1,410 research outputs found
Irreducible modules over finite simple Lie conformal superalgebras of type K
We construct all finite irreducible modules over Lie conformal superalgebras
of type KComment: Accepted for publication in J. Math. Phys
Reversible and Irreversible Spacetime Thermodynamics for General Brans-Dicke Theories
We derive the equations of motion for Palatini F(R) gravity by applying an
entropy balance law T dS= \delta Q+\delta N to the local Rindler wedge that can
be constructed at each point of spacetime. Unlike previous results for metric
F(R), there is no bulk viscosity term in the irreversible flux \delta N. Both
theories are equivalent to particular cases of Brans-Dicke scalar-tensor
gravity. We show that the thermodynamical approach can be used ab initio also
for this class of gravitational theories and it is able to provide both the
metric and scalar equations of motion. In this case, the presence of an
additional scalar degree of freedom and the requirement for it to be dynamical
naturally imply a separate contribution from the scalar field to the heat flux
\delta Q. Therefore, the gravitational flux previously associated to a bulk
viscosity term in metric F(R) turns out to be actually part of the reversible
thermodynamics. Hence we conjecture that only the shear viscosity associated
with Hartle-Hawking dissipation should be associated with irreversible
thermodynamics.Comment: 12 pages, 1 figure; v2: minor editing to clarify Section III, fixed
typos; v3: fixed typo
Theory of gravitation theories: a no-progress report
Already in the 1970s there where attempts to present a set of ground rules,
sometimes referred to as a theory of gravitation theories, which theories of
gravity should satisfy in order to be considered viable in principle and,
therefore, interesting enough to deserve further investigation. From this
perspective, an alternative title of the present paper could be ``why are we
still unable to write a guide on how to propose viable alternatives to general
relativity?''. Attempting to answer this question, it is argued here that
earlier efforts to turn qualitative statements, such as the Einstein
Equivalence Principle, into quantitative ones, such as the metric postulates,
stand on rather shaky grounds -- probably contrary to popular belief -- as they
appear to depend strongly on particular representations of the theory. This
includes ambiguities in the identification of matter and gravitational fields,
dependence of frequently used definitions, such as those of the stress-energy
tensor or classical vacuum, on the choice of variables, etc. Various examples
are discussed and possible approaches to this problem are pointed out. In the
course of this study, several common misconceptions related to the various
forms of the Equivalence Principle, the use of conformal frames and equivalence
between theories are clarified.Comment: Invited paper in the Gravity Research Foundation 2007 special issue
to be published by Int. J. Mod. Phys.
Antibiotic Prophylaxis to Reduce Respiratory Tract Infections and Mortality in Adults Receiving Intensive Care
BACKGROUND: Pneumonia is an important cause of mortality in intensive care units (ICUs). The incidence of pneumonia in ICU patients ranges between 7% and 40%, and the crude mortality from ventilator-associated pneumonia may exceed 50%. Although not all deaths in patients with this form of pneumonia are directly attributable to pneumonia, it has been shown to contribute to mortality in ICUs independently of other factors that are also strongly associated with such deaths. OBJECTIVES: To assess the effects of prophylactic antibiotic regimens, such as selective decontamination of the digestive tract (SDD) for the prevention of respiratory tract infections (RTIs) and overall mortality in adults receiving intensive care. SEARCH STRATEGY: We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2009, issue 1), which contains the Cochrane Acute Respiratory Infections (ARI) Group's Specialised Register; MEDLINE (January 1966 to March 2009); and EMBASE (January 1990 to March 2009). SELECTION CRITERIA: Randomised controlled trials (RCTs) of antibiotic prophylaxis for RTIs and deaths among adult ICU patients. DATA COLLECTION AND ANALYSIS: At least two review authors independently extracted data and assessed trial quality. MAIN RESULTS: We included 36 trials involving 6914 people. There was variation in the antibiotics used, patient characteristics and risk of RTIs and mortality in the control groups. In trials comparing a combination of topical and systemic antibiotics, there was a significant reduction in both RTIs (number of studies = 16, odds ratio (OR) 0.28, 95% confidence interval (CI) 0.20 to 0.38) and total mortality (number of studies = 17, OR 0.75, 95% CI 0.65 to 0.87) in the treated group. In trials comparing topical antimicrobials alone (or comparing topical plus systemic versus systemic alone) there was a significant reduction in RTIs (number of studies = 17, OR 0.44, 95% CI 0.31 to 0.63) but not in total mortality (number of studies = 19, OR 0.97, 95% CI 0.82 to 1.16) in the treated group. AUTHORS' CONCLUSIONS: A combination of topical and systemic prophylactic antibiotics reduces RTIs and overall mortality in adult patients receiving intensive care. Treatment based on the use of topical prophylaxis alone reduces respiratory infections but not mortality. The risk of resistance occurring as a negative consequence of antibiotic use was appropriately explored only in one trial which did not show any such effect
Divergences problem in black hole brick-wall model
In this work we review, in the framework of the so-called brick wall model,
the divergence problem arising in the one loop calculations of various
thermodynamical quantities, like entropy, internal energy and heat capacity.
Particularly we find that, if one imposes that entanglement entropy is equal to
the Bekenstein-Hawking one, the model gives problematic results. Then a
proposal of solution to the divergence problem is made following the zeroth law
of black hole mechanics.Comment: 19 pages, reviseted-extended version accepted by Phys. Rev.
Modified Dispersion Relations from the Renormalization Group of Gravity
We show that the running of gravitational couplings, together with a suitable
identification of the renormalization group scale can give rise to modified
dispersion relations for massive particles. This result seems to be compatible
with both the frameworks of effective field theory with Lorentz invariance
violation and deformed special relativity. The phenomenological consequences
depend on which of the frameworks is assumed. We discuss the nature and
strength of the available constraints for both cases and show that in the case
of Lorentz invariance violation, the theory would be strongly constrained.Comment: revtex4, 9 pages, updated to match published versio
Acoustic horizons for axially and spherically symmetric fluid flow
We investigate the formation of acoustic horizons for an inviscid fluid
moving in a pipe in the case of stationary and axi-symmetric flow. We show
that, differently from what is generally believed, the acoustic horizon forms
in correspondence of either a local minimum or maximum of the flux tube
cross-section. Similarly, the external potential is required to have either a
maximum or a minimum at the horizon, so that the external force has to vanish
there. Choosing a power-law equation of state for the fluid, , we solve the equations of the fluid dynamics and show that the two
possibilities are realized respectively for and . These results
are extended also to the case of spherically symmetric flow.Comment: 6 pages, 3 figure
The universal viscosity to entropy density ratio from entanglement
We present evidence that the universal Kovtun-Son-Starinets shear viscosity
to entropy density ratio of 1/4\pi can be associated with a Rindler causal
horizon in flat spacetime. Since there is no known holographic (gauge/gravity)
duality for this spacetime, a natural microscopic explanation for this
viscosity is in the peculiar properties of quantum entanglement. In particular,
it is well-known that the Minkowski vacuum state is a thermal state and carries
an area entanglement entropy density in the Rindler spacetime. Based on the
fluctuation-dissipation theorem, we expect a similar notion of viscosity
arising from vacuum fluctuations. Therefore, we propose a holographic Kubo
formula in terms of a two-point function of the stress tensor of matter fields
in the bulk. We calculate this viscosity assuming a minimally coupled scalar
field theory and find that the ratio with respect to the entanglement entropy
density is exactly 1/4\pi in four dimensions. The issues that arise in
extending this result to non-minimally coupled scalar fields, higher spins, and
higher dimensions provide interesting hints about the relationship between
entanglement entropy and black hole entropy.Comment: 30 pages; v2: footnote added, minor editin
Superradiant scattering from a hydrodynamic vortex
We show that sound waves scattered from a hydrodynamic vortex may be
amplified. Such superradiant scattering follows from the physical analogy
between spinning black holes and hydrodynamic vortices. However a sonic horizon
analogous to the black hole event horizon does not exist unless the vortex
possesses a central drain, which is challenging to produce experimentally. In
the astrophysical domain, superradiance can occur even in the absence of an
event horizon: we show that in the hydrodynamic analogue, a drain is not
required and a vortex scatters sound superradiantly. Possible experimental
realization in dilute gas Bose-Einstein condensates is discussed.Comment: 10 pages, 1 figur
Scharnhorst effect at oblique incidence
We consider the Scharnhorst effect (anomalous photon propagation in the
Casimir vacuum) at oblique incidence, calculating both photon speed and
polarization states as functions of angle. The analysis is performed in the
framework of nonlinear electrodynamics and we show that many features of the
situation can be extracted solely on the basis of symmetry considerations.
Although birefringence is common in nonlinear electrodynamics it is not
universal; in particular we verify that the Casimir vacuum is not birefringent
at any incidence angle. On the other hand, group velocity is typically not
equal to phase velocity, though the distinction vanishes for special directions
or if one is only working to second order in the fine structure constant. We
obtain an ``effective metric'' that is subtly different from previous results.
The disagreement is due to the way that ``polarization sums'' are implemented
in the extant literature, and we demonstrate that a fully consistent
polarization sum must be implemented via a bootstrap procedure using the
effective metric one is attempting to define. Furthermore, in the case of
birefringence, we show that the polarization sum technique is intrinsically an
approximation.Comment: 11 pages double-column format, 2 figures, RevTeX 4.0 (beta 2). Final
versio
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