6,684 research outputs found
CMB statistics in noncommutative inflation
Noncommutative geometry can provide effective description of physics at very
short distances taking into account generic effects of quantum gravity.
Inflation amplifies tiny quantum fluctuations in the early universe to
macroscopic scales and may thus imprint high energy physics signatures in the
cosmological perturbations that could be detected in the CMB. It is shown here
that this can give rise to parity-violating modulations of the primordial
spectrum and odd non-Gaussian signatures. The breaking of rotational invariance
of the CMB provides constraints on the scale of noncommutativity that are
competitive with the existing noncosmological bounds, and could explain the
curious hemispherical asymmetry that has been claimed to be observed in the
sky. This introduces also non-Gaussianity with peculiar shape- and
scale-dependence, which in principle allows an independent cross-check of the
presence of noncommutativity at inflation.Comment: 9 pages, no figure
DBI Galileons in the Einstein Frame: Local Gravity and Cosmology
It is shown that a disformally coupled theory in which the gravitational
sector has the Einstein-Hilbert form is equivalent to a quartic DBI Galileon
Lagrangian, possessing non-linear higher derivative interactions, and hence
allowing for the Vainshtein effect. This Einstein Frame description
considerably simplifies the dynamical equations and highlights the role of the
different terms. The study of highly dense, non-relativistic environments
within this description unravels the existence of a disformal screening
mechanism, while the study of static vacuum configurations reveals the
existence of a Vainshtein radius, at which the asymptotic solution breaks down.
Disformal couplings to matter also allow the construction of Dark Energy
models, which behave differently than conformally coupled ones and introduce
new effects on the growth of Large Scale Structure over cosmological scales, on
which the scalar force is not screened. We consider a simple Disformally
Coupled Dark Matter model in detail, in which standard model particles follow
geodesics of the gravitational metric and only Dark Matter is affected by the
disformal scalar field. This particular model is not compatible with
observations in the linearly perturbed regime. Nonetheless, disformally coupled
theories offer enough freedom to construct realistic cosmological scenarios,
which can be distinguished from the standard model through characteristic
signatures.Comment: Discussion on the Vainshtein effect added. 25 pages, 6 figures, 2
tables. Accepted for publication in PR
Degeneracies between Modified Gravity and Baryonic Physics
In order to determine the observable signatures of modified gravity theories,
it is important to consider the effect of baryonic physics. We use a modified
version of the ISIS code to run cosmological hydrodynamic simulations to study
degeneracies between modified gravity and radiative hydrodynamical processes.
Of these, one was the standard CDM model and four were variations of
the Symmetron model. For each model we ran three variations of baryonic
processes: non-radiative hydrodynamics; cooling and star formation; and
cooling, star formation, and supernova feedback. We construct stacked gas
density, temperature, and dark matter density profiles of the halos in the
simulations, and study the differences between them. We find that both
radiative variations of the models show degeneracies between their processes
and at least two of the three parameters defining the Symmetron model.Comment: 9 pages, 4 figures, matches version accepted to A&
An analytic model for the transition from decelerated to accelerated cosmic expansion
We consider the scenario where our observable universe is devised as a
dynamical four-dimensional hypersurface embedded in a five-dimensional bulk
spacetime, with a large extra dimension, which is the {\it generalization of
the flat FRW cosmological metric to five dimensions}. This scenario generates a
simple analytical model where different stages of the evolution of the universe
are approximated by distinct parameterizations of the {\it same} spacetime. In
this model the evolution from decelerated to accelerated expansion can be
interpreted as a "first-order" phase transition between two successive stages.
The dominant energy condition allows different parts of the universe to evolve,
from deceleration to acceleration, at different redshifts within a narrow era.
This picture corresponds to the creation of bubbles of new phase, in the middle
of the old one, typical of first-order phase transitions. Taking today, we find that the cross-over from deceleration to acceleration
occurs at , regardless of the equation of state in the very
early universe. In the case of primordial radiation, the model predicts that
the deceleration parameter "jumps" from to at . At the present time and the equation of state of the
universe is , in agreement with observations and some
theoretical predictions.Comment: The abstract and introduction are improved and the discussion section
is expanded. A number of references are adde
Matter instabilities in general Gauss-Bonnet gravity
We study the evolution of cosmological perturbations in f(G) gravity, where
the Lagrangian is the sum of a Ricci scalar R and an arbitrary function f in
terms of a Gauss-Bonnet term G. We derive the equations for perturbations
assuming matter to be described by a perfect fluid with a constant equation of
state w. We show that density perturbations in perfect fluids exhibit negative
instabilities during both the radiation and the matter domination, irrespective
of the form of f(G). This growth of perturbations gets stronger on smaller
scales, which is difficult to be compatible with the observed galaxy spectrum
unless the deviation from General Relativity is very small. Thus f(G)
cosmological models are effectively ruled out from this Ultra-Violet
instability, even though they can be compatible with the late-time cosmic
acceleration and local gravity constraints.Comment: 9 pages, 2 figures, published PRD versio
A generalization of the S-function method applied to a Duffing-Van der Pol forced oscillator
In [1,2] we have developed a method (we call it the S-function method) that
is successful in treating certain classes of rational second order ordinary
differential equations (rational 2ODEs) that are particularly `resistant' to
canonical Lie methods and to Darbouxian approaches. In this present paper, we
generalize the S-function method making it capable of dealing with a class of
elementary 2ODEs presenting elementary functions. Then, we apply this method to
a Duffing-Van der Pol forced oscillator, obtaining an entire class of first
integrals
Magnetic breakdown in a normal-metal - superconductor proximity sandwich
We study the magnetic response of a clean normal-metal slab of finite
thickness in proximity with a bulk superconductor. We determine its free energy
and identify two (meta-)stable states, a diamagnetic one where the applied
field is effectively screened, and a second state, where the field penetrates
the normal-metal layer. We present a complete characterization of the first
order transition between the two states which occurs at the breakdown field,
including its spinodals, the jump in the magnetization, and the latent heat.
The bistable regime terminates at a critical temperature above which the sharp
transition is replaced by a continuous cross-over. We compare the theory with
experiments on normal-superconducting cylinders.Comment: 7 pages Revtex, 3 Postscript figures, needs psfig.te
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