657 research outputs found
Purely gravito-magnetic vacuum space-times
It is shown that there are no vacuum space-times (with or without
cosmological constant) for which the Weyl-tensor is purely gravito-magnetic
with respect to a normal and timelike congruence of observers.Comment: 4 page
An intrinsic characterization of 2+2 warped spacetimes
We give several equivalent conditions that characterize the 2+2 warped
spacetimes: imposing the existence of a Killing-Yano tensor subject to
complementary algebraic restrictions; in terms of the projector (or of the
canonical 2-form ) associated with the 2-planes of the warped product. These
planes are principal planes of the Weyl and/or Ricci tensors and can be
explicitly obtained from them. Therefore, we obtain the necessary and
sufficient (local) conditions for a metric tensor to be a 2+2 warped product.
These conditions exclusively involve explicit concomitants of the Riemann
tensor. We present a similar analysis for the conformally 2+2 product
spacetimes and give an invariant classification of them. The warped products
correspond to two of these invariant classes. The more degenerate class is the
set of product metrics which are also studied from an invariant point of view.Comment: 18 pages; submitted to Class. Quantum Grav
Space-times which are asymptotic to certain Friedman-Robertson-Walker space-times at timelike infinity
We define space-times which are asymptotic to radiation dominant
Friedman-Robertson-Walker space-times at timelike infinity and study the
asymptotic structure. We discuss the local asymptotic symmetry and give a
definition of the total energy from the electric part of the Weyl tensor.Comment: 8 pages, Revte
Research Support in Australian Academic Libraries: Services, Resources, and Relationships
In the last decade Australian academic libraries have increasingly aligned their research support services with assessment criteria used in the national research evaluation exercise (Excellence for Research in Australia). The same period has seen growing interest in research impact outside of traditional measures, such as bibliometrics. Social media has provided opportunities for research dissemination and new tools, altmetrics, to measure these activities have emerged. This article reports on research into the extent and nature of research support services at Australian academic libraries, how the services are managed, and the factors that influence their development and delivery. Quantitative and qualitative research methods were used to compare the findings with an earlier study and to provide a deeper understanding of research support in Australia. Three key themes, services, staff and resourcing, and relationships, are discussed in relation to the management and challenges faced in providing research support
Thermoresponsive TriblockâCopolymers of Polyethylene Oxide and Polymethacrylates: Linking Chemistry, Nanoscale Morphology, and Rheological Properties
© 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, https://creativecommons.org/licenses/by/4.0/Abstract: Thermoreversible gels switch from a freeâflowing liquid state to an elastic gel mesophase upon warming, displaying the reverse transition upon cooling. While this phenomenon makes these advanced materials highly attractive in numerous fields, the generation of optimal materials of tailored rheology and transition temperatures is stifled by the lack of design principles. To address this need, a library of ABA copolymers has been prepared with âAâ blocks exhibiting thermoresponsive behavior and âBâ blocks of poly(ethylene glycol). This library evaluates the effect of âAâ chemistry, probing three polymer classes, and A/B block molecular weight on thermallyâinduced phase changes in solutions of the polymers. An exploration by rheometry coupled to SmallâAngle Neutron Scattering (SANS) elucidates temperatureâdependent hierarchical selfâassembly processes occurring on the nanoscale as well as bulk rheology. This process deciphered links between rheology and supracolloidal assemblies (sphere, ellipses, and cylinders) within the gel state with interactions probed further via structure factors. Several design principles are identified to inform the genesis of nextâgeneration thermoreversible gels, alongside novel materials exhibited thermoresponsive behavior in the solution state for use in applied healthcare technologies.Peer reviewedFinal Published versio
Geometric Interpretation of the Mixed Invariants of the Riemann Spinor
Mixed invariants are used to classify the Riemann spinor in the case of
Einstein-Maxwell fields and perfect fluids. In the Einstein-Maxwell case these
mixed invariants provide information as to the relative orientation of the
gravitational and electromagnetic principal null directions. Consideration of
the perfect fluid case leads to some results about the behaviour of the
Bel-Robinson tensor regarded as a quartic form on unit timelike vectors.Comment: 31 pages, AMS-LaTe
Ricci Collineations for type B warped space-times
We present the general structure of proper Ricci Collineations (RC) for type
B warped space-times. Within this framework, we give a detailed description of
the most general proper RC for spherically symmetric metrics. As examples,
static spherically symmetric and Friedmann-Robertson-Walker space-times are
considered.Comment: 18 pages, Latex, To appear in GR
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