1,353 research outputs found
Resistive and multi-fluid RMHD on graphics processing units
No full textIn this work we present a proof of concept of CUDA-capable, resistive, multi-fluid models of relativistic magnetohydrodynamics (RMHD). Resistive and multi-fluid codes for simulating models of RMHD suffer from stiff source terms, so it is common to implement a set of semi-implicit time integrators to maintain numerical stability. We show, for the first time, that finite volume IMEX schemes for resistive and two-fluid models of RMHD can be accelerated by execution on graphics processing units, significantly reducing the demand set by these kinds of problems. We report parallel speed-ups of over 21× using double-precision floating-point accuracy, and highlight the optimization strategies required for these schemes, and how they differ from ideal RMHD models. The impact of these results is discussed in the context of the next-generation simulations of neutron star mergers.</p
Dynamics of primordial black hole formation
No full textWe examine numerically the formation of small black holes from primordial density fluctuations in a radiation-dominated spatially flat Friedmann–Robertson–Walker spacetime. Large amplitude fluctuations might be expected to form black holes, while smaller fluctuations will be washed out by the expansion of the universe. We have studied the interface between these two types of behaviour. Unlike earlier studies which suggested that there was no lower limit to the mass of a black hole, this work suggests that there is a minimum mass for a primordial black hole of the order of one ten thousandth of the mass contained within the horizon. We discuss the implications for critical collapse studies
Numerical simulations of interfaces in relativistic hydrodynamics
No full textWe consider models of relativistic matter containing sharp interfaces across which the matter model changes. These models will be relevant for neutron stars with crusts, phase transitions, or for viscous boundaries where the length scale is too short to be modelled smoothly. In particular we look at numerical techniques that allow us to evolve stable interfaces, for the interfaces to merge, and for strong waves and shocks to interact with the interfaces. We test these techniques for ideal hydrodynamics in special and general relativity for simple equations of state, finding that simple level set-based methods extend well to relativistic hydrodynamic
Can ChatGPT Be Considered an Author of a Medical Article?
No full textKazuki Ide, Philip Hawke, Takeo Nakayama, Can ChatGPT Be Considered an Author of a Medical Article?, Journal of Epidemiology, 2023, Volume 33, Issue 7, Pages 381-382, Released on J-STAGE July 05, 2023, Advance online publication April 08, 2023, Online ISSN 1349-9092, Print ISSN 0917-5040, https://doi.org/10.2188/jea.JE20230030, https://www.jstage.jst.go.jp/article/jea/33/7/33_JE20230030/_article/-char/e
A variational approach to resistive relativistic plasmas
No full textWe develop an action principle to construct the field equations for a multi-fluid system containing charge-neutral fluids, plasmas, and dissipation (via resistive interactions), by combining the standard, Maxwell action and minimal coupling of the electromagnetic field with a recently developed action for relativistic dissipative fluids. We use a pull-back formalism from spacetime to abstract matter spaces to build unconstrained variations for both the charge-neutral fluids and currents making up the plasmas. Using basic linear algebra techniques, we show that a general 'relabeling' invariance exists for the abstract matter spaces. With the field equations in place, a phenomenological model for the resistivity is developed, using as constraints charge conservation and the Second Law of Thermodynamics. A minimal model for a system of electrons, protons, and heat is developed using the Onsager procedure for incorporating dissipation
Thermal aspects of neutron star mergers
No full textIn order to extract maximal information from neutron-star merger signals, both gravitational and electromagnetic, we need to ensure that our theoretical models/numerical simulations faithfully represent the extreme physics involved. This involves a range of issues, with the finite temperature effects regulating many of the relevant phenomena. As a step toward understanding these issues, we explore the conditions for β-equilibrium in neutron star matter for the densities and temperatures reached in a binary neutron star merger. Using the results from our out-of-equilibrium merger simulation, we consider how different notions of equilibrium may affect the merger dynamics, raising issues that arise when attempting to account for these conditions in future simulations. These issues are both computational and conceptual. We show that the effects lead to, in our case, a softening of the equation of state in some density regions, and to composition changes that affect processes that rely on deviation from equilibrium, such as bulk viscosity, both in terms of the magnitude and the equilibration timescales inherent to the relevant set of reactions. We also demonstrate that it is difficult to determine exactly which equilibrium conditions are relevant in which regions of the matter due to the dependence on neutrino absorption, further complicating the calculation of the reactions that work to restore the matter to equilibrium.</p
Impact of nuclear reactions on gravitational waves from neutron star mergers
No full textNuclear reactions may affect gravitational-wave signals from neutron-star mergers, but the impact is uncertain. To indicate the significance of this effect, we compare two numerical simulations representing intuitive extremes. In one case, reactions happen instantaneously. In the other case, they occur on timescales much slower than the evolutionary timescale. We show that, while the differences in the two gravitational-wave signals are small, the mismatch between them satisfies the condition for distinguishability using the Einstein Telescope noise curve, assuming that the neutron-star equation of state can be well constrained by experiments or by the postmerger signal of the event. This suggests that, to avoid systematic errors in equation of state parameters inferred from observed signals, we need to accurately implement nuclear reactions in future simulations.</p
Portrait of Kenneth Hawke
No full textSepia-toned image of a young man in a United States Navy uniform from World War I. The banner on his sailor cap states he served on the armored cruiser USS San Diego. Pulled from the Pacific fleet and assigned to escort merchant ships through the North Atlantic with the entry of the U. S. into World War I, the San Diego was struck by a German mine off the coast of New York States on July 19, 1918, sinking less than thirty minutes later, the only major U. S. warship to be lost in the first World War. Though born in Tillamook, Hawke's father, C. E. Hawke, was a physician with a general practice in Forest Grove.[back] Kenneth Hawke, 1918
Ceros / drawn on stone by W. Westail ; from an original sketch by E.H. Locker. ; printed by C. Hullmandel
No full textPossiblement el mateix gravat dins: Views in Spain / by Edward Hawke Locker. - London : John Murray, 1824. -- A la part sup. dreta: Cataluña. -- Vista panoràmica. Al fons es dibuixa la silueta de la poblaci
"I know! We'll have an aids summit!" [Bob Hawke] [picture] /
No full textTitle devised by cataloguer.; Published in the Canberra Times on 21 August 1985.; Part of the Pryor collection of cartoons and drawings. The Hawke government introduces a tight Budget, intended to convey the impression to the business and finance community of its responsible economic management credentials. Included in the Budget is $8.4m for research into AIDS. Given his penchant for national summits, no doubt Prime Minister Hawke will be on the lookout for opportunities.--Information provided by Geoff Pryor
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