5,729 research outputs found
Flat 3-Brane with Tension in Cascading Gravity
In the Cascading Gravity brane-world scenario, our 3-brane lies within a
succession of lower-codimension branes, each with their own induced gravity
term, embedded into each other in a higher-dimensional space-time. In the
6+1-dimensional version of this scenario, we show that a 3-brane with tension
remains flat, at least for sufficiently small tension that the weak-field
approximation is valid. The bulk solution is nowhere singular and remains in
the perturbative regime everywhere.Comment: 4 pages, 3 figures. v2: version to appear in PR
Cascading Gravity and Degravitation
We construct a cascading brane model of gravity in which the behavior of the
gravitational force law interpolates from (n+4)-dimensional to
(n+3)-dimensional all the way down to 4-dimensional from longer to shorter
length scales. We show that at the linearized level, this model exhibits the
features necessary for degravitation of the cosmological constant. The model is
shown to be ghost free with the addition of suitable brane kinetic operators,
and we demonstrate this using a number of independent procedures. Consequently
this is a consistent IR modification of gravity, providing a promising
framework for a dynamical, degravitating solution of the cosmological constant
problem.Comment: 31 pages, 1 figur
Cascading Gravity is Ghost Free
We perform a full perturbative stability analysis of the 6D cascading gravity
model in the presence of 3-brane tension. We demonstrate that for sufficiently
large tension on the (flat) 3-brane, there are no ghosts at the perturbative
level, consistent with results that had previously only been obtained in a
specific 5D decoupling limit. These results establish the cascading gravity
framework as a consistent infrared modification of gravity.Comment: 24 pages, minor changes, argument on vector perturbations improve
Chameleon Dark Energy
Chameleons are scalar fields whose mass depends on the environment,
specifically on the ambient matter density. While nearly massless in the
cosmos, where the matter density is tiny, their mass is of order of an inverse
millimeter on Earth, where the density is high. In this note, we review how
chameleons can satisfy current experimental constraints on deviations from
General Relativity (GR). Moreover, we study the cosmological evolution with a
chameleon field and show the existence of an attractor solution, akin to the
tracker solution in quintessence models. We discuss how chameleons can
naturally drive the observed acceleration of the universeComment: 5 pages, 2 figures. To appear in the proceedings of the "Phi in the
Sky" conference, 8-10 July 2004, Porto, Portugua
Cascading DGP
We present a higher codimension generalization of the DGP scenario which,
unlike previous attempts, is free of ghost instabilities. The 4D propagator is
made regular by embedding our visible 3-brane within a 4-brane, each with their
own induced gravity terms, in a flat 6D bulk. The model is ghost-free if the
tension on the 3-brane is larger than a certain critical value, while the
induced metric remains flat. The gravitational force law `cascades' from a 6D
behavior at the largest distances followed by a 5D and finally a 4D regime at
the shortest scales.Comment: 4 pages; one reference added and a typo correcte
The Wilson-Polchinski Renormalization Group Equation in the Planar Limit
We derive the Wilson-Polchinski RG equation in the planar limit. We explain
that the equation necessarily involves also non-planar amplitudes with sphere
topology, which represent multi-trace contributions to the effective action.
The resulting RG equation turns out to be of the Hamilton-Jacobi type since
loop effects manifest themselves through terms which are linear in first order
derivatives of the effective action with respect to the sources. We briefly
outline applications to renormalization of non-commutative field theories,
matrix models with external sources and holography.Comment: 22 pages, Latex, 3 eps figure
Self-regulation processes in the life-course : primary and secondary control
Este artigo discute conceitos relevantes à perspectiva do curso de vida, porém pouco difundidos no Brasil: controle primário e controle secundário. O primeiro se refere aos esforços que o indivíduo empreende para adaptar o ambiente às suas necessidades; o segundo, para se adaptar ao ambiente. Apresenta-se a formulação original dos conceitos como modelo de dois processos de controle, em oposição a modelos de processo único, como o do desamparo aprendido. Em seguida, discute-se revisão conceitual que trouxe modificação e ampliação para estes construtos, concebendo-os em um modelo bidimensional que articula controle primário e secundário com os conceitos de seleção e compensação. Nesse processo, apresentam-se contribuições no intuito de estimular a reflexão e expandir a discussão teórico-conceitual que envolve estes construtos. ________________________________________________________________________________________________________ ABSTRACTThis article discusses relevant concepts of life course perspective which are not very much diffused in Brazil: primary and secondary control. The former refers to efforts to change the environment so that it fits individual's needs. The latter involves efforts to fit in with the environment. The original definitions are presented as a two-process model of control in contrast to one-process models such as learned helplessness. Furthermore, it discusses a conceptual revision, which modified and expanded these constructs as a bi-dimensional model that relates primary and secondary control to the concepts of selection and compensation. In process of arguing about those, suggestions are presented for thinking and expanding the theoretical and conceptual discussion of these constructs
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