2,511 research outputs found
Software de simulación y estudio de una Mini-Evolución Artificial
En el presente trabajo se buscó simular de manera simple y representativa la evolución de una población de microorganismos, que viven en un mundo plano, en el cual se mueven, alimentan y reproducen. El diseño y desarrollo del mismo se realizó con una metodología orientada a objetos. Se utilizaron tarjetas de clases, responsabilidad y colaboración (CRC) y diagramas de jerarquía y colaboración. La implementación se realizó en lenguaje C++11. El resultado final incluye una interfaz de usuario que permite modificar ciertos parámetros iniciales de la población de microorganismos, y ver, de manera gráfica, los movimientos de los mismos y su interacción con otros objetos del mundo donde viven. El software permite almacenar datos de la simulación y facilita el análisis de la evolución de los organismos, mediante histogramas y curvas gaussianas, que reflejan los distintos niveles de inteligencia alcanzadas en el proceso evolutivo.Sociedad Argentina de Informática e Investigación Operativ
Higher derivative theories with constraints : Exorcising Ostrogradski's Ghost
We prove that the linear instability in a non-degenerate higher derivative
theory, the Ostrogradski instability, can only be removed by the addition of
constraints if the original theory's phase space is reduced.Comment: 17 pages, no figures, version published in JCA
The vacuum bubbles in de Sitter background and black hole pair creation
We study the possible types of the nucleation of vacuum bubbles. We classify
vacuum bubbles in de Sitter background and present some numerical solutions.
The thin-wall approximation is employed to obtain the nucleation rate and the
radius of vacuum bubbles. With careful analysis we confirm that Parke's formula
is also applicable to the large true vacuum bubbles. The nucleation of the
false vacuum bubble in de Sitter background is also evaluated. The tunneling
process in the potential with degenerate vacua is analyzed as the limiting
cases of the large true vacuum bubble and false vacuum bubble. Next, we
consider the pair creation of black holes in the background of bubble
solutions. We obtain static bubble wall solutions of junction equation with
black hole pair. The masses of created black holes are uniquely determined by
the cosmological constant and surface tension on the wall. Finally, we obtain
the rate of pair creation of black holes.Comment: 3 figures, minor including errors and typos corrected, and refs.
adde
Vainshtein Mechanism in Binary Pulsars
We compute the scalar gravitational radiation from a binary pulsar system in
the simplest model that exhibits the Vainshtein mechanism. The mechanism is
successful in screening the effect from scalar fields conformally coupled to
matter, although gravitational radiation is less suppressed relative to its
general relativity predictions than static fifth forces effects within the
pulsar system. This is due to a combination of two effects: firstly the
existence of monopole and dipole radiation; secondly the Vainshtein suppression
comes from the hierarchy of scales between the inverse frequency scale and the
Vainshtein radius, rather than the orbital radius of the pulsar system.
Extensions of these results will have direct relevance to infrared
modifications of gravity, such as massive gravity theories, which are known to
exhibit a Vainshtein mechanism. Generalization to Galileon models with higher
order interactions are likely to provide stronger constraints.Comment: minor revisions to match published version in pr
Spatially Covariant Theories of a Transverse, Traceless Graviton, Part I: Formalism
General relativity is a covariant theory of two transverse, traceless
graviton degrees of freedom. According to a theorem of Hojman, Kuchar, and
Teitelboim, modifications of general relativity must either introduce new
degrees of freedom or violate the principle of general covariance. In this
paper, we explore modifications of general relativity that retain the same
number of gravitational degrees of freedom, and therefore explicitly break
general covariance. Motivated by cosmology, the modifications of interest
maintain spatial covariance. Demanding consistency of the theory forces the
physical Hamiltonian density to obey an analogue of the renormalization group
equation. In this context, the equation encodes the invariance of the theory
under flow through the space of conformally equivalent spatial metrics. This
paper is dedicated to setting up the formalism of our approach and applying it
to a realistic class of theories. Forthcoming work will apply the formalism
more generally.Comment: 55 pages, no figure
Crossing the cosmological constant line in a dilatonic brane-world model with and without curvature corrections
We construct a new brane-world model composed of a bulk -with a dilatonic
field-, plus a brane -with brane tension coupled to the dilaton-, cold dark
matter and an induced gravity term. It is possible to show that depending on
the nature of the coupling between the brane tension and the dilaton this model
can describe the late-time acceleration of the brane expansion (for the normal
branch) as it moves within the bulk. The acceleration is produced together with
a mimicry of the crossing of the cosmological constant line (w=-1) on the
brane, although this crossing of the phantom divide is obtained without
invoking any phantom matter neither on the brane nor in the bulk. The role of
dark energy is played by the brane tension, which reaches a maximum positive
value along the cosmological expansion of the brane. It is precisely at that
maximum that the crossing of the phantom divide takes place. We also show that
these results remain valid when the induced gravity term on the brane is
switched off.Comment: 12 pages, 2 figures, RevTeX
Brane cosmological solutions in six-dimensional warped flux compactifications
We study cosmology on a conical brane in the six-dimensional
Einstein-Maxwell-dilaton system, where the extra dimensions are compactified by
a magnetic flux. We systematically construct exact cosmological solutions using
the fact that the system is equivalently described by (6+n)-dimensional pure
Einstein-Maxwell theory via dimensional reduction. In particular, we find a
power-law inflationary solution for a general dilatonic coupling. When the
dilatonic coupling is given by that of Nishino-Sezgin chiral supergravity, this
reduces to the known solution which is not inflating. The power-law solution is
shown to be the late-time attractor. We also investigate cosmological tensor
perturbations in this model using the (6+n)-dimensional description. We obtain
the separable equation of motion and find that there always exist a zero mode,
while tachyonic modes are absent in the spectrum. The mass spectrum of
Kaluza-Klein modes is obtained numerically.Comment: 12 pages, 2 figures; v2: references added; v3: version published in
JCA
Scaling Cosmologies from Duality Twisted Compactifications
Oscillating moduli fields can support a cosmological scaling solution in the
presence of a perfect fluid when the scalar field potential satisfies
appropriate conditions. We examine when such conditions arise in
higher-dimensional, non-linear sigma-models that are reduced to four dimensions
under a generalized Scherk-Schwarz compactification. We show explicitly that
scaling behaviour is possible when the higher-dimensional action exhibits a
global SL(n,R) or O(2,2) symmetry. These underlying symmetries can be exploited
to generate non-trivial scaling solutions when the moduli fields have
non-canonical kinetic energy. We also consider the compactification of
eleven-dimensional vacuum Einstein gravity on an elliptic twisted torus.Comment: 21 pages, 3 figure
Key Success Factors and Guidance for International Collaborative Design Projects
In the built environment (BE) sector, the co-creation process of design demands understanding of requirements (as viewed by parties involved), mobilisation of tacit knowledge, negotiation, and complex exchange of information. The need to collaborate over distance has further exacerbated the complexity of the process, and, in itself, represents a significant challenge for BE professionals who are increasingly expected to undertake this process within globally distributed virtual teams. The research aims to identify key success factors and develop guidance for international collaborative design projects, via the implementation of collaborative design courses in UK and Canadian universities over three academic years. Questionnaire surveys, focus groups, observation of online meetings, personal reflections provided data for the analysis. The findings reveal the significance of the perceived risk of collaboration and a difference in preferred communication mode between architects and civil/structural engineers. These findings suggest the impact of training in the subject discipline, and that the opportunity for co-located working has helped the development of trust. The guidance is aimed at BE educators who wish to implement this activity in their courses
- …