320 research outputs found
Non-abelian Yang-Mills in Kundt spacetimes
We present new exact solutions of the Einstein-Yang-Mills system. The
solutions are described by a null Yang-Mills field in a Kundt spacetime. They
generalize a previously known solution for a metric of wave type. The
solutions are formally of Petrov type III.Comment: Talk presented at the XXVIII Spanish Relativity Meeting E.R.E. 2005,
Oviedo, September 6-10, 2005, to be published by AIP Conference Proceedings,
5 page
Berwald spacetimes and very special relativity
In this work we study Berwald spacetimes and their vacuum dynamics, where the
latter are based on a Finsler generalization of the Einstein's equations
derived from an action on the unit tangent bundle. In particular, we consider a
specific class of spacetimes which are non-flat generalizations of the very
special relativity (VSR) line element, to which we refer as very general
relativity (VGR). We derive necessary and sufficient conditions for the VGR
line element to be of Berwald type. We present two novel examples with the
corresponding vacuum field equations: a Finslerian generalization of vanishing
scalar invariant (VSI) spacetimes in Einstein's gravity as well as the most
general homogeneous and isotropic VGR spacetime.Comment: 17 pages, example section updated, journal references adde
On the non metrizability of Berwald Finsler spacetimes
We investigate whether Szabo's metrizability theorem can be extended to
Finsler spaces of indefinite signature. For smooth, positive definite Finsler
metrics, this important theorem states that, if the metric is of Berwald type
(i.e., its Chern-Rund connection defines an affine connection on the underlying
manifold), then it is affinely equivalent to a Riemann space, meaning that its
affine connection is the Levi-Civita connection of some Riemannian metric. We
show for the first time that this result does not extend to Finsler spacetimes.
More precisely, we find a large class of Berwald spacetimes for which the Ricci
tensor of the affine connection is not symmetric. The fundamental difference
from positive definite Finsler spaces that makes such an asymmetry possible, is
the fact that generally, Finsler spacetimes satisfy certain smoothness
properties only on a proper conic subset of the slit tangent bundle. Indeed, we
prove that when the Finsler Lagrangian is smooth on the entire slit tangent
bundle, the Ricci tensor must necessarily be symmetric. For large classes of
Finsler spacetimes, however, the Berwald property does not imply that the
affine structure is equivalent to the affine structure of a pseudo-Riemannian
metric. Instead, the affine structure is that of metric-affine geometry with
vanishing torsion.Comment: 12 pages, contribution to the Special Issue "Finsler Modification of
Classical General Relativity" in the Journal Univers
Finsler gravitational waves of -type and their observational signature
We introduce a new class of -type exact solutions in Finsler
gravity closely related to the well-known pp-waves in general relativity. Our
class contains most of the exact solutions currently known in the literature as
special cases. The linearized versions of these solutions may be interpretted
as Finslerian gravitational waves, and we investigate the physical effect of
such waves. More precisely, we compute the Finslerian correction to the radar
distance along an nterferometer arm at the moment a Finslerian gravitational
wave passes a detector. We come to the remarkable conclusion that the effect of
a Finslerian gravitational wave on an interferometer is indistinguishable from
that of standard gravitational wave in general relativity. Along the way we
also physically motivate a modification of the Randers metric and prove that it
has some very interesting properties
Identifying Berwald Finsler Geometries
Berwald geometries are Finsler geometries close to (pseudo)-Riemannian
geometries. We establish a simple first order partial differential equation as
necessary and sufficient condition, which a given Finsler Lagrangian has to
satisfy to be of Berwald type. Applied to -Finsler spaces,
respectively -Finsler spacetimes, this reduces to a necessary and
sufficient condition for the Levi-Civita covariant derivative of the defining
-form. We illustrate our results with novel examples of
-Berwald geometries which represent Finslerian versions of
Kundt (constant scalar invariant) spacetimes. The results generalize earlier
findings by Tavakol and van den Bergh, as well as the Berwald conditions for
Randers and m-Kropina resp. very special/general relativity geometries.Comment: 17 pages, results on -Finsler geometries extended,
explicit examples added, updated to journal versio
Randers pp-waves
In this work we study Randers spacetimes of Berwald type and analyze Pfeifer
and Wohlfarth's vacuum field equation of Finsler gravity for this class. We
show that in this case the field equation is equivalent to the vanishing of the
Finsler Ricci tensor, analogously to Einstein gravity. This implies that the
considered vacuum field equation and Rutz's equation coincide in this scenario.
We also construct all exact solutions of Berwald-Randers type to vacuum Finsler
gravity, which turn out to be composed of a CCNV (covariantly constant null
vector) Lorentzian spacetime, commonly known as pp-wave, and a 1-form given by
the pp-wave distinguished null vector. We therefore refer to the found
solutions as \textit{Randers pp-waves}.Comment: 11 pagers, updated to journal versio
Type III Einstein-Yang-Mills solutions
We construct two distinct classes of exact type III solutions of the D=4
Einstein-Yang-Mills system. The solutions are embeddings of the non-abelian
plane waves in spacetimes in Kundt's class. Reduction of the solutions to type
N leads to generalized and Kundt waves. The geodesic equations are briefly
discussed.Comment: revtex, 4 pages, minor changes, some factors and references
corrected, footnote adde
A Cosmological Unicorn Solution to Finsler Gravity
We present a new family of exact vacuum solutions to Pfeifer and Wohlfarth's
field equation in Finsler gravity, consisting of Finsler metrics that are
Landsbergian but not Berwaldian, also known as unicorns due to their rarity.
Interestingly we find that these solutions have a physically viable light cone
structure, even though in some cases the signature is not Lorentzian but
positive definite. We furthermore find a promising analogy between our
solutions and classical FLRW cosmology. One of our solutions in particular has
cosmological symmetry, i.e. it is spatially homogeneous and isotropic, and it
is additionally conformally flat, with conformal factor depending only on the
timelike coordinate. We show that this conformal factor can be interpreted as
the scale factor, we compute it as a function of cosmological time, and we show
that it corresponds to a linearly expanding (or contracting) Finsler universe
Studio de la bioactividad potencial de extractos hemicelulósicos de la cascarilla de arroz
[ES] La cascarilla de arroz es un subproducto agroalimentario cuya revalorización tiene gran interés
en el contexto de la economÃa circular. Es un material lignocelulósico con una importante
fracción de hemicelulosas formada por arabinoxilanos sustituidos cuya ruptura da lugar a xilooligomeros
con aplicaciones alimentarias, médicas y farmacéuticas. En este sentido, se han
estudiado diversas metodologÃas que permitan la extracción de dichos compuestos a partir de
subproductos de la industria alimentaria como la extracción alcalina y la extracción con agua
subcrÃtica. A diferencia del proceso alcalino, la extracción con agua subcrÃtica no solo es una
tecnologÃa sostenible medioambientalmente, sino que además mantiene la funcionalidad de las
fracciones hemicelulósicas aisladas de la materia prima. En este contexto resulta interesante la
valorización de hemicelulosas a partir de cascarilla de arroz mediante ambas tecnologÃas a fin
de poder comprar su funcionalidad en términos de actividad antioxidante y poder
antimicrobiano. Este estudio contempla la determinación de la potencial actividad antioxidante
de los diferentes extractos, asà como el estudio de su potencial actividad antimicrobiana frente
a bacterias tanto Gram – como Gram +. Para la determinación de la actividad antioxidante se ha
utilizado el método de reducción del radical libre 2,2-difenil-1-picrilhidracilo (DPPH), lo cual
permite comparar el poder antioxidante de ambos extractos con compuestos antioxidantes de
referencia. En cuanto a la potencial actividad antimicrobiana de los extractos, se ha determinado
la concentración mÃnima inhibitoria de ambos productos mediante el ensayo MTT que permite
probar múltiples concentraciones diferentes al mismo tiempo de forma rápida y sencilla. De esta
forma, se obtenido la información necesaria para su futura aplicación tanto en materiales de
envasado de alimentos, como en la propia formulación de alimentos, ya que por sus propiedades
activas permitirÃan prolongar su vida útil a la vez que se enriquecen en compuestos
antioxidantes.[EN] Rice Husk is an agro-food by-product whose valorization is of great interest in the context of the
circular economy. It is a lignocellulosic material with an important fraction of hemicelluloses
constituted by substituted arabinoxylans whose rupture, gives rise to Xilo-oligomers with
alimentary, medical and pharmaceutical applications. In this sense, several methodologies have
been studied for allowing the extraction of these compounds from food industry byproducts,
such as alkaline extraction and subcritical water extraction. Unlike the alkaline process, the
extraction with subcritical water is not only an environmentally sustainable technology, but also
maintains the functionality of the hemicellulose fractions isolated from the raw material. In this
context, the valorization of hemicelluloses from rice husks through both technologies is
interesting to compare their effectiveness at maintaining the hemicellulose functionality in
terms of the antioxidant activity and antimicrobial capacity. This study includes the
determination of the potential antioxidant activity of the different extracts, as well as the study
of their potential antimicrobial activity against both Gram (-) and Gram (+) bacteria. For the
determination of the antioxidant activity the method of reduction of the free radical 2.2-
diphenyl-1-Picrilhidracilo (DPPH) was used, which allow for comparing the antioxidant capacity
of both extracts with other known antioxidant compounds. As concerns the antimicrobial
activity of the extracts, the minimum inhibitory concentration of both products was determined
by means of the MTT test, which allows fortesting multiple different concentrations at the same
time, in a quick and easy manner. In this way, the information necessary for its future application
was obtained both in food packaging materials, as well as in food formulation, due to that fact
that their active properties would allow for extending food shelf life, while food are enriched in
antioxidant compounds.Almarche Fuster, A. (2018). Studio de la bioactividad potencial de extractos hemicelulósicos de la cascarilla de arroz. http://hdl.handle.net/10251/107252TFG
A simplified algorithm for inverting higher order diffusion tensors
In Riemannian geometry, a distance function is determined by an inner product on the tangent space. In Riemann-Finsler geometry, this distance function can be determined by a norm. This gives more freedom on the form of the so-called indicatrix or the set of unit vectors. This has some interesting applications, e.g., in medical image analysis, especially in diffusion weighted imaging (DWI). An important application of DWI is in the inference of the local architecture of the tissue, typically consisting of thin elongated structures, such as axons or muscle fibers, by measuring the constrained diffusion of water within the tissue. From high angular resolution diffusion imaging (HARDI) data, one can estimate the diffusion orientation distribution function (dODF), which indicates the relative diffusivity in all directions and can be represented by a spherical polynomial. We express this dODF as an equivalent spherical monomial (higher order tensor) to directly generalize the (second order) diffusion tensor approach. To enable efficient computation of Riemann-Finslerian quantities on diffusion weighted (DW)-images, such as the metric/norm tensor, we present a simple and efficient algorithm to invert even order spherical monomials, which extends the familiar inversion of diffusion tensors, i.e., symmetric matrices.</p
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