261 research outputs found
A note on the equivalence of a barotropic perfect fluid with a K-essence scalar field
In this short note, we obtain the necessary and sufficient condition for a
class of non-canonical single scalar field models to be exactly equivalent to
barotropic perfect fluids, under the assumption of an irrotational fluid flow.
An immediate consequence of this result is that the non-adiabatic pressure
perturbation in this class of scalar field systems vanishes exactly at all
orders in perturbation theory and on all scales. The Lagrangian for this
general class of scalar field models depends on both the kinetic term and the
value of the field. However, after a field redefinition, it can be effectively
cast in the form of a purely kinetic K-essence model.Comment: 4 pages, 1 figure; v2: References and footnotes 3 and 4 added.
Replaced to match published versio
CMB statistical anisotropy from noncommutative gravitational waves
Primordial statistical anisotropy is a key indicator to investigate early
Universe models and has been probed by the cosmic microwave background (CMB)
anisotropies. In this paper, we examine tensor-mode CMB fluctuations generated
from anisotropic gravitational waves, parametrised by , where
is the usual scale-invariant power spectrum. Such anisotropic
tensor fluctuations may arise from an inflationary model with noncommutativity
of fields. It is verified that in this model, an isotropic component and a
quadrupole asymmetry with are created and
hence highly red-tilted off-diagonal components arise in the CMB power spectra,
namely in , , and , and in and . We find that B-mode polarisation is more
sensitive to such signals than temperature and E-mode polarisation due to the
smallness of large-scale cosmic variance and we can potentially measure and at 68% CL in a cosmic-variance-limited experiment. Such
a level of signal may be measured in a PRISM like experiment, while the
instrumental noise contaminates it in the experiment. These results
imply that it is impossible to measure the noncommutative parameter if it is
small enough for the perturbative treatment to be valid. Our formalism and
methodology for dealing with the CMB tensor statistical anisotropy are general
and straightforwardly applicable to other early Universe models.Comment: 13 pages, 2 figures, 1 table. Accepted for publication in JCA
Living with ghosts in Horava-Lifshitz gravity
We consider the branch of the projectable Horava-Lifshitz model which
exhibits ghost instabilities in the low energy limit. It turns out that, due to
the Lorentz violating structure of the model and to the presence of a finite
strong coupling scale, the vacuum decay rate into photons is tiny in a wide
range of phenomenologically acceptable parameters. The strong coupling scale,
understood as a cutoff on ghosts' spatial momenta, can be raised up to TeV. At lower momenta, the projectable Horava-Lifshitz gravity is
equivalent to General Relativity supplemented by a fluid with a small positive
sound speed squared () , that could
be a promising candidate for the Dark Matter. Despite these advantages, the
unavoidable presence of the strong coupling obscures the implementation of the
original Horava's proposal on quantum gravity. Apart from the Horava-Lifshitz
model, conclusions of the present work hold also for the mimetic matter
scenario, where the analogue of the projectability condition is achieved by a
non-invertible conformal transformation of the metric.Comment: 33 pages, 1 figure. The proof of an equivalence between the IR limit
of the projectable Horava-Lifshitz gravity and the mimetic matter scenario is
given in Appendix A. Version accepted for publication in JHE
Biodegradation kinetics of winery wastewater from Port Wine production
Winery wastewaters are characterised by seasonality and variable volume and organic load. This fact together with their high content of biodegradable compounds often results in problems in the operation of biological systems as they may lead to poor sludge settleability, floc disintegration and increased presence of solids in the treated effluent. Biodegradation of winery wastewater from a Port Wine production industry was studied in aerobic batch assays, varying substrate and biomass concentrations. More than 90 % of COD was removed in all cases, in a short period when biomass concentration was higher than 3 g VSS L–1. Data was correlated to several kinetic models, and Haldane model best fitted the experimental data, particularly for lower biomass concentrations. Therefore, an initial high biomass concentration should be present in aerobic treatment of winery wastewater, in order to cope with the large fluctuations in their organic loads. These batch assays are valuable for winery wastewater treatment, as they may simulate typical start-ups after short and long shutdown periods often observed in the winery industry.publishe
Dynamics of intersecting brane systems -- Classification and their applications --
We present dynamical intersecting brane solutions in higher-dimensional
gravitational theory coupled to dilaton and several forms. Assuming the forms
of metric, form fields, and dilaton field, we give a complete classification of
dynamical intersecting brane solutions with/without M-waves and Kaluza-Klein
monopoles in eleven-dimensional supergravity. We apply these solutions to
cosmology and black holes. It is shown that these give FRW cosmological
solutions and in some cases Lorentz invariance is broken in our world. If we
regard the bulk space as our universe, we may interpret them as black holes in
the expanding universe. We also discuss lower-dimensional effective theories
and point out naive effective theories may give us some solutions which are
inconsistent with the higher-dimensional Einstein equations.Comment: 44 pages; v2: minor corrections, references adde
Pathological behaviour of the scalar graviton in Ho\v{r}ava-Lifshitz gravity
We confirm the recent claims that, in the infrared limit of
Ho\v{r}ava-Lifshitz gravity, the scalar graviton becomes a ghost if the sound
speed squared is positive on the flat de Sitter and Minkowski background. In
order to avoid the ghost and tame the instability, the sound speed squared
should be negative and very small, which means that the flow parameter
should be very close to its General Relativity (GR) value. We
calculate the cubic interactions for the scalar graviton which are shown to
have a similar structure with those of the curvature perturbation in
k-inflation models. The higher order interactions become increasing important
for a smaller sound speed squared, that is, when the theory approaches GR. This
invalidates any linearized analysis and any predictability is lost in this
limit as quantum corrections are not controllable. This pathological behaviour
of the scalar graviton casts doubt on the validity of the projectable version
of the theory.Comment: 7 pages, references added; v3: Typos corrected, minor changes to text
and precise determination of the strong coupling scale. Replaced to match
published version
Planck 2018 results IX. Constraints on primordial non-Gaussianity
We analyse the Planck full-mission cosmic microwave background (CMB) temperature and E-mode polarization maps to obtain constraints on primordial non-Gaussianity (NG). We compare estimates obtained from separable template-fitting, binned, and optimal modal bispectrum estimators, finding consistent values for the local, equilateral, and orthogonal bispectrum amplitudes. Our combined temperature and polarization analysis produces the following final results: fNLlocal = −0.9 ± 5.1; fNLequil = −26 ± 47; and fNLortho = −38 ± 24 (68% CL, statistical)
Uniqueness of the gauge invariant action for cosmological perturbations
In second order perturbation theory different definitions are known of gauge
invariant perturbations in single field inflationary models. Consequently the
corresponding gauge invariant cubic actions do not have the same form. Here we
show that the cubic action for one choice of gauge invariant variables is
unique in the following sense: the action for any other, non-linearly related
variable can be brought to the same bulk action, plus additional boundary
terms. These boundary terms correspond to the choice of hypersurface and
generate extra, disconnected contributions to the bispectrum. We also discuss
uniqueness of the action with respect to conformal frames. When expressed in
terms of the gauge invariant curvature perturbation on uniform field
hypersurfaces the action for cosmological perturbations has a unique form,
independent of the original Einstein or Jordan frame. Crucial is that the gauge
invariant comoving curvature perturbation is frame independent, which makes it
extremely helpful in showing the quantum equivalence of the two frames, and
therefore in calculating quantum effects in nonminimally coupled theories such
as Higss inflation.Comment: 27 page
Gauge-invariant perturbations at second order in two-field inflation
We study the second-order gauge-invariant adiabatic and isocurvature
perturbations in terms of the scalar fields present during inflation, along
with the related fully non-linear space gradient of these quantities. We
discuss the relation with other perturbation quantities defined in the
literature. We also construct the exact cubic action of the second-order
perturbations (beyond any slow-roll or super-horizon approximations and
including tensor perturbations), both in the uniform energy density gauge and
the flat gauge in order to settle various gauge-related issues. We thus provide
the tool to calculate the exact non-Gaussianity beyond slow-roll and at any
scale.Comment: 28 pages, no figures. v2: Added a summary subsection 4.3 with further
discussion of the results. Generalized all super-horizon results of section 4
and appendix A to exact ones. Other minor textual changes and references
added. Conclusions unchanged. Matches published versio
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