5,683 research outputs found
Age Problem in Lemaitre-Tolman-Bondi Void Models
As is well known, one can explain the current cosmic acceleration by
considering an inhomogeneous and/or anisotropic universe (which violates the
cosmological principle), without invoking dark energy or modified gravity. The
well-known one of this kind of models is the so-called
Lema\^{\i}tre-Tolman-Bondi (LTB) void model, in which the universe is
spherically symmetric and radially inhomogeneous, and we are living in a
locally underdense void centered nearby our location. In the present work, we
test various LTB void models with some old high redshift objects (OHROs).
Obviously, the universe cannot be younger than its constituents. We find that
an unusually large (characterizing the size of the void) is required to
accommodate these OHROs in LTB void models. There is a serious tension between
this unusually large and the much smaller inferred from other
observations (e.g. SNIa, CMB and so on). However, if we instead consider the
lowest limit 1.7\,Gyr for the quasar APM 08279+5255 at redshift , this
tension could be greatly alleviated.Comment: 17 pages, 9 figures, revtex4; v2: discussions added, Phys. Lett. B in
press; v3: published versio
Quasi-Rip: A New Type of Rip Model without Cosmic Doomsday
The fate of our universe is an unceasing topic of cosmology and the human
being. The discovery of the current accelerated expansion of the universe
significantly changed our view of the fate of the universe. Recently, some
interesting scenarios concerning the fate of the universe attracted much
attention in the community, namely the so-called "Little Rip" and "Pseudo-Rip".
It is worth noting that all the Big Rip, Little Rip and Pseudo-Rip arise from
the assumption that the dark energy density is monotonically
increasing. In the present work, we are interested to investigate what will
happen if this assumption is broken, and then propose a so-called "Quasi-Rip"
scenario, which is driven by a type of quintom dark energy. In this work, we
consider an explicit model of Quasi-Rip in detail. We show that Quasi-Rip has
an unique feature different from Big Rip, Little Rip and Pseudo-Rip. Our
universe has a chance to be rebuilt from the ashes after the terrible rip. This
might be the last hope in the "hopeless" rip.Comment: 9 pages, 2 figures, 1 table, revtex4; v2: discussions added, Phys.
Rev. D in press; v3: published versio
New Generalizations of Cosmography Inspired by the Pade Approximant
The current accelerated expansion of the universe has been one of the most
important fields in physics and astronomy since 1998. Many cosmological models
have been proposed in the literature to explain this mysterious phenomenon.
Since the nature and cause of the cosmic acceleration are still unknown,
model-independent approaches to study the evolution of the universe are
welcome. One of the powerful model-independent approaches is the so-called
cosmography. It only relies on the cosmological principle, without postulating
any underlying theoretical model. However, there are several shortcomings in
the usual cosmography. For instance, it is plagued with the problem of
divergence (or an unacceptably large error), and it fails to predict the future
evolution of the universe. In the present work, we try to overcome or at least
alleviate these problems, and we propose two new generalizations of cosmography
inspired by the Pad\'e approximant. One is to directly parameterize the
luminosity distance based on the Pad\'e approximant, while the other is to
generalize cosmography with respect to a so-called -shift
, which is also inspired by the Pad\'e approximant.
Then, we confront them with the observational data with the help of the Markov
chain Monte Carlo (MCMC) code emcee, and find that they work fairly well.Comment: 16 pages, 3 tables, 5 figures, revtex4; v2: discussions added, Eur.
Phys. J. C in press; v3: published versio
Fully Automated Bone Age Assessment On Large-Scale Hand X-Ray Dataset
Bone age assessment (BAA) is an essential topic in the clinical practice of evaluating the biological maturity of children. Because the manual method is time-consuming and prone to observer variability, it is attractive to develop computer-aided and automated methods for BAA. In this paper, we present a fully automatic BAA method. To eliminate noise in a raw X-ray image, we start with using U-Net to precisely segment hand mask image from a raw X-ray image. Even though U-Net can perform the segmentation with high precision, it needs a bigger annotated dataset. To alleviate the annotation burden, we propose to use deep active learning (AL) to select unlabeled data samples with sufficient information intentionally. These samples are given to Oracle for annotation. After that, they are then used for subsequential training. In the beginning, only 300 data are manually annotated and then the improved U-Net within the AL framework can robustly segment all the 12611 images in RSNA dataset. The AL segmentation model achieved a Dice score at 0.95 in the annotated testing set. To optimize the learning process, we employ six off-the-shell deep Convolutional Neural Networks (CNNs) with pretrained weights on ImageNet. We use them to extract features of preprocessed hand images with a transfer learning technique. In the end, a variety of ensemble regression algorithms are applied to perform BAA. Besides, we choose a specific CNN to extract features and explain why we select that CNN. Experimental results show that the proposed approach achieved discrepancy between manual and predicted bone age of about 6.96 and 7.35 months for male and female cohorts, respectively, on the RSNA dataset. These accuracies are comparable to state-of-the-art performance
Model-Free Approach to Fair Solar PV Curtailment Using Reinforcement Learning
The rapid adoption of residential solar photovoltaics (PV) has resulted in
regular overvoltage events, due to correlated reverse power flows. Currently,
PV inverters prevent damage to electronics by curtailing energy production in
response to overvoltage. However, this disproportionately affects households at
the far end of the feeder, leading to an unfair allocation of the potential
value of energy produced. Globally optimizing for fair curtailment requires
accurate feeder parameters, which are often unknown. This paper investigates
reinforcement learning, which gradually optimizes a fair PV curtailment
strategy by interacting with the system. We evaluate six fairness metrics on
how well they can be learned compared to an optimal solution oracle. We show
that all definitions permit efficient learning, suggesting that reinforcement
learning is a promising approach to achieving both safe and fair PV
coordination
Phenomenological study on the significance of the scalar potential and Lamb shift
We indicated in our previous work that for QED the contributions of the
scalar potential which appears at the loop level is much smaller than that of
the vector potential and in fact negligible. But the situation may be different
for QCD, one reason is that the loop effects are more significant because
is much larger than , and secondly the non-perturbative QCD
effects may induce the scalar potential. In this work, we phenomenologically
study the contribution of the scalar potential to the spectra of charmonia.
Taking into account both vector and scalar potentials, by fitting the well
measured charmonia spectra, we re-fix the relevant parameters and test them by
calculating other states of the charmonia family. We also consider the role of
the Lamb shift and present the numerical results with and without involving the
Lamb shift
Canonical interpretation of the state within the model
Recently, the LHCb Collaboration has confirmed the state , with a
mass ~MeV, and a much larger width
~MeV than the previous experimental measurements,
which has confused the understanding of its nature. We will investigate the
possibility of the interpretation for the ,
considering the mass spectra predicted in the quark model, and the strong decay
properties within the model. We also predict the strong decay
properties of the charmonium states and . Our
results show that the state with the small width given in PDG can be
explained as the charmonium state in the model, and
high precision measurement of the width of the is crucial to
understand its nature.Comment: 9 pages, 3 figures, the version to be published in EPJ
Pengaruh Penerapan Strategi Concept Mapping terhadap Hasil Belajar Siswa di Sekolah Dasar
This study aimed to analyze the influence of concept mapping strategy towards the learning result students in social science study on the V B grade of SD Negeri 17 Pontianak Kota. This study used an experimental method with Pre-Experimental design form using One-Group Pretest-Posttest Design. The population in this research were 68 students. The samples in this research was V B as a research class. Based on the t-test, the calculation result obtained t test (7,29) > t table (1,699) with significance level α = 5% means a significant influence using concept mapping strategy. The value of effect size (ES) is 0.39 with moderate category. It means that concept mapping strategy give a moderate effect to the learning result students in social science study on the V B grade of SD Negeri 17 Pontianak Kota
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