2,260 research outputs found
Scalar Field Cosmologies With Inverted Potentials
Regular bouncing solutions in the framework of a scalar-tensor gravity model
were found in a recent work. We reconsider the problem in the Einstein frame
(EF) in the present work. Singularities arising at the limit of physical
viability of the model in the Jordan frame (JF) are either of the Big Bang or
of the Big Crunch type in the EF. As a result we obtain integrable scalar field
cosmological models in general relativity (GR) with inverted double-well
potentials unbounded from below which possess solutions regular in the future,
tending to a de Sitter space, and starting with a Big Bang. The existence of
the two fixed points for the field dynamics at late times found earlier in the
JF becomes transparent in the EF.Comment: 18 pages, 4 figure
Bouncing Universes in Scalar-Tensor Gravity Models admitting Negative Potentials
We consider the possibility to produce a bouncing universe in the framework
of scalar-tensor gravity models in which the scalar field potential may be
negative, and even unbounded from below. We find a set of viable solutions with
nonzero measure in the space of initial conditions passing a bounce, even in
the presence of a radiation component, and approaching a constant gravitational
coupling afterwards. Hence we have a model with a minimal modification of
gravity in order to produce a bounce in the early universe with gravity tending
dynamically to general relativity (GR) after the bounce.Comment: 12 pages, Improved presentation with 4 figures, Results and
conclusions unchange
How Mayors Perceive the Influence of Social Media on the Policy Cycle
Information technology clearly influences the decision-making behaviour of individuals, groups, and organisations. In particular, social media and Web 2.0 technologies can affect the rationality and effectiveness of the policy cycle in the public sector. This growing influence deserves to be analysed. This work aims to understand how the influence of social media in the different phases of the policy cycle is perceived by mayors, the main decision-makers in local governments
Morphogenetic Characteristics of \u3cem\u3ePanicum Maximum\u3c/em\u3e cv. Aruana Subjected to Five Defoliation Stubble Heights and Two Frequencies
Tillers in grass swards are subject to size density compensation and this mechanism has been observed to follow the -3/2 self thinning rule. This theory assumes that tiller components (leaf lamina and stems) have a constant geometry as the sward is taller or shorter. In a re examination of this rule (SackvilleHamilton et al., 1995) observed that in grass swards the slope can be different from -3/2 depending on the extremes of defoliation (Hernandez-Garay et al., 1999). Therefore, the dimensionless measure, R (ratio tiller leaf area : volume) was proposed to isolate the tiller geometry component from the tiller size. Guinea grass is the second most sown grass in Brazilian pastures and cv. Aruana has been used successfully in the past ten years in sheep grazing systems. Since this bunch grass presents numerous aerial tillers that contribute to yield, the objective of this paper was to evaluate changes in tiller leaf area and volume on plants subjected to high frequency (simulating a continuous grazing), and low frequency defoliation (simulating an intermittent grazing), under different defoliation stubble heights
Laccase-Carrying Polylactic Acid Electrospun Fibers, Advantages and Limitations in Bio-Oxidation of Amines and Alcohols
Laccases are oxidative enzymes that could be good candidates for the functionalization of biopolymers with several applications as biosensors for the determination of bioactive amine and alcohols, for bioremediation of industrial wastewater, and for greener catalysts in oxidation reactions in organic synthesis, especially used for non-phenolic compounds in combination with redox mediators in the so-called Laccase Mediator System (LMS). In this work, we describe the immobilization of Laccase from Trametes versicolor (LTv) in poly-L-lactic acid (PLLA) nanofibers and its application in LMS oxidation reactions. The PLLA-LTv catalysts were successfully produced by electrospinning of a water-in-oil emulsion with an optimized method. Different enzyme loadings (1.6, 3.2, and 5.1% (w)/(w)) were explored, and the obtained mats were thoroughly characterized. The actual amount of the enzyme in the fibers and the eventual enzyme leaching in different solvents were evaluated. Finally, the PLLA-LTv mats were successfully applied as such in the oxidation reaction of catechol, and in the LMS method with TEMPO as mediator in the oxidation of amines with the advantage of easier work-up procedures by the immobilized enzyme. However, the PLLA-LTv failed the oxidation of alcohols with respect to the free enzyme. A tentative explanation was provided
A no-go theorem on the nature of the gravitational field beyond quantum theory
Recently, table-top experiments involving massive quantum systems have been
proposed to test the interface of quantum theory and gravity. In particular,
the crucial point of the debate is whether it is possible to conclude anything
on the quantum nature of the gravitational field, provided that two quantum
systems become entangled due to solely the gravitational interaction.
Typically, this question has been addressed by assuming an underlying physical
theory to describe the gravitational interaction, but no systematic approach to
characterise the set of possible gravitational theories which are compatible
with the observation of entanglement has been proposed. Here, we introduce the
framework of Generalised Probabilistic Theories (GPTs) to the study of the
nature of the gravitational field. This framework has the advantage that it
only relies on the set of operationally accessible states, transformations, and
measurements, without presupposing an underlying theory. Hence, it provides a
framework to systematically study all theories compatible with the detection of
entanglement generated via the gravitational interaction between two
non-classical systems. Assuming that such entanglement is observed we prove a
no-go theorem stating that the following statements are incompatible: i) the
two non-classical systems are independent subsystems, ii) the gravitational
field is a physical degree of freedom which mediates the interaction and iii)
the gravitational field is classical. Moreover we argue that conditions i) and
ii) should be met, and hence that the gravitational field is non-classical.
Non-classicality does not imply that the gravitational field is quantum, and to
illustrate this we provide examples of non-classical and non-quantum theories
which are logically consistent with the other conditions.Comment: 12 pages main text; 23 pages Appendices; many diagrams. Improved
presentation compared to the first versio
Bifurcation diagram and stability for a one-parameter family of planar vector fields
We consider the 1-parameter family of planar quintic systems, , , introduced by A. Bacciotti in 1985. It is known
that it has at most one limit cycle and that it can exist only when the
parameter is in . In this paper, using the Bendixon-Dulac
theorem, we give a new unified proof of all the previous results, we shrink
this to , and we prove the hyperbolicity of the limit cycle. We
also consider the question of the existence of polycycles. The main interest
and difficulty for studying this family is that it is not a semi-complete
family of rotated vector fields. When the system has a limit cycle, we also
determine explicit lower bounds of the basin of attraction of the origin.
Finally we answer an open question about the change of stability of the origin
for an extension of the above systems.Comment: 27 pages, 11 figure
Any consistent coupling between classical gravity and quantum matter is fundamentally irreversible
When gravity is sourced by a quantum system, there is tension between its
role as the mediator of a fundamental interaction, which is expected to acquire
nonclassical features, and its role in determining the properties of spacetime,
which is inherently classical. Fundamentally, this tension should result in
breaking one of the fundamental principles of quantum theory or general
relativity, but it is usually hard to assess which one without resorting to a
specific model. Here, we answer this question in a theory-independent way using
General Probabilistic Theories (GPTs). We consider the interactions of the
gravitational field with a single matter system, and derive a no-go theorem
showing that when gravity is classical at least one of the following
assumptions needs to be violated: (i) Matter degrees of freedom are described
by fully non-classical degrees of freedom; (ii) Interactions between matter
degrees of freedom and the gravitational field are reversible; (iii) Matter
degrees of freedom back-react on the gravitational field. We argue that this
implies that theories of classical gravity and quantum matter must be
fundamentally irreversible, as is the case in the recent model of Oppenheim et
al. Conversely if we require that the interaction between quantum matter and
the gravitational field are reversible, then the gravitational field must be
non-classical.Comment: 5 pages main text; 8 pages Appendices (many diagrams
Machine learning and multidrug-resistant gram-negative bacteria: An interesting combination for current and future research
The dissemination of multidrug-resistant Gram-negative bacteria (MDR-GNB) is associated with increased morbidity and mortality in several countries. Machine learning (ML) is a branch of artificial intelligence that consists of conferring on computers the ability to learn from data. In this narrative review, we discuss three existing examples of the application of ML algorithms for assessing three different types of risk: (i) the risk of developing a MDR-GNB infection, (ii) the risk of MDR-GNB etiology in patients with an already clinically evident infection, and (iii) the risk of anticipating the emergence of MDR in GNB through the misuse of antibiotics. In the next few years, we expect to witness an increasingly large number of research studies perfecting the application of ML techniques in the field of MDR-GNB infections. Very importantly, this cannot be separated from the availability of a continuously refined and updated ethical framework allowing an appropriate use of the large datasets of medical data needed to build efficient ML-based support systems that could be shared through appropriate standard infrastructures
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