15,207 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.
Toward solving the cosmological constant problem by embedding
The typical scalar field theory has a cosmological constant problem. We
propose a generic mechanism by which this problem is avoided at tree level by
embedding the theory into a larger theory. The metric and the scalar field
coupling constants in the original theory do not need to be fine-tuned, while
the extra scalar field parameters and the metric associated with the extended
theory are fine-tuned dynamically. Hence, no fine-tuning of parameters in the
full Lagrangian is needed for the vacuum energy in the new physical system to
vanish at tree level. The cosmological constant problem can be solved if the
method can be extended to quantum loops.Comment: published versio
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
Cosmological coincidence problem in interacting dark energy models
An interacting dark energy model with interaction term is considered. By studying the model near the
transition time, in which the system crosses the w=-1 phantom-divide-line, the
conditions needed to overcome the coincidence problem is investigated. The
phantom model, as a candidate for dark energy, is considered and for two
specific examples, the quadratic and exponential phantom potentials, it is
shown that it is possible the system crosses the w=-1 line, meanwhile the
coincidence problem is alleviated, the two facts that have root in
observations.Comment: 15 pages, LaTeX. Some minor explanations are added. To be published
in Phys. Rev.
Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly
There are evidences that the cosmic microwave background (CMB) large-angle
anomalies imply a departure from statistical isotropy and hence from the
standard cosmological model. We propose a LCDM model extension whose dark
energy component preserves its nondynamical character but wield anisotropic
vacuum pressure. Exact solutions for the cosmological scale factors are
presented, upper bounds for the deformation parameter are evaluated and its
value is estimated considering the elliptical universe proposal to solve the
quadrupole anomaly. This model can be constructed from a Bianchi I cosmology
with cosmological constant from two different ways: i) a straightforward
anisotropic modification of the vacuum pressure consistently with
energy-momentum conservation; ii) a Poisson structure deformation between
canonical momenta such that the dynamics remain invariant under scale factors
rescalings.Comment: 8 pages, 2 columns, 1 figure. v2: figure improved, added comments on
higher eccentricity powers and references. v3: typos corrected, version to
appear in PR
New Interaction between Dark Energy and Dark Matter Changes Sign during Cosmological Evolution
It is found by Cai and Su that the interaction between dark energy and cold
dark matter is likely to change the sign during the cosmological evolution.
Motivated by this, we suggest a new form of interaction between dark energy and
dark matter, which changes from negative to positive as the expansion of our
universe changes from decelerated to accelerated. We find that the interacting
model is consistent with the second law of thermodynamics and the observational
constraints. And, we also discuss the unified adiabatic-squared sound speed of
the model.Comment: 16 pages, 3 figure, 1 table. Final version in PR
Bose-enhanced chemistry: Amplification of selectivity in the dissociation of molecular Bose-Einstein condensates
We study the photodissociation chemistry of a quantum degenerate gas of
bosonic triatomic molecules, assuming two open rearrangement channels
( or ). The equations of motion are equivalent to those of a
parametric multimode laser, resulting in an exponential buildup of macroscopic
mode populations. By exponentially amplifying a small differential in the
single-particle rate-coefficients, Bose stimulation leads to a nearly complete
selectivity of the collective -body process, indicating a novel type of
ultra-selective quantum degenerate chemistry.Comment: 5 pages, 3 figure
Density perturbations in f(R) gravity theories in metric and Palatini formalisms
We make a detailed study of matter density perturbations in both metric and
Palatini formalisms in theories whose Lagrangian density is a general function,
f(R), of the Ricci scalar. We derive these equations in a number of gauges. We
show that for viable models that satisfy cosmological and local gravity
constraints (LGC), matter perturbation equations derived under a sub-horizon
approximation are valid even for super-Hubble scales provided the oscillating
mode (scalaron) does not dominate over the matter-induced mode. Such
approximate equations are especially reliable in the Palatini formalism because
of the absence of scalarons.
Using these equations we make a comparative study of the behaviour of density
perturbations as well as gravitational potentials for a number of classes of
theories. In the metric formalism the parameter m=Rf_{,RR}/f_{,R}
characterising the deviation from the Lambda CDM model is constrained to be
very small during the matter era in order to ensure compatibility with LGC, but
the models in which m grows to the order of 10^{-1} around the present epoch
can be allowed. These models also suffer from an additional fine tuning due to
the presence of scalaron modes which are absent in the Palatini case.
In Palatini formalism LGC and background cosmological constraints provide
only weak bounds on |m| by constraining it to be smaller than ~ 0.1. This is in
contrast to matter density perturbations which, on galactic scales, place far
more stringent constraints on the present deviation parameter m of the order of
|m| < 10^{-5} - 10^{-4}. This is due to the peculiar evolution of matter
perturbations in the Palatini case which exhibits a rapid growth or a damped
oscillation depending on the sign of m.Comment: 36 pages including 8 figures. Accepted for publication in Physical
Review
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
Cosmic Acceleration Data and Bulk-Brane Energy Exchange
We consider a braneworld model with bulk-brane energy exchange. This allows
for crossing of the w=-1 phantom divide line without introducing phantom energy
with quantum instabilities. We use the latest SnIa data included in the Gold06
dataset to provide an estimate of the preferred parameter values of this
braneworld model. We use three fitting approaches which provide best fit
parameter values and hint towards a bulk energy component that behaves like
relativistic matter which is propagating in the bulk and is moving at a speed v
along the fifth dimension, while the bulk-brane energy exchange component
corresponds to negative pressure and signifies energy flowing from the bulk
into the brane. We find that the best fit effective equation of state parameter
marginally crosses the phantom divide line w=-1. Thus, we have
demonstrated both the ability of this class of braneworld models to provide
crossing of the phantom divide and also that cosmological data hint towards
natural values for the model parameters.Comment: 12 pages, 2 figures, added comments, references update
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