15,404 research outputs found
On (Not)-Constraining Heavy Asymmetric Bosonic Dark Matter
Recently, constraints on bosonic asymmetric dark matter have been imposed
based on the existence of old neutron stars excluding the dark matter masses in
the range from keV up to several GeV. The constraints are based on the
star destruction scenario where the dark matter particles captured by the star
collapse forming a black hole that eventually consumes the host star. In
addition, there were claims in the literature that similar constraints can be
obtained for dark matter masses heavier than a few TeV. Here we argue that it
is not possible to extend to these constraints. We show that in the case of
heavy dark matter, instead of forming a single large black hole that consumes
the star, the collapsing dark matter particles form a series of small black
holes that evaporate fast without leading to the destruction of the star. Thus,
no constraints arise for bosonic asymmetric dark matter particles with masses
of a few TeV or higher
The implementation of a multimedia learning environment for graduate civil engineers
This paper examines the underpinning theory, design and implementation of a computer‐based Multimedia Learning Environment (MLE) for graduate civil engineers. The MLE brings together multimedia technology and Intelligent Tutoring Systems techniques for the purpose of developing in the graduate engineer the skills and understanding needed to produce initial design proposals for real‐world dam spillway design problems
Growth of Black Holes in the interior of Rotating Neutron Stars
Mini-black holes made of dark matter that can potentially form in the
interior of neutron stars have been always thought to grow by accreting the
matter of the core of the star via a spherical Bondi accretion. However,
neutron stars have sometimes significant angulal velocities that can in
principle stall the spherical accretion and potentially change the conclusions
derived about the time it takes for black holes to destroy a star. We study the
effect of the star rotation on the growth of such black holes and the evolution
of the black hole spin. Assuming no mechanisms of angular momentum evacuation,
we find that even moderate rotation rates can in fact destroy spherical
accretion at the early stages of the black hole growth. However, we demonstrate
that the viscosity of nuclear matter can alleviate the effect of rotation,
making it possible for the black hole to maintain spherical accretion while
impeding the black hole from becoming maximally rotating.Comment: 9 page
Non-linear Group Actions with Polynomial Invariant Rings and a Structure Theorem for Modular Galois Extensions
Let be a finite -group and a field of characteristic . We
show that has a \emph{non-linear} faithful action on a polynomial ring
of dimension such that the invariant ring is also
polynomial. This contrasts with the case of \emph{linear and graded} group
actions with polynomial rings of invariants, where the classical theorem of
Chevalley-Shephard-Todd and Serre requires to be generated by
pseudo-reflections.
Our result is part of a general theory of "trace surjective -algebras",
which, in the case of -groups, coincide with the Galois ring-extensions in
the sense of \cite{chr}. We consider the \emph{dehomogenized symmetric algebra}
, a polynomial ring with non-linear -action, containing as a
retract and we show that is a polynomial ring. Thus turns out to be
\emph{universal} in the sense that every trace surjective -algebra can be
constructed from by "forming quotients and extending invariants". As a
consequence we obtain a general structure theorem for Galois-extensions with
given -group as Galois group and any prescribed commutative -algebra
as invariant ring. This is a generalization of the Artin-Schreier-Witt theory
of modular Galois field extensions of degree .Comment: 20 page
10 Reasons to Oppose Virginia Sales Tax Increases
Northern and southeastern Virginians will vote in referenda this November to approve or reject increases in the retail sales tax to fund transportation projects. Northern Virginians will decide whether to increase the sales tax from 4.5 percent to 5.0 percent, an 11 percent increase. Virginians in the Hampton Roads area will decide whether to increase the sales tax from 4.5 percent to 5.5 percent, a 22 percent increase. Proponents of tax increases point to unmet transportation needs to support their cause. Yet state spending increased 13 percent in 1999, 7 percent in 2000, and 9 percent in 2001. If key transportation needs have not been met, the problem is not a lack of funds but legislators who have not properly prioritized the budget. If the sales tax referenda are passed, the state government will have a strong incentive to reduce what it would otherwise spend on transportation in northern Virginia and Hampton Roads. By some measures, northern Virginia already gets the short end of the stick with regard to the state budget. Tax increases are not just bad budget policy; they are also bad economic policy. Since higher taxes reduce economic growth, an added cost of higher sales taxes would be lower incomes for Virginians. During the 1990s Virginia taxes grew faster than incomes, and local property taxes have soared recently. Even modest restraint in nontransportation spending could save enough money to fund priority highway projects without tax increases. Further, the state could adopt a spending growth cap that channels excess future tax revenues to transportation needs and tax cuts
Earth's middle age
This research was funded through Natural Environment Research Council (grant NE/J021822/1) and the APC was paid through the RCUK OA block grant.Earth's middle age, extending from 1.7 to 0.75 Ga, was characterized by environmental, evolutionary, and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of preserved passive margins, an absence of a significant Sr anomaly in the paleoseawater record and in the εHf(t) in detrital zircon, a lack of orogenic gold and volcanic-hosted massive sulfide deposits, and an absence of glacial deposits and iron formations. In contrast, anorthosites and kindred bodies are well developed and major pulses of Mo and Cu mineralization, including the world's largest examples of these deposits, are features of this period. These trends are attributed to a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by ca. 1.7 Ga and continued until breakup of its closely related successor, Rodinia, ca. 0.75 Ga. The overall low abundance of passive margins is consistent with a stable continental configuration, which also provided a framework for environmental and evolutionary stability. A series of convergent margin accretionary orogens developed along the edge of the supercontinent. Abundant anorthosites and related rocks developed inboard of the plate margin. Their temporal distribution appears to link with the secular cooling of the mantle, at which time the overlying continental lithosphere was strong enough to be thickened and to support the emplacement of large plutons into the crust, yet the underlying mantle was still warm enough to result in widespread melting of the lower thickened crust.Publisher PDFPeer reviewe
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