1,030 research outputs found
Disformal invariance of continuous media with linear equation of state
We show that the effective theory describing single component continuous
media with a linear and constant equation of state of the form is
invariant under a 1-parameter family of continuous disformal transformations.
In the special case of (ultrarelativistic gas), such a family reduces
to conformal transformations. As examples, perfect fluids, homogeneous and
isotropic solids are discussed.Comment: latex, 7 page
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
Massive Gravity on a Brane
At present no theory of a massive graviton is known that is consistent with
experiments at both long and short distances. The problem is that consistency
with long distance experiments requires the graviton mass to be very small.
Such a small graviton mass however implies an ultraviolet cutoff for the theory
at length scales far larger than the millimeter scale at which gravity has
already been measured. In this paper we attempt to construct a model which
avoids this problem. We consider a brane world setup in warped AdS spacetime
and we investigate the consequences of writing a mass term for the graviton on
a the infrared brane where the local cutoff is of order a large (galactic)
distance scale. The advantage of this setup is that the low cutoff for physics
on the infrared brane does not significantly affect the predictivity of the
theory for observers localized on the ultraviolet brane. For such observers the
predictions of this theory agree with general relativity at distances smaller
than the infrared scale but go over to those of a theory of massive gravity at
longer distances. A careful analysis of the graviton two-point function,
however, reveals the presence of a ghost in the low energy spectrum. A mode
decomposition of the higher dimensional theory reveals that the ghost
corresponds to the radion field. We also investigate the theory with a brane
localized mass for the graviton on the ultraviolet brane, and show that the
physics of this case is similar to that of a conventional four dimensional
theory with a massive graviton, but with one important difference: when the
infrared brane decouples and the would-be massive graviton gets heavier than
the regular Kaluza--Klein modes, it becomes unstable and it has a finite width
to decay off the brane into the continuum of Kaluza-Klein states.Comment: 26 pages, LaTeX. v2: extended version with an appendix added about
non Fierz-Pauli mass terms. Few typos corrected. Final version appeared in
PR
Data analytics for profiling low‐voltage customers with smart meter readings
The energy transition for decarbonization requires consumers’ and producers’ active par-ticipation to give the power system the necessary flexibility to manage intermittency and non‐pro-grammability of renewable energy sources. The accurate knowledge of the energy demand of every single customer is crucial for accurately assessing their potential as flexibility providers. This topic gained terrific input from the widespread deployment of smart meters and the continuous development of data analytics and artificial intelligence. The paper proposes a new technique based on advanced data analytics to analyze the data registered by smart meters to associate to each customer a typical load profile (LP). Different LPs are assigned to low voltage (LV) customers belonging to nominal homogeneous category for overcoming the inaccuracy due to non‐existent coincident peaks, arising by the common use of a unique LP per category. The proposed methodology, starting from two large databases, constituted by tens of thousands of customers of different categories, clusters their consumption profiles to define new representative LPs, without a priori preferring a specific clustering technique but using that one that provides better results. The paper also proposes a method for associating the proper LP to new or not monitored customers, considering only few features easily available for the distribution systems operator (DSO)
Quasilocalized gravity without asymptotic flatness
We present a toy model of a generic five-dimensional warped geometry in which
the 4D graviton is not fully localized on the brane. Studying the tensor sector
of metric perturbation around this background, we find that its contribution to
the effective gravitational potential is of 4D type (1/r) at the intermediate
scales and that at the large scales it becomes 1/r^{1+alpha}, 0<alpha=< 1 being
a function of the parameters of the model (alpha=1 corresponds to the
asymptotically flat geometry). Large-distance behavior of the potential is
therefore not necessarily five-dimensional. Our analysis applies also to the
case of quasilocalized massless particles other than graviton.Comment: 9 pages, 1 figure; to be published in Phys. Rev.
UV slope of z3 bright () Lyman-break galaxies in the COSMOS field
We analyse a unique sample of 517 bright () LBGs at redshift
z3 in order to characterise the distribution of their UV slopes
and infer their dust extinction under standard assumptions. We exploited
multi-band observations over 750 arcmin of the COSMOS field that were
acquired with three different ground-based facilities: the Large Binocular
Camera (LBC) on the Large Binocular Telescope (LBT), the Suprime-Cam on the
SUBARU telescope, and the VIRCAM on the VISTA telescope (ULTRAVISTA DR2). Our
multi-band photometric catalogue is based on a new method that is designed to
maximise the signal-to-noise ratio in the estimate of accurate galaxy colours
from images with different point spread functions (PSF). We adopted an improved
selection criterion based on deep Y-band data to isolate a sample of galaxies
at to minimise selection biases. We measured the UV slopes ()
of the objects in our sample and then recovered the intrinsic probability
density function of values (PDF()), taking into account the
effect of observational uncertainties through detailed simulations. The
galaxies in our sample are characterised by mildly red UV slopes with
throughout the enitre luminosity range that is probed by
our data (). The resulting dust-corrected
star formation rate density (SFRD) is , corresponding to a contribution of about 25% to the
total SFRD at z3 under standard assumptions. Ultra-bright LBGs at match the known trends, with UV slopes being redder at decreasing redshifts,
and brighter galaxies being more highly dust extinct and more frequently
star-forming than fainter galaxies. [abridged]Comment: Matched to journal version. 11 pages, 13 figures, Astronomy &
Astrophysics in pres
Gauss-Bonnet gravity renders negative tension braneworlds unstable
We show that the Gauss-Bonnet correction to Einstein gravity induces a
gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the
Randall-Sundrum I model. We demonstrate that this instability is generically
related to the presence of a negative tension brane in the set-up, with or
without -symmetry across it. Indeed it is shown that the tachyon mode is a
bound state localised on any negative tension brane of co-dimension one,
embedded in anti-de Sitter background. We discuss the possible resolution of
this instability by the inclusion of induced gravity terms on the branes or by
an effective four-dimensional cosmological constant.Comment: published versio
Synergistic Gravity and the Role of Resonances in GRS-Inspired Braneworlds
We consider 5D braneworld models of quasi-localized gravity in which 4D
gravity is reproduced at intermediate scales while the extra dimension opens up
at both the very short and the very long distances, where the geometry is flat.
Our main interest is the interplay between the zero mode of these models,
whenever a normalizable zero mode exists, and the effects of zero energy
graviton resonant modes coming from the contributions of massive KK modes. We
first consider a compactified version of the GRS model and find that
quasi-localized gravity is characterized by a scale for which both the
resonance and the zero mode have significant contribution to 4D gravity. Above
this scale, gravity is primarily mediated by the zero mode, while the resonance
gives only minor corrections. Next, we consider an asymmetric version of the
standard non-compact GRS model, characterized by different cosmological
constants on each AdS side. We show that a resonance is present but the
asymmetry, through the form of the localizing potential, can weaken it,
resulting in a shorter lifetime and, thus, in a shorter distance scale for 4D
gravity. As a third model exhibiting quasi-localization, we consider a version
of the GRS model in which the central positive tension brane has been replaced
by a configuration of a scalar field propagating in the bulk.Comment: 18 pages, 3 figures, added 1 figure, revised version as published in
Class. Quant. Gra
Spontaneous localization of bulk matter fields
We study models compactified on S^1/Z_2 with bulk and brane matter fields
charged under U(1) gauge symmetry. We calculate the FI-terms and show by
minimizing the resulting potential that supersymmetry or gauge symmetry is
spontaneously broken if the sum of the charges does not vanish. Even if this
sum vanishes, there could be an instability as a consequence of localized
FI-terms. This leads to a spontaneous localization of charged bulk fields on
respective branes.Comment: 11 pages, LaTeX, 1 figur
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