1,059 research outputs found
The galileon as a local modification of gravity
In the DGP model, the ``self-accelerating'' solution is plagued by a ghost
instability, which makes the solution untenable. This fact as well as all
interesting departures from GR are fully captured by a four-dimensional
effective Lagrangian, valid at distances smaller than the present Hubble scale.
The 4D effective theory involves a relativistic scalar \pi, universally coupled
to matter and with peculiar derivative self-interactions. In this paper, we
study the connection between self-acceleration and the presence of ghosts for a
quite generic class of theories that modify gravity in the infrared. These
theories are defined as those that at distances shorter than cosmological,
reduce to a certain generalization of the DGP 4D effective theory. We argue
that for infrared modifications of GR locally due to a universally coupled
scalar, our generalization is the only one that allows for a robust
implementation of the Vainshtein effect--the decoupling of the scalar from
matter in gravitationally bound systems--necessary to recover agreement with
solar system tests. Our generalization involves an internal ``galilean''
invariance, under which \pi's gradient shifts by a constant. This symmetry
constrains the structure of the \pi Lagrangian so much so that in 4D there
exist only five terms that can yield sizable non-linearities without
introducing ghosts. We show that for such theories in fact there are
``self-accelerating'' deSitter solutions with no ghost-like instabilities. In
the presence of compact sources, these solutions can support spherically
symmetric, Vainshtein-like non-linear perturbations that are also stable
against small fluctuations. [Short version for arxiv]Comment: 35 pages; minor modifications, a typo corrected in eq. (114
Radiation reaction and quantum damped harmonic oscillator
By taking a Klein-Gordon field as the environment of an harmonic oscillator
and using a new method for dealing with quantum dissipative systems (minimal
coupling method), the quantum dynamics and radiation reaction for a quantum
damped harmonic oscillator investigated. Applying perturbation method, some
transition probabilities indicating the way energy flows between oscillator,
reservoir and quantum vacuum, obtainedComment: 12 pages. Accepted for publication in Mod. Phys. Lett.
Probabilistic and thermodynamic aspects of dynamical systems
The probabilistic approach to dynamical systems giving rise to irreversible behavior at the macroscopic, mesoscopic, and microscopic levels of description is outlined. Signatures of the complexity of the underlying dynamics on the spectral properties of the Liouville, Frobenius-Perron, and Fokker-Planck operators are identified. Entropy and entropy production-like quantities are introduced and the connection between their properties in nonequilibrium steady states and the characteristics of the dynamics in phase space are explored.info:eu-repo/semantics/publishe
Stochastic Resonance in Two Dimensional Landau Ginzburg Equation
We study the mechanism of stochastic resonance in a two dimensional Landau
Ginzburg equation perturbed by a white noise. We shortly review how to
renormalize the equation in order to avoid ultraviolet divergences. Next we
show that the renormalization amplifies the effect of the small periodic
perturbation in the system. We finally argue that stochastic resonance can be
used to highlight the effect of renormalization in spatially extended system
with a bistable equilibria
Comparison of Entropy Production Rates in Two Different Types of Self-organized Flows: B\'{e}nard Convection and Zonal flow
Entropy production rate (EPR) is often effective to describe how a structure
is self-organized in a nonequilibrium thermodynamic system. The "minimum EPR
principle" is widely applicable to characterizing self-organized structures,
but is sometimes disproved by observations of "maximum EPR states." Here we
delineate a dual relation between the minimum and maximum principles; the
mathematical representation of the duality is given by a Legendre
transformation. For explicit formulation, we consider heat transport in the
boundary layer of fusion plasma [Phys. Plasmas {\bf 15}, 032307 (2008)]. The
mechanism of bifurcation and hysteresis (which are the determining
characteristics of the so-called H-mode, a self-organized state of reduced
thermal conduction) is explained by multiple tangent lines to a pleated graph
of an appropriate thermodynamic potential. In the nonlinear regime, we have to
generalize Onsager's dissipation function. The generalized function is no
longer equivalent to EPR; then EPR ceases to be the determinant of the
operating point, and may take either minimum or maximum values depending on how
the system is driven
A note on the wellposedness of scalar brane world cosmological perturbations
We discuss scalar brane world cosmological perturbations for a 3-brane world
in a maximally symmetric 5D bulk. We show that Mukoyama's master equations
leads, for adiabatic perturbations of a perfect fluid on the brane and for
scalar field matter on the brane, to a well posed problem despite the "non
local" aspect of the boundary condition on the brane. We discuss in relation to
the wellposedness the way to specify initial data in the bulk.Comment: 14 pages, one figure, v2 minor change
Synergetics in multiple exciton generation effect in quantum dots
We present detailed analysis of the non-Poissonian population of excitons
produced by MEG effect in quantum dots on the base of statistic theory of MEG
and synergetic approach for chemical reactions. From the analysis we can
conclude that a non-Poissonian distribution of exciton population is evidence
of non-linear and non-equilibrium character of the process of multiple
generation of excitons in quantum dots at a single photon absorptio
Self-organized patterns of coexistence out of a predator-prey cellular automaton
We present a stochastic approach to modeling the dynamics of coexistence of
prey and predator populations. It is assumed that the space of coexistence is
explicitly subdivided in a grid of cells. Each cell can be occupied by only one
individual of each species or can be empty. The system evolves in time
according to a probabilistic cellular automaton composed by a set of local
rules which describe interactions between species individuals and mimic the
process of birth, death and predation. By performing computational simulations,
we found that, depending on the values of the parameters of the model, the
following states can be reached: a prey absorbing state and active states of
two types. In one of them both species coexist in a stationary regime with
population densities constant in time. The other kind of active state is
characterized by local coupled time oscillations of prey and predator
populations. We focus on the self-organized structures arising from
spatio-temporal dynamics of the coexistence. We identify distinct spatial
patterns of prey and predators and verify that they are intimally connected to
the time coexistence behavior of the species. The occurrence of a prey
percolating cluster on the spatial patterns of the active states is also
examined.Comment: 19 pages, 11 figure
Stochastic resonance for nonequilibrium systems
Stochastic resonance (SR) is a prominent phenomenon in many natural and engineered noisy systems, whereby the response to a periodic forcing is greatly amplified when the intensity of the noise is tuned to within a specific range of values. We propose here a general mathematical framework based on large deviation theory and, specifically, on the theory of quasipotentials, for describing SR in noisy
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-dimensional nonequilibrium systems possessing two metastable states and undergoing a periodically modulated forcing. The drift and the volatility fields of the equations of motion can be fairly general, and the competing attractors of the deterministic dynamics and the edge state living on the basin boundary can, in principle, feature chaotic dynamics. Similarly, the perturbation field of the forcing can be fairly general. Our approach is able to recover as special cases the classical results previously presented in the literature for systems obeying detailed balance and allows for expressing the parameters describing SR and the statistics of residence times in the two-state approximation in terms of the unperturbed drift field, the volatility field, and the perturbation field. We clarify which specific properties of the forcing are relevant for amplifying or suppressing SR in a system and classify forcings according to classes of equivalence. Our results indicate a route for a detailed understanding of SR in rather general systems
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