1,173 research outputs found
Entropy bounds in terms of the w parameter
In a pair of recent articles [PRL 105 (2010) 041302 - arXiv:1005.1132; JHEP
1103 (2011) 056 - arXiv:1012.2867] two of the current authors have developed an
entropy bound for equilibrium uncollapsed matter using only classical general
relativity, basic thermodynamics, and the Unruh effect. An odd feature of that
bound, S <= A/2, was that the proportionality constant, 1/2, was weaker than
that expected from black hole thermodynamics, 1/4. In the current article we
strengthen the previous results by obtaining a bound involving the (suitably
averaged) w parameter. Simple causality arguments restrict this averaged
parameter to be <= 1. When equality holds, the entropy bound saturates at the
value expected based on black hole thermodynamics. We also add some clarifying
comments regarding the (net) positivity of the chemical potential. Overall, we
find that even in the absence of any black hole region, we can nevertheless get
arbitrarily close to the Bekenstein entropy.Comment: V1: 14 pages. V2: One reference added. V3: This version accepted for
publication in JHE
Classical and semi-classical energy conditions
The standard energy conditions of classical general relativity are (mostly)
linear in the stress-energy tensor, and have clear physical interpretations in
terms of geodesic focussing, but suffer the significant drawback that they are
often violated by semi-classical quantum effects. In contrast, it is possible
to develop non-standard energy conditions that are intrinsically non-linear in
the stress-energy tensor, and which exhibit much better well-controlled
behaviour when semi-classical quantum effects are introduced, at the cost of a
less direct applicability to geodesic focussing. In this article we will first
review the standard energy conditions and their various limitations. (Including
the connection to the Hawking--Ellis type I, II, III, and IV classification of
stress-energy tensors). We shall then turn to the averaged, nonlinear, and
semi-classical energy conditions, and see how much can be done once
semi-classical quantum effects are included.Comment: V1: 25 pages. Draft chapter, on which the related chapter of the book
"Wormholes, Warp Drives and Energy Conditions" (to be published by Springer),
will be based. V2: typos fixed. V3: small typo fixe
Harmonization of brain PET images in multi-center PET studies using Hoffman phantom scan
Background: Image harmonization has been proposed to minimize heterogeneity in brain PET scans acquired in multi-center studies. However, standard validated methods and software tools are lacking. Here, we assessed the performance of a framework for the harmonization of brain PET scans in a multi-center European clinical trial. / Method: Hoffman 3D brain phantoms were acquired in 28 PET systems and reconstructed using site-specific settings. Full Width at Half Maximum (FWHM) of the Effective Image Resolution (EIR) and harmonization kernels were estimated for each scan. The target EIR was selected as the coarsest EIR in the imaging network. Using “Hoffman 3D brain Analysis tool,” indicators of image quality were calculated before and after the harmonization: The Coefficient of Variance (COV%), Gray Matter Recovery Coefficient (GMRC), Contrast, Cold-Spot RC, and left-to-right GMRC ratio. A COV% ≤ 15% and Contrast ≥ 2.2 were set as acceptance criteria. The procedure was repeated to achieve a 6-mm target EIR in a subset of scans. The method’s robustness against typical dose-calibrator-based errors was assessed. / Results: The EIR across systems ranged from 3.3 to 8.1 mm, and an EIR of 8 mm was selected as the target resolution. After harmonization, all scans met acceptable image quality criteria, while only 13 (39.4%) did before. The harmonization procedure resulted in lower inter-system variability indicators: Mean ± SD COV% (from 16.97 ± 6.03 to 7.86 ± 1.47%), GMRC Inter-Quartile Range (0.040–0.012), and Contrast SD (0.14–0.05). Similar results were obtained with a 6-mm FWHM target EIR. Errors of ± 10% in the DRO activity resulted in differences below 1 mm in the estimated EIR. / Conclusion: Harmonizing the EIR of brain PET scans significantly reduced image quality variability while minimally affecting quantitative accuracy. This method can be used prospectively for harmonizing scans to target sharper resolutions and is robust against dose-calibrator errors. Comparable image quality is attainable in brain PET multi-center studies while maintaining quantitative accuracy
A four-dimensional {\Lambda}CDM-type cosmological model induced from higher dimensions using a kinematical constraint
A class of cosmological solutions of higher dimensional Einstein field
equations with the energy-momentum tensor of a homogeneous, isotropic fluid as
the source are considered with an anisotropic metric that includes the direct
sum of a 3-dimensional (physical, flat) external space metric and an
n-dimensional (compact, flat) internal space metric. A simple kinematical
constraint is postulated that correlates the expansion rates of the external
and internal spaces in terms of a real parameter {\lambda}. A specific solution
for which both the external and internal spaces expand at different rates is
given analytically for n=3. Assuming that the internal dimensions were at
Planck length scales when the external space starts with a Big Bang (t=0), they
expand only 1.49 times and stay at Planck length scales even in the present age
of the universe (13.7 Gyr). The effective four dimensional universe would
exhibit a behavior consistent with our current understanding of the observed
universe. It would start in a stiff fluid dominated phase and evolve through
radiation dominated and pressureless matter dominated phases, eventually going
into a de Sitter phase at late times.Comment: 12 pages, 8 figures; matches the version published in General
Relativity and Gravitatio
Desynchronizing effect of high-frequency stimulation in a generic cortical network model
Transcranial Electrical Stimulation (TCES) and Deep Brain Stimulation (DBS)
are two different applications of electrical current to the brain used in
different areas of medicine. Both have a similar frequency dependence of their
efficiency, with the most pronounced effects around 100Hz. We apply
superthreshold electrical stimulation, specifically depolarizing DC current,
interrupted at different frequencies, to a simple model of a population of
cortical neurons which uses phenomenological descriptions of neurons by
Izhikevich and synaptic connections on a similar level of sophistication. With
this model, we are able to reproduce the optimal desynchronization around
100Hz, as well as to predict the full frequency dependence of the efficiency of
desynchronization, and thereby to give a possible explanation for the action
mechanism of TCES.Comment: 9 pages, figs included. Accepted for publication in Cognitive
Neurodynamic
Model inference for spreadsheets
Many errors in spreadsheet formulas can be avoided if spreadsheets are built automati-
cally from higher-level models that can encode and enforce consistency constraints in the generated
spreadsheets. Employing this strategy for legacy spreadsheets is dificult, because the model has
to be reverse engineered from an existing spreadsheet and existing data must be transferred into
the new model-generated spreadsheet.
We have developed and implemented a technique that automatically infers relational schemas
from spreadsheets. This technique uses particularities from the spreadsheet realm to create better
schemas. We have evaluated this technique in two ways: First, we have demonstrated its appli-
cability by using it on a set of real-world spreadsheets. Second, we have run an empirical study
with users. The study has shown that the results produced by our technique are comparable to
the ones developed by experts starting from the same (legacy) spreadsheet data.
Although relational schemas are very useful to model data, they do not t well spreadsheets as
they do not allow to express layout. Thus, we have also introduced a mapping between relational
schemas and ClassSheets. A ClassSheet controls further changes to the spreadsheet and safeguards
it against a large class of formula errors. The developed tool is a contribution to spreadsheet
(reverse) engineering, because it lls an important gap and allows a promising design method
(ClassSheets) to be applied to a huge collection of legacy spreadsheets with minimal effort.We would like to thank Orlando Belo for his help on running and analyzing the empirical study. We would also like to thank Paulo Azevedo for his help in conducting the statistical analysis of our empirical study. We would also like to thank the anonymous reviewers for their suggestions which helped us to improve the paper. This work is funded by ERDF - European Regional Development Fund through the COMPETE Programme (operational programme for competitiveness) and by National Funds through the FCT - Fundacao para a Ciencia e a Tecnologia (Portuguese Foundation for Science and Technology) within project FCOMP-01-0124-FEDER-010048. The first author was also supported by FCT grant SFRH/BPD/73358/2010
Mapping of Submerged Aquatic Vegetation in Rivers From Very High Resolution Image Data, Using Object Based Image Analysis Combined with Expert Knowledge
The use of remote sensing for monitoring of submerged aquatic vegetation (SAV) in fluvial environments has been limited by the spatial and spectral resolution of available image data. The absorption of light in water also complicates the use of common image analysis methods. This paper presents the results of a study that uses very high resolution (VHR) image data, collected with a Near Infrared sensitive DSLR camera, to map the distribution of SAV species for three sites along the Desselse Nete, a lowland river in Flanders, Belgium. Plant species, including Ranunculus aquatilis L., Callitriche obtusangula Le Gall, Potamogeton natans L., Sparganium emersum L. and Potamogeton crispus L., were classified from the data using Object-Based Image Analysis (OBIA) and expert knowledge. A classification rule set based on a combination of both spectral and structural image variation (e.g. texture and shape) was developed for images from two sites. A comparison of the classifications with manually delineated ground truth maps resulted for both sites in 61% overall accuracy. Application of the rule set to a third validation image, resulted in 53% overall accuracy. These consistent results show promise for species level mapping in such biodiverse environments, but also prompt a discussion on assessment of classification accuracy
Spin-orbit density wave induced hidden topological order in URu2Si2
The conventional order parameters in quantum matters are often characterized
by 'spontaneous' broken symmetries. However, sometimes the broken symmetries
may blend with the invariant symmetries to lead to mysterious emergent phases.
The heavy fermion metal URu2Si2 is one such example, where the order parameter
responsible for a second-order phase transition at Th = 17.5 K has remained a
long-standing mystery. Here we propose via ab-initio calculation and effective
model that a novel spin-orbit density wave in the f-states is responsible for
the hidden-order phase in URu2Si2. The staggered spin-orbit order 'spontaneous'
breaks rotational, and translational symmetries while time-reversal symmetry
remains intact. Thus it is immune to pressure, but can be destroyed by magnetic
field even at T = 0 K, that means at a quantum critical point. We compute
topological index of the order parameter to show that the hidden order is
topologically invariant. Finally, some verifiable predictions are presented.Comment: (v2) Substantially modified from v1, more calculation and comparison
with experiments are include
Feed intake and production parameters of lactating crossbred cows fed maize-based diets of stover, silage or quality protein silage
Thirty-six Boran × Friesian dairy cows (392 ± 12 kg; mean ± SD) in early parity were used in a randomised complete block design. Cows were blocked by parity into three blocks of 12 animals and offered normal maize (NM) stover (T1), NM silage (T2) or quality protein maize (QPM) silage (T3) basal diets supplemented with a similar concentrate mix. Feed intake, body weight and condition changes and milk yield and composition were assessed. The daily intake of DM, OM, NDF and ADF for cows fed the NM stover-based diet was higher (P < 0.05) than for the cows fed the NM silage and QPM silage-based diets. However, the daily intake of DOM (9.3 kg) and ME (140.8 MJ) for cows on QPM silage-based diet was higher (P < 0.05) than for cows on NM stover-based diet (8.4 kg and 124.2 MJ) and NM silage-based diet (7.9 kg and 119.1 MJ). Body weight of cows was affected (P < 0.05) by the diet, but diet had no effect (P > 0.05) on body condition score, milk yield and milk composition. The digestible organic matter in the NM stover-based diet (724 g/kg DM) was lower (P < 0.05) than that in the NM (770 g/kg DM) and QPM silage-based diet (762 g/kg DM). It was concluded that the performances of the cows on the NM silage and QPM silage diets were similar and were not superior to that of the NM stover-based diet
Interplay between pleiotropy and secondary selection determines rise and fall of mutators in stress response
Dramatic rise of mutators has been found to accompany adaptation of bacteria
in response to many kinds of stress. Two views on the evolutionary origin of
this phenomenon emerged: the pleiotropic hypothesis positing that it is a
byproduct of environmental stress or other specific stress response mechanisms
and the second order selection which states that mutators hitchhike to fixation
with unrelated beneficial alleles. Conventional population genetics models
could not fully resolve this controversy because they are based on certain
assumptions about fitness landscape. Here we address this problem using a
microscopic multiscale model, which couples physically realistic molecular
descriptions of proteins and their interactions with population genetics of
carrier organisms without assuming any a priori fitness landscape. We found
that both pleiotropy and second order selection play a crucial role at
different stages of adaptation: the supply of mutators is provided through
destabilization of error correction complexes or fluctuations of production
levels of prototypic mismatch repair proteins (pleiotropic effects), while rise
and fixation of mutators occur when there is a sufficient supply of beneficial
mutations in replication-controlling genes. This general mechanism assures a
robust and reliable adaptation of organisms to unforeseen challenges. This
study highlights physical principles underlying physical biological mechanisms
of stress response and adaptation
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