1,363 research outputs found
A detailed analysis of structure growth in theories of gravity
We investigate the connection between dark energy and fourth order gravity by
analyzing the behavior of scalar perturbations around a
Friedmann-Robertson-Walker background. The evolution equations for scalar
perturbation are derived using the covariant and gauge invariant approach and
applied to two widely studied gravity models. The structure of the
general fourth order perturbation equations and the analysis of scalar
perturbations lead to the discovery of a characteristic signature of fourth
order gravity in the matter power spectrum, the details of which have not seen
before in other works in this area. This could provide a crucial test for
fourth order gravity on cosmological scales.Comment: 27 pages and 35 figure
Symbolic Implementation of Connectors in BIP
BIP is a component framework for constructing systems by superposing three
layers of modeling: Behavior, Interaction, and Priority. Behavior is
represented by labeled transition systems communicating through ports.
Interactions are sets of ports. A synchronization between components is
possible through the interactions specified by a set of connectors. When
several interactions are possible, priorities allow to restrict the
non-determinism by choosing an interaction, which is maximal according to some
given strict partial order.
The BIP component framework has been implemented in a language and a
tool-set. The execution of a BIP program is driven by a dedicated engine, which
has access to the set of connectors and priority model of the program. A key
performance issue is the computation of the set of possible interactions of the
BIP program from a given state.
Currently, the choice of the interaction to be executed involves a costly
exploration of enumerative representations for connectors. This leads to a
considerable overhead in execution times. In this paper, we propose a symbolic
implementation of the execution model of BIP, which drastically reduces this
overhead. The symbolic implementation is based on computing boolean
representation for components, connectors, and priorities with an existing BDD
package
Cosmo-dynamics and dark energy with a quadratic EoS: anisotropic models, large-scale perturbations and cosmological singularities
In general relativity, for fluids with a linear equation of state (EoS) or
scalar fields, the high isotropy of the universe requires special initial
conditions, and singularities are anisotropic in general. In the brane world
scenario anisotropy at the singularity is suppressed by an effective quadratic
equation of state. There is no reason why the effective EoS of matter should be
linear at the highest energies, and a non-linear EoS may describe dark energy
or unified dark matter (Paper I, astro-ph/0512224). In view of this, here we
study the effects of a quadratic EoS in homogenous and inhomogeneous
cosmological models in general relativity, in order to understand if in this
context the quadratic EoS can isotropize the universe at early times. With
respect to Paper I, here we use the simplified EoS P=alpha rho + rho^2/rho_c,
which still allows for an effective cosmological constant and phantom behavior,
and is general enough to analyze the dynamics at high energies. We first study
anisotropic Bianchi I and V models, focusing on singularities. Using dynamical
systems methods, we find the fixed points of the system and study their
stability. We find that models with standard non-phantom behavior are in
general asymptotic in the past to an isotropic fixed point IS, i.e. in these
models even an arbitrarily large anisotropy is suppressed in the past: the
singularity is matter dominated. Using covariant and gauge invariant variables,
we then study linear perturbations about the homogenous and isotropic spatially
flat models with a quadratic EoS. We find that, in the large scale limit, all
perturbations decay asymptotically in the past, indicating that the isotropic
fixed point IS is the general asymptotic past attractor for non phantom
inhomogeneous models with a quadratic EoS. (Abridged)Comment: 16 pages, 6 figure
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