82 research outputs found
Nonminimal isotropic cosmological model with Yang-Mills and Higgs fields
We establish a nonminimal Einstein-Yang-Mills-Higgs model, which contains six
coupling parameters. First three parameters relate to the nonminimal coupling
of non-Abelian gauge field and gravity field, two parameters describe the
so-called derivative nonminimal coupling of scalar multiplet with gravity
field, and the sixth parameter introduces the standard coupling of scalar field
with Ricci scalar. The formulated six-parameter nonminimal
Einstein-Yang-Mills-Higgs model is applied to cosmology. We show that there
exists a unique exact cosmological solution of the de Sitter type for a special
choice of the coupling parameters. The nonminimally extended Yang-Mills and
Higgs equations are satisfied for arbitrary gauge and scalar fields, when the
coupling parameters are specifically related to the curvature constant of the
isotropic spacetime. Basing on this special exact solution we discuss the
problem of a hidden anisotropy of the Yang-Mills field, and give an explicit
example, when the nonminimal coupling effectively screens the anisotropy
induced by the Yang-Mills field and thus restores the isotropy of the model.Comment: 15 pages, revised version accepted to Int. J. Mod. Phys. D, typos
correcte
Covariance properties and regularization of conserved currents in tetrad gravity
We discuss the properties of the gravitational energy-momentum 3-form within
the tetrad formulation of general relativity theory. We derive the covariance
properties of the quantities describing the energy-momentum content under
Lorentz transformations of the tetrad. As an application, we consider the
computation of the total energy (mass) of some exact solutions of Einstein's
general relativity theory which describe compact sources with asymptotically
flat spacetime geometry. As it is known, depending on the choice of tetrad
frame, the formal total integral for such configurations may diverge. We
propose a natural regularization method which yields finite values for the
total energy-momentum of the system and demonstrate how it works on a number of
explicit examples.Comment: 36 pages, Revtex, no figures; small changes, published versio
Wave propagation in linear electrodynamics
The Fresnel equation governing the propagation of electromagnetic waves for
the most general linear constitutive law is derived. The wave normals are found
to lie, in general, on a fourth order surface. When the constitutive
coefficients satisfy the so-called reciprocity or closure relation, one can
define a duality operator on the space of the two-forms. We prove that the
closure relation is a sufficient condition for the reduction of the fourth
order surface to the familiar second order light cone structure. We finally
study whether this condition is also necessary.Comment: 13 pages. Phys. Rev. D, to appea
Invariant conserved currents in gravity theories: diffeomorphisms and local gauge symmetries
Previously, we have developed a general method to construct invariant
conserved currents and charges in gravitational theories with Lagrangians that
are invariant under spacetime diffeomorphisms and local Lorentz
transformations. This approach is now generalized to the case when the local
Lorentz group is replaced by an arbitrary local gauge group. The particular
examples include the Maxwell and Yang-Mills fields coupled to gravity with
Abelian and non-Abelian local internal symmetries, and the metric-affine
gravity in which the local Lorentz spacetime group is extended to the local
general linear group.Comment: 28 pages, Revte
Application of six detection methods for analysis of paralytic shellfish toxins in shellfish from four regions within Latin America
With the move away from use of mouse bioassay (MBA) to test bivalve mollusc shellfish for paralytic shellfish poisoning (PSP) toxins, countries around the world are having to adopt non-animal-based alternatives that fulfil ethical and legal requirements. Various assays have been developed which have been subjected to single-laboratory and multi-laboratory validation studies, gaining acceptance as official methods of analysis and approval for use in some countries as official control testing methods. The majority of validation studies conducted to date do not, however, incorporate shellfish species sourced from Latin America. Consequently, this study sought to investigate the performance of five alternative PSP testing methods together with the MBA, comparing the PSP toxin data generated both qualitatively and quantitatively. The methods included a receptor binding assay (RBA), two liquid chromatography with fluorescence detection (LC-FLD) methods including both pre-column and post-column oxidation, liquid chromatography with tandem mass spectrometry (LC-MS/MS) and a commercial lateral flow assay (LFA) from Scotia. A total of three hundred and forty-nine shellfish samples from Argentina, Mexico, Chile and Uruguay were assessed. For the majority of samples, qualitative results compared well between methods. Good statistical correlations were demonstrated between the majority of quantitative results, with a notably excellent correlation between the current EU reference method using pre-column oxidation LC-FLD and LC-MS/MS. The LFA showed great potential for qualitative determination of PSP toxins, although the findings of high numbers of false-positive results and two false negatives highlighted that some caution is still needed when interpreting results. This study demonstrated that effective replacement methods are available for countries that no longer wish to use the MBA, but highlighted the importance of comparing toxin data from the replacement method using local shellfish species of concern before implementing new methods in official control testing programs
Matter loops corrected modified gravity in Palatini formulation
Recently, corrections to the standard Einstein-Hilbert action are proposed to
explain the current cosmic acceleration in stead of introducing dark energy. In
the Palatini formulation of those modified gravity models, there is an
important observation due to Arkani-Hamed: matter loops will give rise to a
correction to the modified gravity action proportional to the Ricci scalar of
the metric. In the presence of such term, we show that the current forms of
modified gravity models in Palatini formulation, specifically, the 1/R gravity
and gravity, will have phantoms. Then we study the possible
instabilities due to the presence of phantom fields. We show that the strong
instability in the metric formulation of 1/R gravity indicated by Dolgov and
Kawasaki will not appear and the decay timescales for the phantom fields may be
long enough for the theories to make sense as effective field theory . On the
other hand, if we change the sign of the modification terms to eliminate the
phantoms, some other inconsistencies will arise for the various versions of the
modified gravity models. Finally, we comment on the universal property of the
Palatini formulation of the matter loops corrected modified gravity models and
its implications.Comment: 11 pages, 1 figures, References adde
Torsion and accelerating expansion of the universe in quadratic gravitation
Several exact cosmological solutions of a metric-affine theory of gravity
with two torsion functions are presented. These solutions give a essentially
different explanation from the one in most of previous works to the cause of
the accelerating cosmological expansion and the origin of the torsion of the
spacetime. These solutions can be divided into two classes. The solutions in
the first class define the critical points of a dynamical system representing
an asymptotically stable de Sitter spacetime. The solutions in the second class
have exact analytic expressions which have never been found in the literature.
The acceleration equation of the universe in general relativity is only a
special case of them. These solutions indicate that even in vacuum the
spacetime can be endowed with torsion, which means that the torsion of the
spacetime has an intrinsic nature and a geometric origin. In these solutions
the acceleration of the cosmological expansion is due to either the scalar
torsion or the pseudoscalar torsion function. Neither a cosmological constant
nor dark energy is needed. It is the torsion of the spacetime that causes the
accelerating expansion of the universe in vacuum. All the effects of the
inflation, the acceleration and the phase transformation from deceleration to
acceleration can be explained by these solutions. Furthermore, the energy and
pressure of the matter without spin can produce the torsion of the spacetime
and make the expansion of the universe decelerate as well as accelerate.Comment: 20 pages. arXiv admin note: text overlap with gr-qc/0604006,
arXiv:1110.344
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
Star Formation and Dynamics in the Galactic Centre
The centre of our Galaxy is one of the most studied and yet enigmatic places
in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre
(GC) is the ideal environment to study the extreme processes that take place in
the vicinity of a supermassive black hole (SMBH). Despite the hostile
environment, several tens of early-type stars populate the central parsec of
our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and
inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the
SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The
formation of such early-type stars has been a puzzle for a long time: molecular
clouds should be tidally disrupted by the SMBH before they can fragment into
stars. We review the main scenarios proposed to explain the formation and the
dynamical evolution of the early-type stars in the GC. In particular, we
discuss the most popular in situ scenarios (accretion disc fragmentation and
molecular cloud disruption) and migration scenarios (star cluster inspiral and
Hills mechanism). We focus on the most pressing challenges that must be faced
to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in
expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A.,
'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201
Pectinmethyesterases Modulate Plant Homogalacturonan Status in Defenses Against the Aphid Myzus persicae.
Because they suck phloem sap and act as vectors for phytopathogenic viruses, aphids pose a threat to crop yields worldwide. Pectic homogalacturonan (HG) has been described as a defensive element for plants during infections with phytopathogens. However, its role during aphid infestation remains unexplored. Using immunofluorescence assays and biochemical approaches, the HG methylesterification status and associated modifying enzymes during the early stage of Arabidopsis thaliana infestation with the green peach aphid Myzus persicae were analyzed. Additionally, the influence of PME activity on aphid settling and feeding behavior was evaluated by free choice assays and the Electrical Penetration Graph technique, respectively. Our results revealed that HG status and HG-modifying enzymes are significantly altered during the early stage of the plant-aphid interaction. Aphid infestation induced a significant increase in total PME activity and methanol emissions, concomitant with a decrease in the degree of HG methylesterification. Conversely, inhibition of PME activity led to a significant decrease in the settling and feeding preference of aphids. Furthermore, we demonstrate that the PME inhibitor AtPMEI13 has a defensive role during aphid infestation, since pmei13 mutants are significantly more susceptible to M. persicae in terms of settling preference, phloem access, and phloem sap drainage
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