16,280 research outputs found

### Constraining Generalized Non-local Cosmology from Noether Symmetries

We study a generalized nonlocal theory of gravity which, in specific limits,
can become either the curvature non-local or teleparallel non-local theory.
Using the Noether Symmetry Approach, we find that the coupling functions coming
from the non-local terms are constrained to be either exponential or linear in
form. It is well known that in some non-local theories, a certain kind of
exponential non-local couplings are needed in order to achieve a renormalizable
theory. In this paper, we explicitly show that this kind of coupling does not
need to by introduced by hand, instead, it appears naturally from the
symmetries of the Lagrangian in flat Friedmann-Robertson-Walker cosmology.
Finally, we find de-Sitter and power law cosmological solutions for different
nonlocal theories. The symmetries for the generalized non-local theory is also
found and some cosmological solutions are also achieved under the full theory.Comment: 15 pages, to be published in Eur.Phys.J.

### Cosmography of f(R) - brane cosmology

Cosmography is a useful tool to constrain cosmological models, in particular
dark energy models. In the case of modified theories of gravity, where the
equations of motion are generally quite complicated, cosmography can contribute
to select realistic models without imposing arbitrary choices a priori. Indeed,
its reliability is based on the assumptions that the universe is homogeneous
and isotropic on large scale and luminosity distance can be "tracked" by the
derivative series of the scale factor a(t). We apply this approach to induced
gravity brane-world models where an f(R)-term is present in the brane effective
action. The virtue of the model is to self-accelerate the normal and healthy
DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We
show that the model, coming from a fundamental theory, is consistent with the
LCDM scenario at low redshift. We finally estimate the cosmographic parameters
fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors
from Baryon Acoustic Oscillations (BAO) and then provide constraints on the
present day values of f(R) and its second and third derivatives.Comment: 9 pages, 3 figures, to appear in Phys. Rev.

### Noether Symmetries in Gauss-Bonnet-teleparallel cosmology

A generalized teleparallel cosmological model, $f(T_\mathcal{G},T)$,
containing the torsion scalar $T$ and the teleparallel counterpart of the
Gauss-Bonnet topological invariant $T_{\mathcal{G}}$, is studied in the
framework of the Noether Symmetry Approach. As $f(\mathcal{G}, R)$ gravity,
where $\mathcal{G}$ is the Gauss-Bonnet topological invariant and $R$ is the
Ricci curvature scalar, exhausts all the curvature information that one can
construct from the Riemann tensor, in the same way, $f(T_\mathcal{G},T)$
contains all the possible information directly related to the torsion tensor.
In this paper, we discuss how the Noether Symmetry Approach allows to fix the
form of the function $f(T_\mathcal{G},T)$ and to derive exact cosmological
solutions.Comment: 6 page

### Breaking the Vainshtein screening in clusters of galaxies

In this work we will test an alternative model of gravity belonging to the
large family of galileon models. It is characterized by an intrinsic breaking
of the Vainshtein mechanism inside large astrophysical objects, thus having
possibly detectable observational signatures. We will compare theoretical
predictions from this model with the observed total mass profile for a sample
of clusters of galaxies. The profiles are derived using two complementary
tools: X-ray hot intra-cluster gas dynamics, and strong and weak gravitational
lensing. We find that a dependence with the dynamical internal status of each
cluster is possible; for those clusters which are very close to be relaxed, and
thus less perturbed by possible astrophysical local processes, the galileon
model gives a quite good fit to both X-ray and lensing observations. Both
masses and concentrations for the dark matter halos are consistent with earlier
results found in numerical simulations and in the literature, and no compelling
statistical evidence for a deviation from general relativity is detectable from
the present observational state. Actually, the characteristic galileon
parameter $\Upsilon$ is always consistent with zero, and only an upper limit
($\lesssim0.086$ at $1\sigma$, $\lesssim0.16$ at $2\sigma$, and $\lesssim0.23$
at $3\sigma$) can be established. Some interesting distinctive deviations might
be operative, but the statistical validity of the results is far from strong,
and better data would be needed in order to either confirm or reject a
potential tension with general relativity.Comment: 26 pages, 3 tables, 4 figures. Accepted for publication on Phys. Rev.

### Ultrasound shear wave elastography for liver disease. A critical appraisal of the many actors on the stage

In the last 12\u200a-\u200a18 months nearly all ultrasound manufacturers have arrived to implement ultrasound shear wave elastography modality in their equipment for the assessment of chronic liver disease; the few remaining players are expected to follow in 2016.When all manufacturers rush to a new technology at the same time, it is evident that the clinical demand for this information is of utmost value. Around 1990, there was similar demand for color Doppler ultrasound; high demand for contrast-enhanced ultrasonography was evident at the beginning of this century, and around 2010 demand increased for strain elastography. However, some issues regarding the new shear wave ultrasound technologies must be noted to avoid misuse of the resulting information for clinical decisions. As new articles are expected to appear in 2016 reporting the findings of the new technologies from various companies, we felt that the beginning of this year was the right time to present an appraisal of these issues. We likewise expect that in the meantime EFSUMB will release a new update of the existing guidelines 1 2.The first ultrasound elastography method became available 13 years ago in the form of transient elastography with Fibroscan(\uae) 3. It was the first technique providing non-invasive quantitive information about the stiffness of the liver and hence regarding the amount of fibrosis in chronic liver disease 3. The innovation was enormous, since a non-invasive modality was finally available to provide findings otherwise achievable only by liver biopsy. In fact, prior to ultrasound elastography, a combination of conventional and Doppler ultrasound parameters were utilized to inform the physician about the presence of cirrhosis and portal hypertension 4. However, skilled operators were required, reproducibility and diagnostic accuracy were suboptimal, and it was not possible to differentiate the pre-cirrhotic stages of fibrosis. All these limitations were substantially improved by transient elastography, performed with Fibroscan(\uae), a technology dedicated exclusively to liver elastography. Since then, more than 1300 articles dealing with transient elastography have been listed in PubMed, some describing results with more than 10,000 patients 5. The technique has been tested in nearly all liver disease etiologies, with histology as the reference standard. Meta-analysis of data, available in many etiologies 6, showed good performance and reproducibility as well as some situations limiting reliability 5. Thresholds for the different fibrosis stages (F0 to F4) have been provided by many large-scale studies utilizing histology as the reference standard 7. Transient elastography tracks the velocity of shear waves generated by the gentle hit of a piston on the skin, with the resulting compression wave traveling in the liver along its longitudinal axis. The measurement is made in a 4\u200acm long section of the liver, thus able to average slightly inhomogeneous fibrotic deposition.In 2008 a new modality became available, Acoustic Radiation Force Impulse (ARFI) quantification, and classified by EFSUMB 1 as point shear wave elastography (pSWE), since the speed of the shear wave (perpendicular to the longitudinal axis) is measured in a small region (a "point", few millimeters) at a freely-choosen depth within 8\u200acm from the skin. This technology was the first to be implemented in a conventional ultrasound scanner by Siemens(\uae) 8. Several articles have been published regarding this technology, most with the best reference standards 9, some including findings on more than 1000 hepatitis C patients 10 or reporting meta-analysis of data 11. Although the correlation between Siemens pSWE and transient elastography appeared high 12 13, the calculated thresholds for the different fibrosis stages and the stiffness ranges between the two techniques are not superimposable.Interestingly, pSWE appears to provide greater applicability than transient elastography for measuring both liver 13 and spleen stiffness, which is a new application of elastography 14, of interest for the prediction of the degree of portal hypertension 15 16.Nowadays other companies have started producing equipment with pSWE technology, but only very few articles have been published so far, for instance describing the use of Philips(\uae) equipment, which was the second to provide pSWE. These articles show preliminary good results also in comparison with TE 17 18. Not enough evidence is currently available in the literature about the elastographic performance of the products most recently introduced to the market. Furthermore, with some products the shear wave velocities generated by a single ultrasound acoustic push pulse can be measured in a bidimensional area (a box in the range of 2\u200a-\u200a3\u200acm per side) rather than in a single small point, producing a so-called bidimensional 2D-SWE 1. The stiffness is depicted in color within the area and refreshing of the measurement occurs every 1\u200a-\u200a2 seconds. Once the best image is acquired, the operator chooses a Region Of Interest (ROI) within the color box, where the mean stiffness is then calculated. 2D-SWE can be performed as a "one shot" technique or as a semi-"real-time" technique for a few seconds (at about 1 frame per second) in order to obtain a stable elastogram. With either technique, there should be no motion/breathing during image acquisition. A bidimensional averaged area should overcome the limitation of pSWE to inadvertently investigate small regions of greater or lesser stiffness than average. A shear wave quality indicator could be useful to provide real-time feedback and optimize placement of the sampling ROIs, a technology recently presented by Toshiba(\uae), but which is still awaiting validation in the literature.Supersonic Imagine by Aixplorer(\uae) which works with a different modality of insonation and video analysis compared to the the previously-mentioned three techniques (i.\u200ae., transient elastography, pSWE and 2D-SWE), leading to a bidimensional assessment of liver stiffness in real time up to 5\u200aHz and in larger regions; thus this technique is also termed real-time 2\u200aD SWE. It has been available on the market for a few years 19 20, and many articles have been published showing stiffness values quite similar to those of Fibroscan(\uae) 21; likewise, defined thresholds based on histological findings have appeared in several articles 19 20 21.After this brief summary of the technological state of the art we would like to mention the following critical issues that we believe every user should note prior to providing liver stiffness reports. \ub7 The thresholds obtained from the "oldest" techniques for the various fibrosis stages based on hundreds of patients with histology as reference standard cannot be straightforwardly applied to the new ultrasound elastography techniques, even if based on the same principle (e.\u200ag. pSWE). In fact, the different manufacturers apply proprietary patented calculation modes, which might result in slightly to moderately different values. It should be kept in mind that the range for intermediate fibrosis stages (F1 to F3) is quite narrow, in the order of 2\u200a-\u200a3 kilopascal (over a total range spanning 2 to 75 kPa with Fibroscan), so that slightly different differences in outputs could shift the assessment of patients from one stage to another. Comparative studies using phantoms and healthy volunteers, as well as patients, are eagerly awaited. In fact, the equipment might not produce linear correlations of measurements at different degrees of severity of fibrosis. As a theoretical example, some equipment might well correlate in their values with an older technique, such as transient elastography, at low levels of liver fibrosis, but not as well in cases of more advanced fibrosis or vice versa. Consequentely, when elastography data are included in a report, the equipment utilized for the measurement should be clearly specified, and conclusions about the fibrosis stage should be withheld if an insufficient number of comparative studies with solid reference standards are available for that specific equipment.. \ub7 Future studies using histology as a reference might be biased in comparison to previous studies, since nowadays fewer patients with chronic hepatitis C or hepatitis B undergo biopsy. In fact, due to wide availability of effective drugs as well as the use of established elastography methods for patients with viral hepatitis, most cases submitted to biopsy today have uncertain etiology or inconsistent and inconclusive clinical data. Therefore, extrapolated thresholds from such inhomogeneous populations applied to more ordinary patients with viral hepatitis might become problematic in the future, although no better solution is currently anticipated. This situation might lead to the adoption of a standard validated elastographic method as reference, but this has to be agreed-upon at an international level.. \ub7 Ultrasound elastography embedded in conventional scanners usually allows the choice of where to place the ROI within the color stiffness box and whether to confirm or exclude each single measurement when determining the final value. Thus, the operator has a greater potential to influence the final findings than with Fibroscan\uae, where these choices are not available. This has to be kept in mind to avoid the possibility that an operator could, even inadvertently, tend to confirm an assumption about that specific patient or to confirm the patient's expectations.. \ub7 Quality criteria for the new technologies following transient elastography are absent (depending on the manufacturer) or have not been satisfactorily defined, so that the information potentially inserted in a report cannot currently be judged for its reliability by the clinician.. (ABSTRACT TRUNCATED

### Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter

Aims. We investigated the gravitational effects of a scalar field within
scalar-tensor gravity as an alternative to dark matter. Motivated by chameleon,
symmetron and f(R)-gravity models, we studied a phenomenological scenario where
the scalar field has both a mass (i.e. interaction length) and a coupling
constant to the ordinary matter which scale with the local properties of the
considered astrophysical system. Methods. We analysed the feasibility of this
scenario using the modified gravitational potential obtained in its context and
applied it to the galactic and hot gas/stellar dynamics in galaxy clusters and
elliptical/spiral galaxies respectively. This is intended to be a first step in
assessing the viability of this new approach in the context of "alternative
gravity" models. Results. The main results are: 1. the velocity dispersion of
elliptical galaxies can be fitted remarkably well by the suggested scalar
field, with model significance similar to a classical Navarro-Frenk-White dark
halo profile; 2. the analysis of the stellar dynamics and the gas equilibrium
in elliptical galaxies has shown that the scalar field can couple with ordinary
matter with different strengths (different coupling constants) producing and/or
depending on the different clustering state of matter components; 3. elliptical
and spiral galaxies, combined with clusters of galaxies, show evident
correlations among theory parameters which suggest the general validity of our
results at all scales and a way toward a possible unification of the theory for
all types of gravitational systems we considered. All these results demonstrate
that the proposed scalar field scenario can work fairly well as an alternative
to dark matter.Comment: 23 pages, 15 figures, 5 tables, accepted for publication on Astronomy
& Astrophysic

### Non-Universality of Density and Disorder in Jammed Sphere Packings

We show for the first time that collectively jammed disordered packings of
three-dimensional monodisperse frictionless hard spheres can be produced and
tuned using a novel numerical protocol with packing density $\phi$ as low as
0.6. This is well below the value of 0.64 associated with the maximally random
jammed state and entirely unrelated to the ill-defined ``random loose packing''
state density. Specifically, collectively jammed packings are generated with a
very narrow distribution centered at any density $\phi$ over a wide density
range $\phi \in [0.6,~0.74048\ldots]$ with variable disorder. Our results
support the view that there is no universal jamming point that is
distinguishable based on the packing density and frequency of occurence. Our
jammed packings are mapped onto a density-order-metric plane, which provides a
broader characterization of packings than density alone. Other packing
characteristics, such as the pair correlation function, average contact number
and fraction of rattlers are quantified and discussed.Comment: 19 pages, 4 figure

- …