368 research outputs found
On Geodesic Motion in Horava-Lifshitz Gravity
We propose an action for a free particle in Horava-Lifshitz gravity based on
Foliation Preserving Diffeomorphisms. The action reduces to the usual
relativistic action in the low energy limit and allows for subluminal and
superluminal motions with upper and lower bounds on velocity respectively. We
find that deviation from general relativity is governed by a position dependent
coupling constant which also depends on the mass of the particle. As a result,
light-like geodesics are not affected whereas massive particles follow
geodesics that become mass dependent and hence the equivalence principle is
violated. We make an exact study for geodesics in flat space and a qualitative
analysis for those in a spherically symmetric curved background.Comment: 12 pages, Dedicated to Farhad Ardalan on his 70th birthda
Semiclassical String Solutions on 1/2 BPS Geometries
We study semiclassical string solutions on the 1/2 BPS geometry of type IIB
string theory characterized by concentric rings on the boundary plane. We
consider both folded rotating strings carrying nonzero R-charge and circular
pulsating strings. We find that unlike rotating strings, as far as circular
pulsating strings are concerned, the dynamics remains qualitatively unchanged
when the concentric rings replace AdS_5\times S^5. Using the gravity dual we
have also studied the Wilson loop of the corresponding gauge theory. The result
is qualitatively the same as that in AdS_5\times S^5 in the global coordinates
where the corresponding gauge theory is defined on S^3\times R. We show that
there is a correction to 1/L leading order behavior of the potential between
external objects.Comment: 17 pages, latex file, v2: refs. adde
Quantum Local Quench, AdS/BCFT and Yo-Yo String
We propose a holographic model for local quench in 1+1 dimensional Conformal
Field Theory (CFT). The local quench is produced by joining two identical CFT's
on semi-infinite lines. When these theories have a zero boundary entropy, we
use the AdS/Boundary CFT proposal to describe this process in terms of bulk
physics. Boundaries of the original CFT's are extended in AdS as dynamical
surfaces. In our holographic picture these surfaces detach from the boundary
and form a closed folded string which can propagate in the bulk. The dynamics
of this string is governed by the tensionless Yo-Yo string solution and its
subsequent evolution determines the time dependence after quench. We use this
model to calculate holographic Entanglement Entropy (EE) of an interval as a
function of time. We propose how the falling string deforms Ryu-Takayanagi's
curves. Using the deformed curves we calculate EE and find complete agreement
with field theory results.Comment: 20 pages, 13 figures, discussion improved, Version to appear in JHE
Temperature in the Throat
We study the temperature of extended objects in string theory. Rotating probe
D-branes admit horizons and temperatures a la Unruh effect. We find that the
induced metrics on slow rotating probe D1-branes in holographic string
solutions including warped Calabi-Yau throats have distinct thermal horizons
with characteristic Hawking temperatures even if there is no black hole in the
bulk Calabi-Yau. Taking the UV/IR limits of the solution, we show that the
world volume black hole nucleation depends on the deformation and the warping
of the throat. We find that world volume horizons and temperatures of expected
features form not in the regular confining IR region but in the singular
nonconfining UV solution. In the conformal limit of the UV, we find horizons
and temperatures similar to those on rotating probes in the AdS throat found in
the literature. In this case, we also find that activating a background gauge
field form the U(1) R--symmetry modifies the induced metric with its
temperature describing two different classes of black hole solutions.Comment: Revised, extended and published versio
Quasi-Normal Modes of Extremal BTZ Black Holes in TMG
We study the spectrum of tensor perturbations on extremal BTZ black holes in
topologically massive gravity for arbitrary values of the coefficient of the
Chern-Simons term, . Imposing proper boundary conditions at the boundary
of the space and at the horizon, we find that the spectrum contains
quasi-normal modes.Comment: 18 pages, Latex, Ref's added, Typos correcte
Hybrid cell-centred/vertex model for multicellular systems
This thesis presents a hybrid vertex/cell-centred approach to mechanically simulate planar cellular monolayers undergoing cell reorganisation. Cell centres are represented by a triangular nodal network, while the cell boundaries are formed by an associated vertex network. The two networks are coupled through a kinematic constraint which we allow to relax progressively. Cell-cell connectivity changes due to cell reorganisation or remodelling events, are accentuated. These situations are handled by using a variable resting length and applying an Equilibrium-Preserving Mapping (EPM) on the new connectivity, which computes a new set of resting lengths that preserve nodal and vertex equilibrium. As a by-product, the proposed technique enables to recover fully vertex or fully cell-centred models in a seamless manner by modifying a numerical parameter of the model. The properties of the model are illustrated by simulating monolayers subjected to imposed extension and during a wound healing process. The evolution of forces and the EPM are analysed during the remodelling events.Esta tesis presenta un modelo híbrido para la simulación mecánica de monocapas celulares. Este modelo combina métodos de vértices y centrados en la célula, y está orientado al análisis de deformaciones con reorganización celular. Los núcleos vienen representados por nodos que forman una malla triangular, mientras que las contornos (membranas y córtex) forman una malla poligonal de vértices. Las dos mallas se acoplan a través de una restricción cinemática que puede ser relajada de forma controlada. El estudio hace especial hincapié en los cambios de conectividad, tanto debidos a la reorganización celular como el remodelado del citoesqueleto. Estas situaciones se abordan a través de una longitud de referencia variable y aplicando un Mapeo con Conservación de Equilibrio (EPM) que minimiza el error en el equilibrio nodal y en los vértices. La técnica resultante puede ser adaptada progresivamente a través de un parámetro, dando lugar a un modelo exclusivamente de vértices o a uno de centros. Sus propiedades se ilustran en simulaciones de monocapas sujetas a una extensión impuesta y durante el proceso de cicatrizado de heridas. La evolución de las fuerzas y los efectos del EPM durante el remodelado se analizan en estos ejemplos
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