6,431 research outputs found
Friedmann cosmology with decaying vacuum density
Among the several proposals to solve the incompatibility between the observed
small value of the cosmological constant and the huge value obtained by quantum
field theories, we can find the idea of a decaying vacuum energy density,
leading from high values at early times of universe evolution to the small
value observed nowadays. In this paper we consider a variation law for the
vacuum density recently proposed by Schutzhold on the basis of quantum field
estimations in the curved, expanding background, characterized by a vacuum
density proportional to the Hubble parameter. We show that, in the context of
an isotropic and homogeneous, spatially flat model, the corresponding solutions
retain the well established features of the standard cosmology, and, in
addition, are in accordance with the observed cosmological parameters. Our
scenario presents an initial phase dominated by radiation, followed by a dust
era long enough to permit structure formation, and by an epoch dominated by the
cosmological term, which tends asymptotically to a de Sitter universe. Taking
the matter density equals to half of the vacuum energy density, as suggested by
observation, we obtain a universe age given by Ht = 1.1, and a decelerating
parameter equals to -1/2.Comment: Accepted for publication in General Relativity and Gravitatio
Bursts in discontinuous Aeolian saltation
Close to the onset of Aeolian particle transport through saltation we find in
wind tunnel experiments a regime of discontinuous flux characterized by bursts
of activity. Scaling laws are observed in the time delay between each burst and
in the measurements of the wind fluctuations at the fluid threshold Shields
number . The time delay between each burst decreases on average with
the increase of the Shields number until sand flux becomes continuous. A
numerical model for saltation including the wind-entrainment from the turbulent
fluctuations can reproduce these observations and gives insight about their
origin. We present here also for the first time measurements showing that with
feeding it becomes possible to sustain discontinuous flux even below the fluid
threshold
Non-adiabatic Chaplygin gas
The split of a generalised Chaplygin gas with an equation of state p =
-A/\rho^{\alpha} into an interacting mixture of pressureless matter and a
dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda}
implies the existence of non-adiabatic pressure perturbations. We demonstrate
that the square of the effective (non-adiabatic) sound speed c_s of the medium
is proportional to the ratio of the perturbations of the dark energy to those
of the dark matter. Since, as demonstrated explicitly for the particular case
\alpha = -1/2, dark-energy perturbations are negligible compared with
dark-matter perturbations on scales that are relevant for structure formation,
we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities
which have plagued previous adiabatic Chaplygin-gas models.Comment: Version to appear in Physics Letters
Sustainability Assessment of indicators for integrated water resources management
The scientific community strongly recommends the adoption of indicators for the evaluation and monitoring of progress towards sustainable development. Furthermore, international organizations consider that indicators are powerful decision-making tools. Nevertheless, the quality and reliability of the indicators depends on the application of adequate and appropriate criteria to assess them. The general objective of this study was to evaluate how indicators related to water use and management perform against a set of sustainability criteria. Our research identified 170 indicators related to water use and management. These indicators were assessed by an international panel of experts that evaluated whether they fulfil the four sustainability criteria: social, economic, environmental, and institutional. We employed an evaluation matrix that classified all indicators according to the DPSIR (Driving Forces, Pressures, States, Impacts and Responses) framework. A pilot study served to test and approve the research methodology before carrying out the full implementation. The findings of the study show that 24 indicators comply with the majority of the sustainability criteria; 59 indicators are bi-dimensional (meaning that they comply with two sustainability criteria); 86 are one-dimensional indicators (fulfilling just one of the four sustainability criteria) and one indicator do not fulfil any of the sustainability criteria.Postprint (author's final draft
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