13,331 research outputs found
Vector field theories in cosmology
Recently proposed theories based on the cosmic presence of a vectorial field
are compared and contrasted. In particular the so called Einstein aether theory
is discussed in parallel with a recent proposal of a strained space-time theory
(Cosmic Defect theory). We show that the latter fits reasonably well the cosmic
observed data with only one, or at most two, adjustable parameters, whilst
other vector theories use much more. The Newtonian limits are also compared.
Finally we show that the CD theory may be considered as a special case of the
aether theories, corresponding to a more compact and consistent paradigm.Comment: 19 pages, 1 figure, to appear on Phys. Rev.
Observational signatures of f(R) dark energy models that satisfy cosmological and local gravity constraints
We discuss observational consequences of f(R) dark energy scenarios that
satisfy local gravity constraints (LGC) as well as conditions of the
cosmological viability. The model we study is given by m(r)=C(-r-1)^p (C>0,
p>1) with m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f, which cover viable f(R) models
proposed so far in a high-curvature region designed to be compatible with LGC.
The equation of state of dark energy exhibits a divergence at a redshift z_c
that can be as close as a few while satisfying sound horizon constraints of
Cosmic Microwave Background (CMB). We study the evolution of matter density
perturbations in details and place constraints on model parameters from the
difference of spectral indices of power spectra between CMB and galaxy
clustering. The models with p>5 can be consistent with those observational
constraints as well as LGC. We also discuss the evolution of perturbations in
the Ricci scalar R and show that an oscillating mode (scalaron) can easily
dominate over a matter-induced mode as we go back to the past. This violates
the stability of cosmological solutions, thus posing a problem about how the
over-production of scalarons should be avoided in the early universe.Comment: 13 pages, 7 figures, version to appear in Physical Review
Electromagnetic Fields of Slowly Rotating Magnetized Gravastars
We study the dipolar magnetic field configuration and present solutions of
Maxwell equations in the internal background spacetime of a a slowly rotating
gravastar. The shell of gravastar where magnetic field penetrated is modeled as
sphere consisting of perfect highly magnetized fluid with infinite
conductivity. Dipolar magnetic field of the gravastar is produced by a circular
current loop symmetrically placed at radius at the equatorial plane.Comment: 5 pages, 2 figures, accepted for publication to Mod. Phys. Lett.
Noncommutative Field Theory and Lorentz Violation
The role of Lorentz symmetry in noncommutative field theory is considered.
Any realistic noncommutative theory is found to be physically equivalent to a
subset of a general Lorentz-violating standard-model extension involving
ordinary fields. Some theoretical consequences are discussed. Existing
experiments bound the scale of the noncommutativity parameter to (10 TeV)^{-2}.Comment: 4 page
Yang-Mills Instantons with Lorentz Violation
An analysis is performed of instanton configurations in pure Euclidean
Yang-Mills theory containing small Lorentz-violating perturbations that
maintain gauge invariance. Conventional topological arguments are used to show
that the general classification of instanton solutions involving the
topological charge is the same as in the standard case. Explicit solutions are
constructed for general gauge invariant corrections to the action that are
quadratic in the curvature. The value of the action is found to be unperturbed
to lowest order in the Lorentz-violating parameters.Comment: 16 page
Dynamical vacuum energy, holographic quintom, and the reconstruction of scalar-field dark energy
When taking the holographic principle into account, the vacuum energy will
acquire dynamical property that its equation of state is evolving. The current
available observational data imply that the holographic vacuum energy behaves
as quintom-type dark energy. We adopt the viewpoint of that the scalar field
models of dark energy are effective theories of an underlying theory of dark
energy. If we regard the scalar field model as an effective description of such
a holographic vacuum theory, we should be capable of using the scalar field
model to mimic the evolving behavior of the dynamical vacuum energy and
reconstructing this scalar field model according to the fits of the
observational dataset. We find the generalized ghost condensate model is a good
choice for depicting the holographic vacuum energy since it can easily realize
the quintom behavior. We thus reconstruct the function of the
generalized ghost condensate model using the best-fit results of the
observational data.Comment: 13 pages, 3 figures; references updated, accepted for publication in
Phys. Rev.
Dual interacting cosmologies and late accelerated expansion
In this paper we show that by considering a universe dominated by two
interacting components a superaccelerated expansion can be obtained from a
decelerated one by applying a dual transformation that leaves the Einstein's
field equations invariant.Comment: 13 pages, 1 figura, version to match published articl
Adiabatic population transfer via multiple intermediate states
This paper discusses a generalization of stimulated Raman adiabatic passage
(STIRAP) in which the single intermediate state is replaced by intermediate
states. Each of these states is connected to the initial state \state{i} with
a coupling proportional to the pump pulse and to the final state \state{f}
with a coupling proportional to the Stokes pulse, thus forming a parallel
multi- system. It is shown that the dark (trapped) state exists only
when the ratio between each pump coupling and the respective Stokes coupling is
the same for all intermediate states. We derive the conditions for existence of
a more general adiabatic-transfer state which includes transient contributions
from the intermediate states but still transfers the population from state
\state{i} to state \state{f} in the adiabatic limit. We present various
numerical examples for success and failure of multi- STIRAP which
illustrate the analytic predictions. Our results suggest that in the general
case of arbitrary couplings, it is most appropriate to tune the pump and Stokes
lasers either just below or just above all intermediate states.Comment: 14 pages, two-column revtex style, 10 figure
Solitary and compact-like shear waves in the bulk of solids
We show that a model proposed by Rubin, Rosenau, and Gottlieb [J. Appl. Phys.
77 (1995) 4054], for dispersion caused by an inherent material characteristic
length, belongs to the class of simple materials. Therefore, it is possible to
generalize the idea of Rubin, Rosenau, and Gottlieb to include a wide range of
material models, from nonlinear elasticity to turbulence. Using this insight,
we are able to fine-tune nonlinear and dispersive effects in the theory of
nonlinear elasticity in order to generate pulse solitary waves and also bulk
travelling waves with compact support
Spacetime-varying couplings and Lorentz violation
Spacetime-varying coupling constants can be associated with violations of
local Lorentz invariance and CPT symmetry. An analytical supergravity cosmology
with time-varying fine-structure constant provides an explicit example.
Estimates are made for some experimental constraints.Comment: 4 page
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