740 research outputs found
Fate of the Universe, Age of the Universe, Dark Matter, and the Decaying Vacuum Energy
It is shown that in the cosmological models based on a vacuum energy decaying
as a^{-2}, where a is the scale factor of the universe, the fate of the
universe in regard to whether it will collapse in future or expand forever is
determined not by the curvature constant k but by an effective curvature
constant k_{eff}. It is argued that a closed universe with k=1 may expand
forever, in other words simulate the expansion dynamics of a flat or an open
universe because of the possibility that k_{eff}=0 or -1, respectively. Two
such models, in one of which the vacuum does not interact with matter and in
another of which it does, are studied. It is shown that the vacuum equation of
state p_{vac}= -\rho_{vac} may be realized in a decaying vacuum cosmology
provided the vacuum interacts wuth matter. The optical depths for gravitational
lensing as a function of the matter density and other parameters in the models
are calculated at a source redshift of 2. The age of the universe is discussed
and shown to be compatible with the new Hipparcos lower limit of 11Gyr. The
possibility that a time-varying vacuum energy may serve as dark matter is
suggested.Comment: AAS LaTex, 29 pages, published in the Astrophysical Journal, 520, 45,
199
Species diversity and distribution of ruderal flora on landfills in Maradi city, Niger
Waste management continues to be a critical environmental issue in cities. It impacts on the well being of the population, the environment and the biodiversity. In the city of Maradi, in Niger, more interest is given to the problem in order to understand the whole waste management system. It is in this context that this study is carried out to investigate on the role of ruderal flora on the municipal solid wastes dumpsites and landfill sites in Maradi city. The specific objectives are to determine the floristic diversity and distribution of ruderal flora on the municipal solid waste disposal sites, and to identify potential species that can play an important role in the phytoremediation of these sites. In total, 65 species belonging to 52 genera and 24 families were recorded. These species can be categorised into two groups containing anthropic and nitrophilic species according to the ascending Hierarchical Classification (AHC) at 25% similarity. Characteristic species of the first group G1 are Amaranthus viridis and Cucurbita pepo, and Datura innoxia and Cucumis melo for the second group G2. Other ruderal species, namely Amaranthus spinosus L., Amaranthus viridis L., Celosia trygina L., Datura innoxia Mill., and an introduced woody species, Cuphea hyssopifolia Kunth., found are not included in the Maradi city list of species. Datura innoxia, Amaranthus viridis and Amaranthus spinosus are species known to tolerate different degrees of pollution and their ecology should be further study to better understand how they can be used for phytoremediation on this kind of sites
Higher spin fields and the problem of cosmological constant
The cosmological evolution of free massless vector or tensor (but not gauge)
fields minimally coupled to gravity is analyzed. It is shown that there are
some unstable solutions for these fields in De Sitter background. The back
reaction of the energy-momentum tensor of such solutions to the original
cosmological constant exactly cancels the latter and the expansion regime
changes from the exponential to the power law one. In contrast to the
adjustment mechanism realized by a scalar field the gravitational coupling
constant in this model is time-independent and the resulting cosmology may
resemble the realistic one.Comment: 15 pages, Latex twic
Relationship between thermohaline and biochemical patterns in the levantine upper and intermediate water masses, Southeastern Mediterranean Sea (2013â2021)
The relationships between the interannual variations of the Levantine intermediate water (LIW) core properties and the corresponding biochemical variations in the euphotic zone were systematically studied in the Southeastern Mediterranean during 2013â2021 and since 2002 based on a previous study. Salinity and temperature interannual fluctuations in the LIW continue to follow the AdriaticâIonian Bimodal Oscillating System (BiOS) mechanism, with salinity and temperature peaks in the years 2008â2010, 2014â2015, and 2018â2019 coinciding with periods of anticyclonic circulation of the North Ionian Gyre (NIG). During these anticyclonic periods, the transport of Atlantic Water into the Levant is reduced together with the transport of LIW out of the basin. These interannual fluctuations are superimposed on a long-term warming trend clearly evident from previous studies, showing a maximal temperature in 2018â2019, higher than the previously mentioned temperature peaks by ~0.7°C and ~0.4°C. The enhanced warming in 2018â2019 has caused a decrease in density (sigma) values of the LIW core, which gave way to the shallowest record of this water mass (~110-m depth), bringing it well within the lower photic zone. We suggest that a higher level of nutrients became available, supporting the observed long-term rise of the intergraded chlorophyll a (Chl.a) (0.89 mg mâ2 yearâ1), with a maximum recorded during 2018â2019. The long-term record of the mixed layer depths shows no significant change; thus, the uplift of nutrients during winter mixing cannot support the trend and variations of the integrated Chl.a. Additional biological parameters of specific pico-phytoplankton populations and integrated bacterial production and abundance were measured in 2013â2021, but the measurements were too sparse to follow a clear interannual dynamics. Yet significantly higher average levels for integrated primary production and bacterial abundances were observed during the anticyclonic period (as for Chl.a). The combined impacts of the BiOS mechanism and global warming, and hence the increase in LIW residence time and buoyancy, may impact the primary producersâ biomass at the photic zone. This latter feedback may slightly counter the enhanced oligotrophication due to enhanced stratification
Exact Black Hole and Cosmological Solutions in a Two-Dimensional Dilaton-Spectator Theory of Gravity
Exact black hole and cosmological solutions are obtained for a special
two-dimensional dilaton-spectator () theory of gravity. We show how
in this context any desired spacetime behaviour can be determined by an
appropriate choice of a dilaton potential function and a ``coupling
function'' in the action. We illustrate several black hole solutions
as examples. In particular, asymptotically flat double- and multiple- horizon
black hole solutions are obtained. One solution bears an interesting
resemblance to the string-theoretic black hole and contains the same
thermodynamic properties; another resembles the Reissner-Nordstrom
solution. We find two characteristic features of all the black hole solutions.
First the coupling constants in must be set equal to constants of
integration (typically the mass). Second, the spectator field and its
derivative both diverge at any event horizon. A test particle with
``spectator charge" ({\it i.e.} one coupled either to or ),
will therefore encounter an infinite tidal force at the horizon or an
``infinite potential barrier'' located outside the horizon respectively. We
also compute the Hawking temperature and entropy for our solutions. In
cosmology, two non-singular solutions which resemble two exact solutions
in string-motivated cosmology are obtained. In addition, we construct a
singular model which describes the standard non-inflationary big bang
cosmology (). Motivated by the
similaritiesbetween and gravitational field equations in
cosmology, we briefly discuss a special dilaton-spectator action
constructed from the bosonic part of the low energy heterotic string action andComment: 34 pgs. Plain Tex, revised version contains some clarifying comments
concerning the relationship between the constants of integration and the
coupling constants
Deciphering Drought Response Mechanisms: Transcriptomic Insights from Drought-Tolerant and Drought-Sensitive Wheat (Triticum aestivum L.) Cultivars
Drought stress poses a significant threat to wheat (Triticum aestivum L.) cultivation, necessitating an in-depth understanding of the molecular mechanisms underpinning drought response in both tolerant and sensitive varieties. In this study, 12 diverse bread wheat cultivars were evaluated for their drought stress responses, with particular emphasis on the contrasting performance of cultivars Atay 85 (sensitive), Gerek 79, and Mufitbey (tolerant). Transcriptomic analysis was performed on the root and leaf tissues of the aforementioned cultivars subjected to 4-hour and 8-hour drought stress and compared with controls. Differentially expressed genes (DEGs) were categorized based on their cellular component, molecular function, and biological function. Notably, there was greater gene expression variability in leaf tissues compared to root tissues. A noticeable trend of decreased gene expression was observed for cellular processes such as protein refolding and cellular metabolic processes like photorespiration as drought stress duration increased (8 hours) in the leaf tissues of drought-tolerant and sensitive cultivars. Metabolic processes related to gene expression were predominantly activated in response to 4-hour and 8-hour drought stress. The drought-tolerant cultivars exhibited increased expression levels of genes related to protein binding, metabolic processes, and cellular functions, indicating their ability to adapt better to drought stress compared to the drought-sensitive cultivar Atay 85. We detected more than 25 differentially expressed TFs in leaf tissues under 4-hour and 8-hour drought stress, while only 4 TFs were identified in the root tissues of sensitive cultivar. In contrast, the tolerant cultivar exhibited more than 80 different TF transcripts in both leaves and roots after 4 hours of drought stress, with this number decreasing to 18 after 8 hours of drought stress. Differentially expressed genes with a focus on metal ion binding, carbohydrate degradation, ABA-related genes, and cell wall-related genes were highlighted. Ferritin (TaFer), TaPME42 and Extensin-like protein (TaExLP), Germin-like protein (TaGLP 9-1), Metacaspase-5 (TaMC5), Arogenate Dehydratase 5 (ADT-5), Phosphoglycerate/ bisphosphoglycerate mutase (TaPGM), Serine/threonine protein phosphatase 2A (TaPP2A), GIGANTEA (TaGI), Polyadenylate-binding protein (TaRBP45B) exhibited differential expression by qRT-PCR in root and leaf tissues of tolerant and sensitive bread wheat cultivars. This study provides valuable insights into the complex molecular mechanisms associated with drought response in wheat, highlighting genes and pathways involved in drought tolerance. Understanding these mechanisms is essential for developing drought-tolerant wheat varieties, enhancing agricultural sustainability, and addressing the challenges posed by water scarcity
Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification
We consider the scenario emerging from the dynamics of a generalized
-brane in a spacetime. The equation of state describing this
system is given in terms of the energy density, , and pressure, , by
the relationship , where is a positive constant and
. We discuss the conditions under which homogeneity arises
and show that this equation of state describes the evolution of a universe
evolving from a phase dominated by non-relativistic matter to a phase dominated
by a cosmological constant via an intermediate period where the effective
equation of state is given by .Comment: 5 pages, 4 figures, revte
Uncertainty estimation for operational ocean forecast products-a multi-model ensemble for the North Sea and the Baltic Sea
Multi-model ensembles for sea surface temperature (SST), sea surface salinity (SSS), sea surface currents (SSC), and water transports have been developed for the North Sea and the Baltic Sea using outputs from several operational ocean forecasting models provided by different institutes. The individual models differ in model code, resolution, boundary conditions, atmospheric forcing, and data assimilation. The ensembles are produced on a daily basis. Daily statistics are calculated for each parameter giving information about the spread of the forecasts with standard deviation, ensemble mean and median, and coefficient of variation. High forecast uncertainty, i.e., for SSS and SSC, was found in the Skagerrak, Kattegat (Transition Area between North Sea and Baltic Sea), and the Norwegian Channel. Based on the data collected, longer-term statistical analyses have been done, such as a comparison with satellite data for SST and evaluation of the deviation between forecasts in temporal and spatial scale. Regions of high forecast uncertainty for SSS and SSC have been detected in the Transition Area and the Norwegian Channel where a large spread between the models might evolve due to differences in simulating the frontal structures and their movements. A distinct seasonal pattern could be distinguished for SST with high uncertainty between the forecasts during summer. Forecasts with relatively high deviation from the multi-model ensemble (MME) products or the other individual forecasts were detected for each region and each parameter. The comparison with satellite data showed that the error of the MME products is lowest compared to those of the ensemble members
Cosmic anti-friction and accelerated expansion
We explain an accelerated expansion of the present universe, suggested from
observations of supernovae of type Ia at high redshift, by introducing an
anti-frictional force that is self-consistently exerted on the particles of the
cosmic substratum. Cosmic anti-friction, which is intimately related to
``particle production'', is shown to give rise to an effective negative
pressure of the cosmic medium. While other explanations for an accelerated
expansion (cosmological constant, quintessence) introduce a component of dark
energy besides ``standard'' cold dark matter (CDM) we resort to a
phenomenological one-component model of CDM with internal self-interactions. We
demonstrate how the dynamics of the LambdaCDM model may be recovered as a
special case of cosmic anti-friction. We discuss the connection with
two-component models and obtain an attractor behavior for the ratio of the
energy densities of both components which provides a possible phenomenological
solution to the coincidence problem.Comment: 19 pages, 7 (3 new) figures, new derivation of kinetic equation with
force term, accepted by Physical Review
Tunneling in Decaying Cosmologies and the Cosmological Constant Problem
The tunneling rate, with exact prefactor, is calculated to first order in
for an empty closed Friedmann-Robertson-Walker (FRW) universe with
decaying cosmological term ( is the scale factor and
is a parameter ). This model is equivalent to a cosmology
with the equation of state . The calculations are
performed by applying the dilute-instanton approximation on the corresponding
Duru-Kleinert path integral.
It is shown that the highest tunneling rate occurs for corresponding to
the cosmic string matter universe. The obtained most probable cosmological
term, like one obtained by Strominger, accounts for a possible solution to the
cosmological constant problem.Comment: 21 pages, REVTEX, The section 3 is considerably completed including
some physical mechanisms supporting the time variation of the cosmological
constant, added references for the section 3. Accepted to be published in
Phys. Rev.
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