4,817 research outputs found
Horava-Lifshitz gravity with detailed balance
Horava-Lifshitz gravity with "detailed balance" but without the
projectability assumption is discussed. It is shown that detailed balance is
quite efficient in limiting the proliferation of couplings in Horava-Lifshitz
gravity, and that its implementation without the projectability assumption
leads to a theory with sensible dynamics. However, the (bare) cosmological
constant is restricted to be large and negative.Comment: Contribution to the proceedings of NEB 15 conference, Chania, 20-23
June 2012 (talk given by D.V.
Rotating black holes in three-dimensional Ho\v{r}ava gravity
We study black holes in the infrared sector of three-dimensional Ho\v{r}ava
gravity. It is shown that black hole solutions with anti-de Sitter asymptotics
are admissible only in the sector of the theory in which the scalar degree of
freedom propagates infinitely fast. We derive the most general class of
stationary, circularly symmetric, asymptotically anti-de Sitter black hole
solutions. We also show that the theory admits black hole solutions with de
Sitter and flat asymptotics, unlike three-dimensional general relativity. For
all these cases, universal horizons may or may not exist depending on the
choice of parameters. Solutions with de Sitter asymptotics can have universal
horizons that lie beyond the de Sitter horizon.Comment: 16 pages, 9 figures, final published versio
Gravity with Auxiliary Fields
Modifications of General Relativity usually include extra dynamical degrees
of freedom, which to date remain undetected. Here we explore the possibility of
modifying Einstein's theory by adding solely nondynamical fields. With the
minimal requirement that the theory satisfies the weak equivalence principle
and admits a covariant Lagrangian formulation, we show that the field equations
generically have to include higher-order derivatives of the matter fields. This
has profound consequences for the viability of these theories. We develop a
parametrization based on a derivative expansion and show that - to next to
leading order - all theories are described by just two parameters. Our approach
can be used to put stringent, theory-independent constraints on such theories,
as we demonstrates using the Newtonian limit as an example.Comment: 5 pages, no figures; v2: clarifications and minor improvements,
matches published versio
Covariant action for bouncing cosmologies in modified Gauss-Bonnet gravity
Cyclic universes with bouncing solutions are candidates for solving the big
bang initial singularity problem. Here we seek bouncing solutions in a modified
Gauss-Bonnet gravity theory, of the type , where is the Ricci
scalar, is the Gauss-Bonnet term, and some function of it. In finding
such a bouncing solution we resort to a technique that reduces the order of the
differential equations of the theory to second order equations. As
general relativity is a theory whose equations are of second order, this order
reduction technique enables one to find solutions which are perturbatively
close to general relativity. We also build the covariant action of the order
reduced theory.Comment: 8 page
Horava-Lifshitz gravity: detailed balance revisited
We attempt a critical reconsideration of "detailed balance" as a principle
that can be used to restrict the proliferation of couplings in Horava-Lifshitz
gravity. We re-examine the shortcomings that have been usually associated with
it in the literature and we argue that easy remedies can be found for all of
them within the framework of detailed balance, and that the most persistent of
them are actually related to projectability. We show that, once projectability
is abandoned, detailed balance reduces the number of independent couplings by
roughly an order of magnitude and imposes only one restriction that constitutes
a phenomenological concern: the size of the (bare) cosmological constant is
unacceptably large. Remarkably, this restriction (which is present in the
projectable version as well) has been so far under-appreciated in the
literature. Optimists might prefer to interpret it as a potential blessing in
disguise, as it allows one to entertain the idea of a miraculous cancelation
between the bare cosmological constant and the (still poorly understood) vacuum
energy contribution.Comment: v1: 9 pages; v2: typos corrected, minor rephrasings, published
versio
Spatial and temporal distribution of mineral nutrients and sugars throughout the lifespan of Hibiscus rosa-sinensis L. flower
Although the physiological and molecular mechanisms of flower development and senescence have been extensively investigated,
a whole-flower partitioning study of mineral concentrations has not been carried out. In this work, the distribution of sucrose, total
reducing sugars, dry and fresh weight and macro and micronutrients were analysed in Hibiscus rosa-sinensis L. petals, stylestigma
including stamens and ovary at different developmental stages (bud, open and senescent flowers). Total reducing sugars
showed the highest value in petals of bud flowers, then fell during the later stages of flower development whereas sucrose showed
the highest value in petals of senescent flowers. In petals, nitrogen and phosphorus content increased during flower opening,
then nitrogen level decreased in senescent flowers. The calcium, phosphorus and boron concentrations were highest in ovary
tissues whatever the developmental stage. Overall, the data presented suggests that the high level of total reducing sugars prior
the onset of flower opening contributes to support petal cells expansion, while the high amount of sucrose at the time of petal
wilting may be viewed as a result of senescence. Furthermore, this study discusses how the accumulation of particular mineral
nutrients can be considered in a tissue specific manner for the activation of processes directly connected with reproduction
UV-B Physiological Changes Under Conditions of Distress and Eustress in Sweet Basil
UV-B radiation has been previously reported to induce protective or deleterious effects on plants depending on the UV-B irradiation doses. To elucidate how these contrasting events are physiologically coordinated, we exposed sweet basil plants to two UV-B doses: low (8.5 kJ m-2 day-1, 30 min exposure) and high (68 kJ m-2 day-1, 4 h exposure), with the plants given both doses once continuously in a single day. Physiological tests during and after both UV-B exposures were performed by comparing the stress-induced damage and adverse effects on photosynthetic activity, the concentration and composition of photosynthetic and non-photosynthetic pigments, and stress-related hormones biosynthesis in basil plants. Our results showed that upon receiving a high UV-B dose, a severe inactivation of oxygen evolving complex (OEC) activity at the PSII donor side and irreversible PSII photodamage caused primarily by limitation of the acceptor side occurred, which overloaded protective mechanisms and finally led to the death of the plants. In contrast, low UV-B levels did not induce any signs of UV-B stress injuries. The OEC partial limitation and the inactivation of the electron transport chain allowed the activation of photoprotective mechanisms, avoiding irreversible damage to PSII. Overall results indicate the importance of a specific response mechanisms regulating photoprotection vs irreversible photoinhibition in basil that were modulated depending on the UV-B doses
Recomendaciones para la prevención de riesgos contra descargas atmosféricas en el patrimonio construido
Con el objeto de preservar los valores tangibles e intangibles del patrimonio cultural construido, se han elaborado numerosos y prestigiosos documentos internacionales, los cuales establecen el marco de una serie de principios teórico doctrinarios que delimitan y guÃan los diferentes tipos de intervenciones en el patrimonio.
Como consecuencia de los daños sufridos por el patrimonio construido, a causa de fenómenos naturales (algunos de ellos catastróficos), los especialistas han advertido la importancia de centrar los esfuerzos en planes estratégicos de prevención y manejo de riesgos.
En algunas regiones, como la zona geográfica a la que pertenece la ciudad de La Plata, las grandes catástrofes naturales (terremotos, tsunamis, erupciones volcánicas, inundaciones, huracanes) son de una probabilidad prácticamente insignificante. Eventualmente, el riesgo a impactos de rayos durante las tormentas eléctricas, constituye el más probable.
En este marco, el propósito del trabajo es advertir sobre la importancia de la prevención ante este tipo de riesgos mediante la evaluación y la instalación de sistemas de protección contra rayos en edificios patrimoniales, analizando algunos casos particulares de nuestra ciudad.
En el documento se describen los posibles efectos dañinos que las descargas atmosféricas pueden tener sobre el patrimonio construido, asà como las medidas preventivas recomendables según la normativa internacional vigente y de aceptación a nivel mundial, elaborada por la Comisión Electrotécnica Internacional (IEC: International Electrotechnical Commission), norma IEC 62305 del año 2006.Facultad de IngenierÃ
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