99 research outputs found
Dynamics of entropy perturbations in assisted dark energy with mixed kinetic terms
We study dynamics of entropy perturbations in the two-field assisted dark
energy model. Based on the scenario of assisted dark energy, in which one
scalar field is subdominant compared with the other in the early epoch, we show
that the entropy perturbations in this two-field system tend to be constant on
large scales in the early epoch and hence survive until the present era for a
generic evolution of both fields during the radiation and matter eras. This
behaviour of the entropy perturbations is preserved even when the fields are
coupled via kinetic interaction. Since, for assisted dark energy, the
subdominant field in the early epoch becomes dominant at late time, the entropy
perturbations can significantly influence the dynamics of density perturbations
in the universe. Assuming correlations between the entropy and curvature
perturbations, the entropy perturbations can enhance the integrated Sachs-Wolfe
(ISW) effect if the signs of the contributions from entropy perturbations and
curvature perturbations are opposite after the matter era, otherwise the ISW
contribution is suppressed. For canonical scalar field the effect of entropy
perturbations on ISW effect is small because the initial value of the entropy
perturbations estimated during inflation cannot be sufficiently large. However,
in the case of k-essence, the initial value of the entropy perturbations can be
large enough to affect the ISW effect to leave a significant imprint on the CMB
power spectrum.Comment: 25 pages, 8 figures, revised version, accepted for publication in
JCA
Predictors of upper limb disorders among a public university workers in Malaysia
Background: Upper Limb Disorder (ULD) includes a wide range of musculoskeletal diseases and syndromes, which are usually associated with pain and discomfort. The major forms of musculoskeletal disease that account for significant work-related morbidity include upper limb disorders. The main objective of this study was to determine the prevalence and associated factors of upper limb disorders among a public university worker.
Methodology: A cross sectional study design and simple random sampling was used to select six out of fifteen faculties. A total 271 participants were selected from the list of workers name during 20th of May 2014 to 1st of September 2014. Data were collected by face-to-face interview based on standardizes Nordic questionnaire. Weight was measured with a digital (TANITA Weighing Scale) and height by using (SECA206 bodymeter).
Results: The results of this study showed that the response rate was 86%, the prevalence of ULDs among staffs of public university in Malaysia was 67.2% in different body regions, while neck had the highest prevalence 147 (54.2%) and elbows had the lowest prevalence 35 (13%). In addition, significant association were found between ULDs with age, smoking, BMI, exercise and duration of employment (P<0.05). Logistic regression analysis indicated that BMI and exercise were significant predictors of ULDs.
Conclusion: The prevalence of upper limb disorders among public university workers is high (67.2%). The major risk factors responsible for ULDs was BMI and exercise among public university workers, so workers should be educated on how to maintain an optimal BMI through the consumption of appropriate meals and regular exercise recommended in order to prevent ULDs and other related musculoskeletal problems
Optimal Rehabilitation of Water Distribution Systems using a Cluster-based Technique
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Optimal rehabilitation of large water distribution system (WDS) with many decision variables, is often time-consuming and computationally expensive. This paper presents a new optimal rehabilitation methodology for WDSs based on graph theory clustering concept. The methodology starts with partitioning the WDS based on its connectivity properties into a number of clusters (small sub-systems). Pipes which might have direct impact on system performance are identified and considered for rehabilitation problem. Three optimisation-based strategies are then considered for pipe rehabilitation in the clustered network: 1) rehabilitation of some of the pipes inside the clusters; 2) rehabilitation of pipes in the path supplying water to the clusters; 3) combination of strategies 1 and 2. In all optimisation strategies, the decision variables for rehabilitation problem are the diameters of duplicated pipes; the objective functions are to minimise the total cost of duplicated pipes and to minimise the number of nodes with pressure deficiency. The performance of proposed strategies was demonstrated in a large WDS with pressure deficiencies. The performance of these strategies were also compared to the full search space optimisation strategy and engineering judgement based optimisation strategy in which all pipes and selection of pipes are considered as decision variables respectively. The results show that strategy 3 is able to generate solutions with similar performance that are cheaper by around 53% and 35% in comparison with the full search space and engineering judgement based optimisation strategies respectively. The results also demonstrate that the cluster-based approach can reduce the computational efforts for achieving optimum solutions compared to the other optimization strategies
Observational constraints on the dark energy density evolution
We constrain the evolution of the dark energy density from Cosmic Microwave
Background, Large Scale Structure and Supernovae Ia measurements. While
Supernovae Ia are most sensitive to the equation of state of dark energy
today, the Cosmic Microwave Background and Large Scale Structure data best
constrains the dark energy evolution at earlier times. For the parametrization
used in our models, we find and the dark energy fraction at very
high redshift at 95 per cent confidence level.Comment: 5 pages, 10 figure
CMB constraints on noncommutative geometry during inflation
We investigate the primordial power spectrum of the density perturbations
based on the assumption that spacetime is noncommutative in the early stage of
inflation. Due to the spacetime noncommutativity, the primordial power spectrum
can lose rotational invariance. Using the k-inflation model and slow-roll
approximation, we show that the deviation from rotational invariance of the
primordial power spectrum depends on the size of noncommutative length scale
L_s but not on sound speed. We constrain the contributions from the spacetime
noncommutativity to the covariance matrix for the harmonic coefficients of the
CMB anisotropies using five-year WMAP CMB maps. We find that the upper bound
for L_s depends on the product of sound speed and slow-roll parameter.
Estimating this product using cosmological parameters from the five-year WMAP
results, the upper bound for L_s is estimated to be less than 10^{-27} cm at
99.7% confidence level.Comment: 8 pages, 1 figure, References added, Accepted for publication in EPJC
(submitted version
Spatial Ricci scalar dark energy model
Inspired by holographic principle, we suggest that the density of dark energy
is proportional to the spatial Ricci scalar curvature (SRDE). Such model is
phenomenologically viable. The best fit values of its parameters at 68%
confidence level are found to be: and
, constrained from the Union+CFA3 sample of 397 SNIa and
the BAO measurement. We find the equation of state of SRDE crosses -1 at
. The present values of the deceleration parameter for
SRDE is found to be . The phase transition from deceleration
to acceleration of the Universe for SRDE occurs at the redshift . After studying on the perturbation of each component of the Universe, we
show that the matter power spectra and cosmic microwave background temperature
anisotropy is slightly affected by SRDE, compared with CDM.Comment: 9 pages, 7 figure
Probing interaction and spatial curvature in the holographic dark energy model
In this paper we place observational constraints on the interaction and
spatial curvature in the holographic dark energy model. We consider three kinds
of phenomenological interactions between holographic dark energy and matter,
i.e., the interaction term is proportional to the energy densities of dark
energy (), matter (), and matter plus dark energy
(). For probing the interaction and spatial curvature in
the holographic dark energy model, we use the latest observational data
including the type Ia supernovae (SNIa) Constitution data, the shift parameter
of the cosmic microwave background (CMB) given by the five-year Wilkinson
Microwave Anisotropy Probe (WMAP5) observations, and the baryon acoustic
oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). Our
results show that the interaction and spatial curvature in the holographic dark
energy model are both rather small. Besides, it is interesting to find that
there exists significant degeneracy between the phenomenological interaction
and the spatial curvature in the holographic dark energy model.Comment: 11 pages, 5 figures; to appear in JCA
Self-interacting holographic dark energy
We investigate a spatially flat Friedmann-Robertson-Walker (FRW) universe
where dark matter exchanges energy with a self-interacting holographic dark
energy (SIHDE). Using the --statistical method on the Hubble function,
we obtain a critical redshift that seems to be consistent with both BAO and CMB
data. We calculate the theoretical distance modulus for confronting with the
observational data of SNe Ia for small redshift and large redshift
. The model gets accelerate faster than the CDM
one and it can be a good candidate to alleviate the coincidence problem. We
also examine the age crisis at high redshift associated with the old quasar APM
08279+5255.Comment: 9 pages, 5 figures. Title and motivation changed. Statistical
analysis improved. Accepted for publication in Modern Physics Letters A
(2012). arXiv admin note: text overlap with arXiv:1012.0883 by other author
Interacting models may be key to solve the cosmic coincidence problem
It is argued that cosmological models that feature a flow of energy from dark
energy to dark matter may solve the coincidence problem of late acceleration
(i.e., "why the energy densities of both components are of the same order
precisely today?"). However, much refined and abundant observational data of
the redshift evolution of the Hubble factor are needed to ascertain whether
they can do the job.Comment: 25 pages, 11 figures; accepted for publication in JCA
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