Cosmic censorship and spherical gravitational collapse with tangential pressure

We study the spherical gravitational collapse of a compact object under the approximation that the radial pressure is identically zero, and the tangential pressure is related to the density by a linear equation of state. It turns out that the Einstein equations can be reduced to the solution of an integral for the evolution of the area radius. We show that for positive pressure there is a finite region near the center which necessarily expands outwards, if collapse begins from rest. This region could be surrounded by an inward moving one which could collapse to a singularity - any such singularity will necessarily be covered by a horizon. For negative pressure the entire object collapses inwards, but any singularities that could arise are not naked. Thus the nature of the evolution is very different from that of dust, even when the ratio of pressure to density is infinitesimally small.Comment: 16 pages, Latex file, two figures, uses epsf.st

Historical ‘signposts’ and other temporal indicators in the Czech lexicon

This article posits that the Czechs employ a great many historical markers, previously applied to other events of national importance, which help to shape collective memory and right the ‘wrongs’ of the past. It is argued that these temporal indicators share a number of clearly defined characteristics, and that their use is too systematic and calculated to be merely a function of the constraints of the lexicon. The first part of the study considers in detail questions of semantics (especially the distinction between denotation and connotation), the lexicographical sources available to the researcher, and the lexical ‘signpost’ in context, while the second part focuses on practical examples of lexical re-appropriation since 1918, with particular reference to dictionaries and the Czech National Corpus.University of Wolverhampto

Stellar Stalling:the view from asteroseismology

Asteroseismology, the study of intrinsic oscillations in stars, can reveal fundamental properties of cool stars critical in our understanding of stellar rotational evolution. Through space missions such as Kepler and TESS, asteroseismology has seen a surge of new data and research in the past decade. These data have contributed to important results in the field of stellar braking particularly for F, G and K stars, thanks to estimates of stellar age for rotating field stars, and the measurement of stellar rotation through oscillation spectra as important comparisons for estimates from star-spots. In this talk, I will provide an introduction to asteroseismology how it can provide important results for your research. This will be followed by a breakdown of how asteroseismologists have used these techniques to establish the presence of weakened magnetic braking on the main sequence using asteroseismic data, and what we should be looking forward to from these techniques with the TESS mission

Unified N=2 Maxwell-Einstein and Yang-Mills-Einstein Supergravity Theories in Four Dimensions

We study unified N=2 Maxwell-Einstein supergravity theories (MESGTs) and unified Yang-Mills Einstein supergravity theories (YMESGTs) in four dimensions. As their defining property, these theories admit the action of a global or local symmetry group that is (i) simple, and (ii) acts irreducibly on all the vector fields of the theory, including the ``graviphoton''. Restricting ourselves to the theories that originate from five dimensions via dimensional reduction, we find that the generic Jordan family of MESGTs with the scalar manifolds [SU(1,1)/U(1)] X [SO(2,n)/SO(2)X SO(n)] are all unified in four dimensions with the unifying global symmetry group SO(2,n). Of these theories only one can be gauged so as to obtain a unified YMESGT with the gauge group SO(2,1). Three of the four magical supergravity theories defined by simple Euclidean Jordan algebras of degree 3 are unified MESGTs in four dimensions. Two of these can furthermore be gauged so as to obtain 4D unified YMESGTs with gauge groups SO(3,2) and SO(6,2), respectively. The generic non-Jordan family and the theories whose scalar manifolds are homogeneous but not symmetric do not lead to unified MESGTs in four dimensions. The three infinite families of unified five-dimensional MESGTs defined by simple Lorentzian Jordan algebras, whose scalar manifolds are non-homogeneous, do not lead directly to unified MESGTs in four dimensions under dimensional reduction. However, since their manifolds are non-homogeneous we are not able to completely rule out the existence of symplectic sections in which these theories become unified in four dimensions.Comment: 47 pages; latex fil