13,238 research outputs found
Cosmic Constraint to DGP Brane Model: Geometrical and Dynamical Perspectives
In this paper, the Dvali-Gabadadze-Porrati (DGP) brane model is confronted by
current cosmic observational data sets from geometrical and dynamical
perspectives. On the geometrical side, the recent released Union2 of type
Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital
Sky Survey and the Two Degree Galaxy Redshift Survey (transverse and radial to
line-of-sight data points), the cosmic microwave background (CMB) measurement
given by the seven-year Wilkinson Microwave Anisotropy Probe observations
(shift parameters , and redshift at the last scatter surface
), ages of high redshifts galaxies, i.e. the lookback time (LT) and the
high redshift Gamma Ray Bursts (GRBs) are used. On the dynamical side, data
points about the growth function (GF) of matter linear perturbations are used.
Using the same data sets combination, we also constrain the flat CDM
model as a comparison. The results show that current geometrical and dynamical
observational data sets much favor flat CDM model and the departure
from it is above () for spatially flat DGP model
with(without) SN systematic errors. The consistence of growth function data
points is checked in terms of relative departure of redshift-distance relation.Comment: 14 pages, 5 figures, 2 tables, accepted for publication in Physical
Review
A study of the application of singular perturbation theory
A hierarchical real time algorithm for optimal three dimensional control of aircraft is described. Systematic methods are developed for real time computation of nonlinear feedback controls by means of singular perturbation theory. The results are applied to a six state, three control variable, point mass model of an F-4 aircraft. Nonlinear feedback laws are presented for computing the optimal control of throttle, bank angle, and angle of attack. Real Time capability is assessed on a TI 9900 microcomputer. The breakdown of the singular perturbation approximation near the terminal point is examined Continuation methods are examined to obtain exact optimal trajectories starting from the singular perturbation solutions
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.
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
Probing the Coupling between Dark Components of the Universe
We place observational constraints on a coupling between dark energy and dark
matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the
five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB)
shift parameter from the three-year Wilkinson Microwave Anisotropy Probe
(WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan
Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling
delta and (ii) varying coupling delta(z) that depends on a redshift z, both of
which have simple parametrizations of the Hubble parameter to confront with
observational data. We find that the combination of the three databases
marginalized over a present dark energy density gives stringent constraints on
the coupling, -0.08 < delta < 0.03 (95% CL) in the constant coupling model and
-0.4 < delta_0 < 0.1 (95% CL) in the varying coupling model, where delta_0 is a
present value. The uncoupled LambdaCDM model (w_X = -1 and delta = 0) still
remains a good fit to the data, but the negative coupling (delta < 0) with the
equation of state of dark energy w_X < -1 is slightly favoured over the
LambdaCDM model.Comment: 9 pages, 7 figures, RevTeX, minor corrections, references added,
accepted for publication in Phys. Rev.
Observational constraints on interacting quintessence models
We determine the range of parameter space of Interacting Quintessence Models
that best fits the recent WMAP measurements of Cosmic Microwave Background
temperature anisotropies. We only consider cosmological models with zero
spatial curvature. We show that if the quintessence scalar field decays into
cold dark matter at a rate that brings the ratio of matter to dark energy
constant at late times,the cosmological parameters required to fit the CMB data
are: \Omega_x = 0.43 \pm 0.12, baryon fraction \Omega_b = 0.08 \pm 0.01, slope
of the matter power spectrum at large scals n_s = 0.98 \pm 0.02 and Hubble
constant H_0 = 56 \pm 4 km/s/Mpc. The data prefers a dark energy component with
a dimensionless decay parameter c^2 =0.005 and non-interacting models are
consistent with the data only at the 99% confidence level. Using the Bayesian
Information Criteria we show that this exra parameter fits the data better than
models with no interaction. The quintessence equation of state parameter is
less constrained; i.e., the data set an upper limit w_x \leq -0.86 at the same
level of significance. When the WMAP anisotropy data are combined with
supernovae data, the density parameter of dark energy increases to \Omega_x
\simeq 0.68 while c^2 augments to 6.3 \times 10^{-3}. Models with quintessence
decaying into dark matter provide a clean explanation for the coincidence
problem and are a viable cosmological model, compatible with observations of
the CMB, with testable predictions. Accurate measurements of baryon fraction
and/or of matter density independent of the CMB data, would support/disprove
these models.Comment: 16 pages, Revtex4, 5 eps figures, to appear in Physical Review
Boundary fields and renormalization group flow in the two-matrix model
We analyze the Ising model on a random surface with a boundary magnetic field
using matrix model techniques. We are able to exactly calculate the disk
amplitude, boundary magnetization and bulk magnetization in the presence of a
boundary field. The results of these calculations can be interpreted in terms
of renormalization group flow induced by the boundary operator. In the
continuum limit this RG flow corresponds to the flow from non-conformal to
conformal boundary conditions which has recently been studied in flat space
theories.Comment: 31 pages, Late
Solution of the dispersionless Hirota equations
The dispersionless differential Fay identity is shown to be equivalent to a
kernel expansion providing a universal algebraic characterization and solution
of the dispersionless Hirota equations. Some calculations based on D-bar data
of the action are also indicated.Comment: Late
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