673 research outputs found
Understanding the suppression of structure formation from dark matter-dark energy momentum coupling
Models in which scalar field dark energy interacts with dark matter via a pure momentum coupling have previously been found to potentially ease the structure formation tension between early-and late-universe observations. In this article we explore the physical mechanism underlying this feature. We argue analytically that the perturbation growth equations imply the suppression of structure growth, illustrating our discussion with numerical calculations. Then we generalise the previously studied quadratic coupling between the dark energy and dark matter to a more general power law case, also allowing for the slope of the dark energy exponential potential to vary. We find that the structure growth suppression is a generic feature of power law couplings and it can, for a range of parameter values, be larger than previously found
Cosmic strings from pseudo-anomalous Fayet-Iliopoulos U(1) in D3/D7 brane inflation
We examine the consequences of recent developments on Fayet-Iliopoulos (FI)
terms for D-term inflationary models. There is currently no known way to couple
constant FI terms to supergravity consistently; only field-dependent FI terms
are allowed. These are natural in string theory and we argue that the FI term
in D3/D7 inflation turns out to be of this type, corresponding to a
pseudo-anomalous U(1). T he anomaly is canceled by the Green-Schwarz mechanism
in 4 dimensions. Inflation proceeds as usual, except that the scale is set by
the GS parameter. Cosmic strings resulting from a pseudo-anomalous U(1) have
potentially interesting characteristics. Originally expected to be global, they
turn out to be local in the string theory context and can support currents. We
outline the nature of these strings, discuss bounds on their formation, and
summarize resulting cosmological consequences.Comment: 10 pages; minor changes to match published versio
A study on the interacting Ricci dark energy in gravity
The present work reports study on the interacting Ricci dark energy in a
modified gravity theory named gravity. The specific model (proposed by R. Myrzakulov, arXiv:1205.5266v2) is considered here. For
this model we have observed a quintom-like behavior of the equation of state
(EoS) parameter and a transition from matter dominated to dark energy density
has been observed through fraction density evolution. The statefinder
parameters reveal that the model interpolates between dust and CDM
phases of the universe.Comment: 12 pages, 5 figure
Ultraviolet singularities in classical brane theory
We construct for the first time an energy-momentum tensor for the
electromagnetic field of a p-brane in arbitrary dimensions, entailing finite
energy-momentum integrals. The construction relies on distribution theory and
is based on a Lorentz-invariant regularization, followed by the subtraction of
divergent and finite counterterms supported on the brane. The resulting
energy-momentum tensor turns out to be uniquely determined. We perform the
construction explicitly for a generic flat brane. For a brane in arbitrary
motion our approach provides a new paradigm for the derivation of the,
otherwise divergent, self-force of the brane. The so derived self-force is
automatically finite and guarantees, by construction, energy-momentum
conservation.Comment: 41 pages, no figures, minor change
Quintessence-the Dark Energy in the Universe?
Quintessence - the energy density of a slowly evolving scalar field - may
constitute a dynamical form of the homogeneous dark energy in the universe. We
review the basic idea and indicate observational tests which may distinguish
quintessence from a cosmological constant.Comment: 13 pages, 3 figures, LaTe
A four-dimensional {\Lambda}CDM-type cosmological model induced from higher dimensions using a kinematical constraint
A class of cosmological solutions of higher dimensional Einstein field
equations with the energy-momentum tensor of a homogeneous, isotropic fluid as
the source are considered with an anisotropic metric that includes the direct
sum of a 3-dimensional (physical, flat) external space metric and an
n-dimensional (compact, flat) internal space metric. A simple kinematical
constraint is postulated that correlates the expansion rates of the external
and internal spaces in terms of a real parameter {\lambda}. A specific solution
for which both the external and internal spaces expand at different rates is
given analytically for n=3. Assuming that the internal dimensions were at
Planck length scales when the external space starts with a Big Bang (t=0), they
expand only 1.49 times and stay at Planck length scales even in the present age
of the universe (13.7 Gyr). The effective four dimensional universe would
exhibit a behavior consistent with our current understanding of the observed
universe. It would start in a stiff fluid dominated phase and evolve through
radiation dominated and pressureless matter dominated phases, eventually going
into a de Sitter phase at late times.Comment: 12 pages, 8 figures; matches the version published in General
Relativity and Gravitatio
Optimal control models of the goal-oriented human locomotion
In recent papers it has been suggested that human locomotion may be modeled
as an inverse optimal control problem. In this paradigm, the trajectories are
assumed to be solutions of an optimal control problem that has to be
determined. We discuss the modeling of both the dynamical system and the cost
to be minimized, and we analyze the corresponding optimal synthesis. The main
results describe the asymptotic behavior of the optimal trajectories as the
target point goes to infinity
Kahler Moduli Inflation Revisited
We perform a detailed numerical analysis of inflationary solutions in Kahler
moduli of type IIB flux compactifications. We show that there are inflationary
solutions even when all the fields play an important role in the overall shape
of the scalar potential. Moreover, there exists a direction of attraction for
the inflationary trajectories that correspond to the constant volume direction.
This basin of attraction enables the system to have an island of stability in
the set of initial conditions. We provide explicit examples of these
trajectories, compute the corresponding tilt of the density perturbations power
spectrum and show that they provide a robust prediction of n_s approximately
0.96 for 60 e-folds of inflation.Comment: 27 pages, 9 figure
Some Recent Developments on Kink Collisions and Related Topics
We review recent works on modeling of dynamics of kinks in 1+1 dimensional
theory and other related models, like sine-Gordon model or
theory. We discuss how the spectral structure of small perturbations can affect
the dynamics of non-perturbative states, such as kinks or oscillons. We
describe different mechanisms, which may lead to the occurrence of the resonant
structure in the kink-antikink collisions. We explain the origin of the
radiation pressure mechanism, in particular, the appearance of the negative
radiation pressure in the and models. We also show that the
process of production of the kink-antikink pairs, induced by radiation is
chaotic.Comment: 26 pages, 9 figures; invited chapter to "A dynamical perspective on
the {\phi}4 model: Past, present and future", Eds. P.G. Kevrekidis and J.
Cuevas-Maraver; Springer book class with svmult.cls include
Inflation, moduli (de)stabilization and supersymmetry breaking
We study the cosmological inflation from the viewpoint of the moduli
stabilization. We study the scenario that the superpotential has a large value
during the inflation era enough to stabilize moduli, but it is small in the
true vacuum. This scenario is discussed by using a simple model, one type of
hybrid models.Comment: 17 pages, 7 figure
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