1,031 research outputs found
Modeling the quantum evolution of the universe through classical matter
It is well known that the canonical quantization of the
Friedmann-Lema\^itre-Robertson-Walker (FLRW) filled with a perfect fluid leads
to nonsingular universes which, for later times, behave as their classical
counterpart. This means that the expectation value of the scale factor
never vanishes and, as , we recover the classical expression for
the scale factor. In this paper, we show that such universes can be reproduced
by classical cosmology given that the universe is filled with an exotic matter.
In the case of a perfect fluid, we find an implicit equation of state (EoS). We
then show that this single fluid with an implict EoS is equivalent to two
non-interacting fluids, one of them representing stiff matter with negative
energy density. In the case of two non-interacting scalar fields, one of them
of the phantom type, we find their potential energy. In both cases we find that
quantum mechanics changes completely the configuration of matter for small
values of time, by adding a fluid or a scalar field with negative energy
density. As time passes, the density of negative energy decreases and we
recover the ordinary content of the classical universe. The more the initial
wave function of the universe is concentrated around the classical big bang
singularity, the more it is necessary to add negative energy, since this type
of energy will be responsible for the removal of the classical singularity.Comment: updated version as accepted by Gen. Relativ. Gravi
Dilaton Quantum Cosmology with a Schrodinger-like equation
A quantum cosmological model with radiation and a dilaton scalar field is
analysed. The Wheeler-deWitt equation in the mini-superspace induces a
Schr\"odinger equation, which can be solved. An explicit wavepacket is
constructed for a particular choice of the ordering factor. A consistent
solution is possible only when the scalar field is a phantom field. Moreover,
although the wavepacket is time dependent, a Bohmian analysis allows to extract
a bouncing behaviour for the scale factor.Comment: 14 pages, 3 figures in eps format. Minors corrections, new figure
Emergence of structural and dynamical properties of ecological mutualistic networks
Mutualistic networks are formed when the interactions between two classes of
species are mutually beneficial. They are important examples of cooperation
shaped by evolution. Mutualism between animals and plants plays a key role in
the organization of ecological communities. Such networks in ecology have
generically evolved a nested architecture independent of species composition
and latitude - specialists interact with proper subsets of the nodes with whom
generalists interact. Despite sustained efforts to explain observed network
structure on the basis of community-level stability or persistence, such
correlative studies have reached minimal consensus. Here we demonstrate that
nested interaction networks could emerge as a consequence of an optimization
principle aimed at maximizing the species abundance in mutualistic communities.
Using analytical and numerical approaches, we show that because of the
mutualistic interactions, an increase in abundance of a given species results
in a corresponding increase in the total number of individuals in the
community, as also the nestedness of the interaction matrix. Indeed, the
species abundances and the nestedness of the interaction matrix are correlated
by an amount that depends on the strength of the mutualistic interactions.
Nestedness and the observed spontaneous emergence of generalist and specialist
species occur for several dynamical implementations of the variational
principle under stationary conditions. Optimized networks, while remaining
stable, tend to be less resilient than their counterparts with randomly
assigned interactions. In particular, we analytically show that the abundance
of the rarest species is directly linked to the resilience of the community.
Our work provides a unifying framework for studying the emergent structural and
dynamical properties of ecological mutualistic networks.Comment: 10 pages, 4 figure
The Dynamics of Nestedness Predicts the Evolution of Industrial Ecosystems
In economic systems, the mix of products that countries make or export has
been shown to be a strong leading indicator of economic growth. Hence, methods
to characterize and predict the structure of the network connecting countries
to the products that they export are relevant for understanding the dynamics of
economic development. Here we study the presence and absence of industries at
the global and national levels and show that these networks are significantly
nested. This means that the less filled rows and columns of these networks'
adjacency matrices tend to be subsets of the fuller rows and columns. Moreover,
we show that nestedness remains relatively stable as the matrices become more
filled over time and that this occurs because of a bias for industries that
deviate from the networks' nestedness to disappear, and a bias for the missing
industries that reduce nestedness to appear. This makes the appearance and
disappearance of individual industries in each location predictable. We
interpret the high level of nestedness observed in these networks in the
context of the neutral model of development introduced by Hidalgo and Hausmann
(2009). We show that, for the observed fills, the model can reproduce the high
level of nestedness observed in these networks only when we assume a high level
of heterogeneity in the distribution of capabilities available in countries and
required by products. In the context of the neutral model, this implies that
the high level of nestedness observed in these economic networks emerges as a
combination of both, the complementarity of inputs and heterogeneity in the
number of capabilities available in countries and required by products. The
stability of nestedness in industrial ecosystems, and the predictability
implied by it, demonstrates the importance of the study of network properties
in the evolution of economic networks.Comment: 26 page
Effect of Biodiversity Changes in Disease Risk: Exploring Disease Emergence in a Plant-Virus System
The effect of biodiversity on the ability of parasites to infect their host and cause disease (i.e. disease risk) is a major question in pathology, which is central to understand the emergence of infectious diseases, and to develop strategies for their management. Two hypotheses, which can be considered as extremes of a continuum, relate biodiversity to disease risk: One states that biodiversity is positively correlated with disease risk (Amplification Effect), and the second predicts a negative correlation between biodiversity and disease risk (Dilution Effect). Which of them applies better to different host-parasite systems is still a source of debate, due to limited experimental or empirical data. This is especially the case for viral diseases of plants. To address this subject, we have monitored for three years the prevalence of several viruses, and virus-associated symptoms, in populations of wild pepper (chiltepin) under different levels of human management. For each population, we also measured the habitat species diversity, host plant genetic diversity and host plant density. Results indicate that disease and infection risk increased with the level of human management, which was associated with decreased species diversity and host genetic diversity, and with increased host plant density. Importantly, species diversity of the habitat was the primary predictor of disease risk for wild chiltepin populations. This changed in managed populations where host genetic diversity was the primary predictor. Host density was generally a poorer predictor of disease and infection risk. These results support the dilution effect hypothesis, and underline the relevance of different ecological factors in determining disease/infection risk in host plant populations under different levels of anthropic influence. These results are relevant for managing plant diseases and for establishing conservation policies for endangered plant species
Sustainability and Corporate Social Responsibility in the Perspective of Social Economy Entities: A Bibliometric Study
We start from the conceptual interconnection between Sustainability and Corporate Social Responsibility, which, although continuously subject to controversy, both within the scope of its definition and in its performance, advocate, as the ultimate goal, human development and of the society in general, promoting the interests of the Communities on a permanent, long-term basis and without compromising the options of the future generations. In this context, we cannot fail to draw a parallel with the entities that constitute the Social Economy Sector. The activities they carry out are of economic and social nature and must be pursued in the general interest of their members, users and beneficiaries, thus in the general interest of the Community. These institutions also reveal concerns about the sustainability in all the dimensions involved (economic, social and environmental), in which the organizational performance is particularly important, as it becomes imperative to guarantee their continuity, fostering and promoting their social action. We will, therefore, start by framing what is meant by Sustainability, Corporate Social Responsibility and Social Economy, with a particular focus on the current requirements of stakeholders regarding the socially responsible behaviour of the institutions as these, in turn, will entail the adoption of more comprehensive management tools, also more efficient and transparent concerning all dimensions (economic, financial and social). It is within this framework that a project called “TFA—TheoFrameAccountability—Theoretical framework for the promotion of accountability in the social economy sector: The IPSS case” emerges, being promoted by the University of Aveiro, with the participation of National Confederation of Solidarity Institutions (CNIS—acronym in Portuguese), and the Polytechnic Institutes of Coimbra and Porto. This project aims to promote the accountability of the social economy sector (economic, financial and social aspects), in the Private Social Solidarity Institutions (IPSS—acronym in Portuguese), assisting them not only in fulfilling their legal obligations, but also facilitating the reporting of results of activities carried out in a more effective manner and promoting transfer of knowledge (for the IPSS and also for the academic community), thus contributing to the development and sustainability of these institutions. Thus, we develop an exploratory and descriptive analysis, of a quantitative-qualitative nature, in which the procedures of data collection determine the result of the search strategy by the defined descriptors. For this purpose, the analysis will focus on the following variables: number of articles published per year; methodologies used; theories of support; identification by sector/area of activity; countries of origin; more representative institutions; authors who publish more and journals with the largest number of publications. The main results indicate a growing concern about sustainability and a growing publication in this area. This paper presents a bibliometric study to evaluate the main trends of current research on sustainability and on corporate social responsibility, thus contributing to the construction of the theoretical basis underlying the “TFA—TheoFrameAccountability” project.info:eu-repo/semantics/publishedVersio
Aharonov-Bohm interferences from local deformations in graphene
One of the most interesting aspects of graphene is the tied relation between
structural and electronic properties. The observation of ripples in the
graphene samples both free standing and on a substrate has given rise to a very
active investigation around the membrane-like properties of graphene and the
origin of the ripples remains as one of the most interesting open problems in
the system. The interplay of structural and electronic properties is
successfully described by the modelling of curvature and elastic deformations
by fictitious gauge fields that have become an ex- perimental reality after the
suggestion that Landau levels can form associated to strain in graphene and the
subsequent experimental confirmation. Here we propose a device to detect
microstresses in graphene based on a scanning-tunneling-microscopy setup able
to measure Aharonov-Bohm inter- ferences at the nanometer scale. The
interferences to be observed in the local density of states are created by the
fictitious magnetic field associated to elastic deformations of the sample.Comment: Some bugs fixe
Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films
The outstanding electrical and mechanical properties of graphene make it very
attractive for several applications, Nanoelectronics above all. However a
reproducible and non destructive way to produce high quality, large-scale area,
single layer graphene sheets is still lacking. Chemical Vapour Deposition of
graphene on Cu catalytic thin films represents a promising method to reach this
goal, because of the low temperatures (T < 900 Celsius degrees) involved during
the process and of the theoretically expected monolayer self-limiting growth.
On the contrary such self-limiting growth is not commonly observed in
experiments, thus making the development of techniques allowing for a better
control of graphene growth highly desirable. Here we report about the local
ablation effect, arising in Raman analysis, due to the heat transfer induced by
the laser incident beam onto the graphene sample.Comment: v1:9 pages, 8 figures, submitted to SpringerPlus; v2: 11 pages,
PDFLaTeX, 9 figures, revised peer-reviewed version resubmitted to
SpringerPlus; 1 figure added, figure 1 and 4 replaced,typos corrected,
"Results and discussion" section significantly extended to better explain
etching mechanism and features of Raman spectra, references adde
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