Bounce and cyclic cosmology in extended nonlinear massive gravity

We investigate non-singular bounce and cyclic cosmological evolutions in a universe governed by the extended nonlinear massive gravity, in which the graviton mass is promoted to a scalar-field potential. The extra freedom of the theory can lead to certain energy conditions violations and drive cyclicity with two different mechanisms: either with a suitably chosen scalar-field potential under a given Stuckelberg-scalar function, or with a suitably chosen Stuckelberg-scalar function under a given scalar-field potential. Our analysis shows that extended nonlinear massive gravity can alter significantly the evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA

DecidArch: Playing Cards as Software Architects

Teaching software architecture is a challenge because of the difficulty to expose students to actual meaningful design situations. Games can provide a useful illustration of the design decision making process, and teach students the power of team interaction for making sound decisions. We introduce a game –DecidArch– developed to achieve three learning objectives: 1) create awareness about the rationale involved in design decision making, 2) enable appreciation of the reasoning behind candidate design decisions proposed by others, and 3) create awareness about interdependencies between design decisions. The game has been played by 22 groups with a total of 83 players, all of them students of the VU software architecture course. We present some of the lessons learned, both from our observation and through participant survey. We conclude that the game well supports our three learning objectives, and we identify several improvement points for future game editions

Black holes in the Einstein -Gauss-Bonnet theory and the geometry of their thermodynamics-II

In the present work we study (i) charged black hole in Einstein-Gauss-Bonnet (EGB) theory, known as Einstein-Maxwell-Gauss-Bonnet (EMGB) black hole and (ii) black hole in EGB gravity with Yang-Mills field. The thermodynamic geometry of these two black hole solutions has been investigated, using the modified entropy in Gauss-Bonnet theory.Comment: 7 page

Gauss-Bonnet Black Holes in dS Spaces

We study the thermodynamic properties associated with black hole horizon and cosmological horizon for the Gauss-Bonnet solution in de Sitter space. When the Gauss-Bonnet coefficient is positive, a locally stable small black hole appears in the case of spacetime dimension $d=5$, the stable small black hole disappears and the Gauss-Bonnet black hole is always unstable quantum mechanically when $d \ge 6$. On the other hand, the cosmological horizon is found always locally stable independent of the spacetime dimension. But the solution is not globally preferred, instead the pure de Sitter space is globally preferred. When the Gauss-Bonnet coefficient is negative, there is a constraint on the value of the coefficient, beyond which the gravity theory is not well defined. As a result, there is not only an upper bound on the size of black hole horizon radius at which the black hole horizon and cosmological horizon coincide with each other, but also a lower bound depending on the Gauss-Bonnet coefficient and spacetime dimension. Within the physical phase space, the black hole horizon is always thermodynamically unstable and the cosmological horizon is always stable, further, as the case of the positive coefficient, the pure de Sitter space is still globally preferred. This result is consistent with the argument that the pure de Sitter space corresponds to an UV fixed point of dual field theory.Comment: Rextex, 17 pages including 8 eps figures, v2: minor changes, to appear in PRD, v3: references adde

Black holes in which the electrostatic or scalar equation is solvable in closed form

We show that the method used in the Schwarzschild black hole for finding the elementary solution of the electrostatic equation in closed form cannot extend in higher dimensions. By contrast, we prove the existence of static, spherically symmetric geometries with a non-degenerated horizon in which the static scalar equation can be solved in closed form. We give the explicit results in 6 dimensions. We determine moreover the expressions of the electrostatic potential and of the static scalar field for a point source in the extremal Reissner-Nordstrom black holes in higher dimensions.Comment: 20 pages, no figur

Hidden symmetries for thermodynamics and emergence of relativity

Erik Verlinde recently proposed an idea about the thermodynamic origin of gravity. Though this is a beautiful idea which may resolve many long standing problems in the theories of gravity, it also raises many other problems. In this article I will comment on some of the problems of Verlinde's proposal with special emphasis on the thermodynamical origin of the principle of relativity. It is found that there is a large group of hidden symmetries of thermodynamics which contains the Poincare group of the spacetime for which space is emergent. This explains the thermodynamic origin of the principle of relativity.Comment: V1: 4 pages, comments/criticisms welcomed; V2: references added; V3: typos and minor corrections? V4? substantial changes in Section 3 and other parts mad

Thermodynamic and gravitational instability on hyperbolic spaces

We study the properties of anti--de Sitter black holes with a Gauss-Bonnet term for various horizon topologies (k=0, \pm 1) and for various dimensions, with emphasis on the less well understood k=-1 solution. We find that the zero temperature (and zero energy density) extremal states are the local minima of the energy for AdS black holes with hyperbolic event horizons. The hyperbolic AdS black hole may be stable thermodynamically if the background is defined by an extremal solution and the extremal entropy is non-negative. We also investigate the gravitational stability of AdS spacetimes of dimensions D>4 against linear perturbations and find that the extremal states are still the local minima of the energy. For a spherically symmetric AdS black hole solution, the gravitational potential is positive and bounded, with or without the Gauss-Bonnet type corrections, while, when k=-1, a small Gauss-Bonnet coupling, namely, \alpha << {l}^2 (where l is the curvature radius of AdS space), is found useful to keep the potential bounded from below, as required for stability of the extremal background.Comment: Shortened to match published (PRD) version, 18 pages, several eps figure

Topological Charged Black Holes in High Dimensional Spacetimes and Their Formation from Gravitational Collapse of a Type II Fluid

Topological charged black holes coupled with a cosmological constant in $R^{2}\times X^{D-2}$ spacetimes are studied, where $X^{D-2}$ is an Einstein space of the form ${}^{(D-2)}R_{AB} = k(D-3) h_{AB}$. The global structure for the four-dimensional spacetimes with $k = 0$ is investigated systematically. The most general solutions that represent a Type $II$ fluid in such a high dimensional spacetime are found, and showed that topological charged black holes can be formed from the gravitational collapse of such a fluid. When the spacetime is (asymptotically) self-similar, the collapse always forms black holes for $k = 0, -1$, in contrast to the case $k = 1$, where it can form either balck holes or naked singularities.Comment: 14 figures, to appear in Phys. Rev.

Redisposition of acremonium-like fungi in Hypocreales

Acremonium is acknowledged as a highly ubiquitous genus including saprobic, parasitic, or endophytic fungi that inhabit a variety of environments. Species of this genus are extensively exploited in industrial, commercial, pharmaceutical, and biocontrol applications, and proved to be a rich source of novel and bioactive secondary metabolites. Acremonium has been recognised as a taxonomically difficult group of ascomycetes, due to the reduced and high plasticity of morphological characters, wide ecological distribution and substrate range. Recent advances in molecular phylogenies, revealed that Acremonium is highly polyphyletic and members of Acremonium s. lat. belong to at least three distinct orders of Sordariomycetes, of which numerous orders, families and genera with acremonium-like morphs remain undefined. To infer the phylogenetic relationships and establish a natural classification for acremonium-like taxa, systematic analyses were conducted based on a large number of cultures with a global distribution and varied substrates. A total of 633 cultures with acremonium-like morphology, including 261 ex-type cultures from 89 countries and a variety of substrates including soil, plants, fungi, humans, insects, air, and water were examined. An overview phylogenetic tree based on three loci (ITS, LSU, rpb2) was generated to delimit the orders and families. Separate trees based on a combined analysis of four loci (ITS, LSU, rpb2, tef-1α) were used to delimit species at generic and family levels. Combined with the morphological features, host associations and ecological analyses, acremonium-like species evaluated in the present study are currently assigned to 63 genera, and 14 families in Cephalothecales, Glomerellales and Hypocreales, mainly in the families Bionectriaceae, Plectosphaerellaceae and Sarocladiaceae and five new hypocrealean families, namely Chrysonectriaceae, Neoacremoniaceae, Nothoacremoniaceae, Pseudoniessliaceae and Valsonectriaceae. Among them, 17 new genera and 63 new combinations are proposed, with descriptions of 65 new species. Furthermore, one epitype and one neotype are designated to stabilise the taxonomy and use of older names. Results of this study demonstrated that most species of Acremonium s. lat. grouped in genera of Bionectriaceae, including the type A. alternatum. A phylogenetic backbone tree is provided for Bionectriaceae, in which 183 species are recognised and 39 well-supported genera are resolved, including 10 new genera. Additionally, rpb2 and tef-1α are proposed as potential DNA barcodes for the identification of taxa in Bionectriaceae