Numerical Method For Modelling Weakly Ionized Plasmas, Stephen O'Sullivan, Turlough Downes
Articles
Astrophysical fluids under the influence of magnetic fields are often subjected to single- or two-fluid approximations. In the case of weakly ionized plasmas, however, this can be inappropriate due to distinct responses from the multiple constituent species to both collisional and non-collisional forces. As a result, in dense molecular clouds and protostellar accretion discs, for instance, the conductivity of the plasma may be highly anisotropic leading to phenomena such as Hall and ambipolar diffusion strongly influencing the dynamics.
Reissner–Nordstrom¨ Expansion, Emil Prodanov, Rossen Ivanov, V.G. Gueorguiev
Reissner–Nordstrom¨ Expansion, Emil Prodanov, Rossen Ivanov, V.G. Gueorguiev
Conference papers
We propose a classical mechanism for the cosmic expansion during the radiation-dominated era, assuming the Universe as a two-component gas. The first component is the ultra-relativistic “standard” fraction described by an equation of state of an ideal quantum gas of massless particles. The second component consist of superheavy charged particles and their interaction with the “standard” fraction drives the expansion. This interaction is described by the Reissner–Nordstr¨om metric purely geometrically — the superheavy charged particles are modeled as zero-dimensional naked singularities which exhibit gravitational repulsion. The radius of a repulsive sphere, surrounding a naked singularity of charge Q, is inversely …
Reissner–Nordstrom Expansion, Emil Prodanov, Rossen Ivanov, Vesselin Gueorguiev
Reissner–Nordstrom Expansion, Emil Prodanov, Rossen Ivanov, Vesselin Gueorguiev
Articles
We propose a classical mechanism for the cosmic expansion during the radiation-dominated era, assuming the Universe as a two-component gas. The first component is the ultra-relativistic “standard” fraction described by an equation of state of an ideal quantum gas of massless particles. The second component consist of superheavy charged particles and their interaction with the “standard” fraction drives the expansion. This interaction is described by the Reissner–Nordstr¨om metric purely geometrically — the superheavy charged particles are modeled as zero-dimensional naked singularities which exhibit gravitational repulsion. The radius of a repulsive sphere, surrounding a naked singularity of charge Q, is inversely …
The Nonlinear Amplification Of Magnetic Fields By Cosmic Rays At Supernova Remnant Shocks, Brian Reville, Peter Duffy, Stephen O'Sullivan
The Nonlinear Amplification Of Magnetic Fields By Cosmic Rays At Supernova Remnant Shocks, Brian Reville, Peter Duffy, Stephen O'Sullivan
Conference papers
A central problem in the theory of cosmic ray acceleration at supernova shock fronts is the generation of turbulent magnetic fields needed to scatter particles across a shock front. In this paper we build on previous studies [1], [2] into the effect of streaming cosmic rays produced by the outer shocks of supernova remnants on the stochastic component of the magnetic field. A three dimensional, MHD code has been constructed which demonstrates the nonlinear growth of the turbulent field.
An Explicit Scheme For Multifluid Magnetohydrodynamics, Stephen O'Sullivan, Turlough Downes
An Explicit Scheme For Multifluid Magnetohydrodynamics, Stephen O'Sullivan, Turlough Downes
Articles
When modelling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation, thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing time-step limit.Within an explicit differencing framework, a multifluid scheme for weakly ionized plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating …
Manev Potential And General Relativity, Rossen Ivanov, Emil Prodanov
Manev Potential And General Relativity, Rossen Ivanov, Emil Prodanov
Articles
Manevis classical potential is known to successfully modify Newtonian celestial mechanics, in accordance with the general-relativistic description. The idea of replacing the exact solution of Einsteinis equations, describing orbiting bodies, with a classical potential is widely used today. These models are not restricted to planetary motion only, but also include numerous interesting astrophysical phenomena such as accretion of matter around black holes or massive stars.
The Exterior Orientation Of Aerial Imagery Using Existing Orthoimagery And Terrain Models:A Performance Test, Finnian O'Cionnaith
The Exterior Orientation Of Aerial Imagery Using Existing Orthoimagery And Terrain Models:A Performance Test, Finnian O'Cionnaith
Masters
In every part of the world the rate of map revision is alarmingly low when compared to the rate of change of many human influenced surface features. There is therefore a requirement to regularly gather up-to-date information about surface features and to incorporate changes in maps both quickly and effectively before it becomes history. When an aerial photograph is taken, it contains errors one category of which is caused by the aircraft taking the photograph being tilted. This aerial photograph by itself is of little commercial value. Accurate maps cannot be created from it, its use in GIS (Geographical Information …
Riemann Tensor Of The Ambient Universe, The Dilaton, And Newton’S Constant, Rossen Ivanov, Emil Prodanov
Riemann Tensor Of The Ambient Universe, The Dilaton, And Newton’S Constant, Rossen Ivanov, Emil Prodanov
Articles
We investigate a four-dimensional world, embedded into a five-dimensional spacetime, and find the five-dimensional Riemann tensor via generalisation of the Gauss (--Codacci) equations. We then derive the generalised equations of the four-dimensional world and also show that the square of the dilaton field is equal to the Newton's constant. We find plausable constant and non-constant solutions for the dilaton. Comment: 5 pages, revtex
Astrophysical Plasma Diagnostics Through Analysis Of Ar Iine Shape Characteristics, Vladimir Milosavljevic, Stevan Djenize
Astrophysical Plasma Diagnostics Through Analysis Of Ar Iine Shape Characteristics, Vladimir Milosavljevic, Stevan Djenize
Articles
On the basis of five accurately recorded neutral argon ( ) line shapes (in the 4s-5p transition) we have recovered the basic plasma parameters i.e. electron temperature ( T) and electron density ( N) using our new line deconvolution procedure in the case of three different plasmas created in a linear, low-pressure, pulsed arc discharge. The mentioned plasma parameters have also been measured using independent experimental diagnostic techniques. An excellent agreement has been found among the two sets of obtained parameters. This recommends our deconvolution procedure for plasma diagnostic purposes, especially in astrophysics where direct measurements of the …
Ion Contribution To The Astrophysical Important 447.15, 587.56 And 667.82 Nm He I Spectral Lines Broadening, Vladimir Milosavljevic, Stevan Djenize
Ion Contribution To The Astrophysical Important 447.15, 587.56 And 667.82 Nm He I Spectral Lines Broadening, Vladimir Milosavljevic, Stevan Djenize
Articles
Characteristics of the astrophysical important Stark broadened 447.15 nm, 587.56 nm and 667.82 nm spectral line profiles have been measured at electron densities between 0.3 10 and 8.2 10 m and electron temperatures between 8000 and 33 000 K in plasmas created in five various discharge conditions using a linear, low-pressure, pulsed arc as an optically thin plasma source operated in a helium-nitrogen-oxygen gas mixture. On the basis of the observed asymmetry of the line profiles we have obtained their ion broadening parameters ( A) caused by influence of the ion microfield on the line broadening mechanism and also …
Stark Shifts And Transition Probabilities In The Ne Ii Spectrum, Stevan Djenize, Vladimir Milosavljevic, Milan S. Dimitrijevic
Stark Shifts And Transition Probabilities In The Ne Ii Spectrum, Stevan Djenize, Vladimir Milosavljevic, Milan S. Dimitrijevic
Articles
Stark shifts (d) and transition probabilities of the spontaneous emission (Einstein's A values) of forty two singly charged neon ( ) ion spectral lines have been measured in a linear, low pressure, pulsed arc at 35 300 K electron temperature and 1.83 10 23 m -3 electron density. Transition probabilities have been obtained using the relative line intensity ratio (RLIR) method. Stark shift values have also been calculated, using the semiclassical perturbation formalism (SCPF). The measured and calculated shift values and the measured A values have been compared to the existing data taken from available data sources.
Bouncing Branes, Emil Prodanov
Bouncing Branes, Emil Prodanov
Articles
Two classical scalar fields are minimally coupled to gravity in the Kachru-Shulz-Silverstein scenario with a rolling fifth radius. A Tolman wormhole solution is found for a R x S^3 brane with Lorentz metric and for a R x AdS_3 brane with positive definite metric.
Numerical Simulations Of Steady And Pulsed Non-Adiabatic Magnetised Jets From Young Stars, Stephen O'Sullivan, Tom Ray
Numerical Simulations Of Steady And Pulsed Non-Adiabatic Magnetised Jets From Young Stars, Stephen O'Sullivan, Tom Ray
Articles
No abstract provided.