Hasil untuk "gr-qc"

R. López-González, Yubing Lu, Tania F. Gendron et al.

Yong-Jie Zhang, Tania F. Gendron, M. Ebbert et al.

David S. Pereira, Francisco S. N. Lobo, José Mimoso

We investigate the observed asymmetry between matter and antimatter by incorporating higher-order matter contributions in gravity, specifically analyzing gravitational baryogenesis within the framework of $ f(R,\mathcal{T}^2) $ gravity, where $ R $ is the Ricci scalar and $ \mathcal{T}^2 \equiv T_{μν}T^{μν} $. We further explore the impact of high-order matter contributions by considering an interaction term analogous to those in gravitational and spontaneous baryogenesis, constructed using $ \mathcal{T}^2 $. The cutoff energy scale of the new interaction term is presented and its implications to Big Bang Nucleosynthesis (BBN) are discussed. The properties and implications of this term are analyzed within the frameworks of General Relativity and $f(R, \mathcal{T}^2)$ gravity. Furthermore, a connection to Big Bang Nucleosynthesis (BBN) is established, providing an observational constraint on the functional form of $f(R, \mathcal{T}^2)$. By introducing $\mathcal{T}^2$ into the gravitational action, we propose that these modifications could significantly influence the early Universe's dynamics, thereby altering the conditions necessary for baryogenesis to occur.

S. Moreno-Grau, I. Rojas, I. Hernández et al.

Large variability among Alzheimer's disease (AD) cases might impact genetic discoveries and complicate dissection of underlying biological pathways.

Jaime de Cabo Martin, Przemysław Małkiewicz, Patrick Peter

A quantum cosmological bouncing model may exhibit an ambiguity stemming from the nonclassical nature of the background evolution: two classically equivalent theories can produce two qualitatively different potentials sourcing the perturbations. We derive explicitly the quantum canonical transformation involving the quantum background to show how it leads to inequivalent theories. We identify the relevant quantum parameter describing the difference and expand upon the ambiguity by calculating the expected power spectra produced for initial quantum vacuum fluctuations in the contracting phase of both potentials. We find that one spectral index corresponds to all values of this parameter but one, while the other thus represents a set of measure zero.

Cosimo Bambi, Dejan Stojkovic

Wormholes are hypothetical topologically-non-trivial structures of the spacetime. From the theoretical point of view, the possibility of their existence is challenging but cannot be ruled out. This article is a compact and non-exhaustive review of past and current efforts to search for astrophysical wormholes in the Universe.

Athanasios Bakopoulos, Theodoros Nakas

In this work, we consider a very simple gravitational theory that contains a scalar field with its kinetic and potential terms minimally coupled to gravity, while the scalar field is assumed to have a coulombic form. In the context of this theory, we study an analytic, asymptotically flat, and regular (ultra-compact) black-hole solutions with non-trivial scalar hair of secondary type. At first, we examine the properties of the static and spherically symmetric black-hole solution -- firstly appeared in 1504.08209 [gr-qc] -- and we find that in the causal region of the spacetime the stress-energy tensor, needed to support our solution, satisfies the strong energy conditions. Then, by using the slow-rotating approximation, we generalize the static solution into a slowly rotating one, and we determine explicitly its angular velocity $ω(r)$. We also find that the angular velocity of our ultra-compact solution is always larger compared to the angular velocity of the corresponding equally massive slow-rotating Schwarzschild black hole. In addition, we investigate the axial perturbations of the derived solutions by determining the Schrödinger-like equation and the effective potential. We show that there is a region in the parameter space of the free parameters of our theory, which allows for the existence of stable ultra-compact black hole solutions. Specifically, we calculate that the most compact and stable black hole solution is 0.551 times smaller than the Schwarzschild one, while it rotates 2.491 times faster compared to the slow-rotating Schwarzschild black hole. Finally, we present without going into details the generalization of the derived asymptotically flat solutions to asymptotically (A)dS solutions.

Hosung Kim, A. Irimia, Sam Hobel et al.

Quantifying, controlling, and monitoring image quality is an essential prerequisite for ensuring the validity and reproducibility of many types of neuroimaging data analyses. Implementation of quality control (QC) procedures is the key to ensuring that neuroimaging data are of high-quality and their validity in the subsequent analyses. We introduce the QC system of the Laboratory of Neuro Imaging (LONI): a web-based system featuring a workflow for the assessment of various modality and contrast brain imaging data. The design allows users to anonymously upload imaging data to the LONI-QC system. It then computes an exhaustive set of QC metrics which aids users to perform a standardized QC by generating a range of scalar and vector statistics. These procedures are performed in parallel using a large compute cluster. Finally, the system offers an automated QC procedure for structural MRI, which can flag each QC metric as being ‘good’ or ‘bad.’ Validation using various sets of data acquired from a single scanner and from multiple sites demonstrated the reproducibility of our QC metrics, and the sensitivity and specificity of the proposed Auto QC to ‘bad’ quality images in comparison to visual inspection. To the best of our knowledge, LONI-QC is the first online QC system that uniquely supports the variety of functionality where we compute numerous QC metrics and perform visual/automated image QC of multi-contrast and multi-modal brain imaging data. The LONI-QC system has been used to assess the quality of large neuroimaging datasets acquired as part of various multi-site studies such as the Transforming Research and Clinical Knowledge in Traumatic Brain Injury (TRACK-TBI) Study and the Alzheimer’s Disease Neuroimaging Initiative (ADNI). LONI-QC’s functionality is freely available to users worldwide and its adoption by imaging researchers is likely to contribute substantially to upholding high standards of brain image data quality and to implementing these standards across the neuroimaging community.

Zahra Riahi Samani, J. A. Alappatt, D. Parker et al.

Quality assessment of diffusion MRI (dMRI) data is essential prior to any analysis, so that appropriate pre-processing can be used to improve data quality and ensure that the presence of MRI artifacts do not affect the results of subsequent image analysis. Manual quality assessment of the data is subjective, possibly error-prone, and infeasible, especially considering the growing number of consortium-like studies, underlining the need for automation of the process. In this paper, we have developed a deep-learning-based automated quality control (QC) tool, QC-Automator, for dMRI data, that can handle a variety of artifacts such as motion, multiband interleaving, ghosting, susceptibility, herringbone, and chemical shifts. QC-Automator uses convolutional neural networks along with transfer learning to train the automated artifact detection on a labeled dataset of ∼332,000 slices of dMRI data, from 155 unique subjects and 5 scanners with different dMRI acquisitions, achieving a 98% accuracy in detecting artifacts. The method is fast and paves the way for efficient and effective artifact detection in large datasets. It is also demonstrated to be replicable on other datasets with different acquisition parameters.

Nir Drucker, S. Gueron

Hao Wu, Huayong Wang

Quasi-cyclic low-density parity-check (QC-LDPC) codes are the choice for data channels in the fifth generation (5G) new radio (NR). At the transmitter side, code bits from the QC-LDPC encoder are delivered to the rate matcher. The task of the rate matcher is to select an appropriate number of code bits via puncturing and/or repetition. Code bits that are not selected do not need to be encoded. At the receiver side, the de-rate matcher combines code bits of different transmission attempts and sends them to the QC-LDPC decoder. The output of the QC-LDPC decoder only needs to include necessary systematic bits. Unnecessary systematic bits and parity bits can be completely removed from the decoding process. Taking these considerations into account, a smaller sub-base matrix instead of a full-base matrix can be used in the encoding and decoding process. In this paper, we propose an efficient implementation of QC-LDPC codes for 5G NR. The full-base matrix is pruned before being used. Compared to the traditional schemes, the proposed scheme improves the throughput of QC-LDPC codes in 5G NR.

Qian Guo, T. Johansson, Paul Stankovski Wagner

Algorithms for secure encryption in a post-quantum world are currently receiving a lot of attention in the research community. One of the most promising such algorithms is the code-based scheme called QC-MDPC, which has excellent performance and a small public key size. In this paper, we present a very efficient key recovery attack on the QC-MDPC scheme using the fact that decryption uses an iterative decoding step, and this can fail with some small probability. We identify a dependence between the secret key and the failure in decoding. This can be used to build what we refer to as a distance spectrum for the secret key, which is the set of all distances between any two ones in the secret key. In a reconstruction step, we then determine the secret key from the distance spectrum. The attack has been implemented and tested on a proposed instance of QC-MDPC for 80-bit security. It successfully recovers the secret key in minutes. A slightly modified version of the attack can be applied on proposed versions of the QC-MDPC scheme that provides IND-CCA security. The attack is a bit more complex in this case, but still very much below the security level. The reason why we can break schemes with proved CCA security is that the model for these proofs typically does not include the decoding error possibility. At last, we present several algorithms for key reconstruction from an empirical distance spectrum. We first improve the naïve algorithm for key reconstruction by a factor of about 3 0000, when the parameters for 80-bit security are implemented. We further develop the algorithm to deal with errors in the distance spectrum. This ultimately reduces the requirement on the number of ciphertexts that need to be collected for a successful key recovery.

Qian Zhou, Xiaoquan Su, Gongchao Jing et al.

RNA-Seq has become one of the most widely used applications based on next-generation sequencing technology. However, raw RNA-Seq data may have quality issues, which can significantly distort analytical results and lead to erroneous conclusions. Therefore, the raw data must be subjected to vigorous quality control (QC) procedures before downstream analysis. Currently, an accurate and complete QC of RNA-Seq data requires of a suite of different QC tools used consecutively, which is inefficient in terms of usability, running time, file usage, and interpretability of the results. We developed a comprehensive, fast and easy-to-use QC pipeline for RNA-Seq data, RNA-QC-Chain, which involves three steps: (1) sequencing-quality assessment and trimming; (2) internal (ribosomal RNAs) and external (reads from foreign species) contamination filtering; (3) alignment statistics reporting (such as read number, alignment coverage, sequencing depth and pair-end read mapping information). This package was developed based on our previously reported tool for general QC of next-generation sequencing (NGS) data called QC-Chain, with extensions specifically designed for RNA-Seq data. It has several features that are not available yet in other QC tools for RNA-Seq data, such as RNA sequence trimming, automatic rRNA detection and automatic contaminating species identification. The three QC steps can run either sequentially or independently, enabling RNA-QC-Chain as a comprehensive package with high flexibility and usability. Moreover, parallel computing and optimizations are embedded in most of the QC procedures, providing a superior efficiency. The performance of RNA-QC-Chain has been evaluated with different types of datasets, including an in-house sequencing data, a semi-simulated data, and two real datasets downloaded from public database. Comparisons of RNA-QC-Chain with other QC tools have manifested its superiorities in both function versatility and processing speed. We present here a tool, RNA-QC-Chain, which can be used to comprehensively resolve the quality control processes of RNA-Seq data effectively and efficiently.

Guohua Zhang, Yulin Hu, Yi Fang et al.

In this paper, we consider the constructions of type-II quasi-cyclic (QC) low-density parity-check (LDPC) codes with girth eight from a Sidon sequence. We first derive the necessary and sufficient conditions guaranteeing a girth-eight type-II QC-LDPC code. By combining these conditions with the concept of the Sidon sequence, three classes of type-II QC-LDPC codes are subsequently proposed with girth eight. To the best of our knowledge, the second and the third classes we proposed are the first series of systematic and algebraic constructions for the girth-eight type-II QC-LDPC codes that have rates being greater than a half and, at the same time, have distance upper bounds being not limited to 12. Via simulations, we show the promising performance of the proposed type-II QC-LDPC codes with girth eight. In addition, a set of general bounds and explicit/random constructions without using a Sidon sequence are also presented for the type-II QC-LDPC codes with girth six or eight. We observe that the proposed codes perform better than or equally well as the random QC-LDPC codes from PEG and quadr.-congr. methods, while the novel codes possess both highly structured parity-check matrices and very flexible choices in circulant size.

Carleton Coffrin, H. Hijazi, P. Van Hentenryck

Convex relaxations of the power flow equations and, in particular, the Semi-Definite Programming (SDP) and Second-Order Cone (SOC) relaxations, have attracted significant interest in recent years. The Quadratic Convex (QC) relaxation is a departure from these relaxations in the sense that it imposes constraints to preserve stronger links between the voltage variables through convex envelopes of the polar representation. This paper is a systematic study of the QC relaxation for AC Optimal Power Flow with realistic side constraints. The main theoretical result shows that the QC relaxation is stronger than the SOC relaxation and neither dominates nor is dominated by the SDP relaxation. In addition, comprehensive computational results show that the QC relaxation may produce significant improvements in accuracy over the SOC relaxation at a reasonable computational cost, especially for networks with tight bounds on phase angle differences. The QC and SOC relaxations are also shown to be significantly faster and reliable compared to the SDP relaxation given the current state of the respective solvers.

Edward Eaton, Matthieu Lequesne, A. Parent et al.

F. Gilchrist, H. Rodd, C. Deery et al.

Existing paediatric oral health-related quality of life (OHRQoL) measures are generic instruments designed to evaluate a range of oral conditions. It has been found that disease-specific measures may be more adept at detecting subtle changes which occur following treatment of the condition in question. Furthermore, existing self-report OHRQoL measures have not involved children at all stages of development of the measure. The aim of this study was to develop a caries-specific measure of quality of life for children. The first stage of the study involved a qualitative enquiry with children, aged 5–16 years, to inform the development of the measure. Children generated the potential items, contributed to item reduction and questionnaire design and participated in the testing of face and content validity. The resulting measure was evaluated in a cross-sectional validation study. Ethical approval was granted for the study. The qualitative study found that children discussed a number of caries-related impacts which affected their daily lives. These were incorporated into a draft measure which was further refined following testing of face and content validity. This resulted in the production of the Caries Impacts and Experiences Questionnaire for Children (CARIES-QC), comprising 16 items and one global question. Two hundred participants with a mean (range) age of 8.1 (5–16) years took part in the further evaluation of CARIES-QC. Four items, which did not fit the Rasch model, were removed from further analysis. The remaining 12 items demonstrated good internal consistency (alpha = 0.9) and the total score showed significant correlations with the number of decayed teeth, presence of pain, pulpal involvement, the Child Perceptions Questionnaire (16-item short form) and the global score (p < 0.01, Spearman’s rho). In conclusion, children’s input allowed the development of a valid and reliable child-centred caries-specific quality of life measure. CARIES-QC can now be used to evaluate which interventions for dental caries are most effective in reducing impacts from the child’s perspective.

Palemkota Maithresh, Tejinder P. Singh

Spontaneous localisation is a falsifiable, phenomenological, mechanism for explaining the absence of macroscopic position superpositions, currently being tested for in the laboratory. The theory of trace dynamics provides a possible theoretical origin for spontaneous localisation. We have recently proposed how to employ non-commutative geometry to include gravity in trace dynamics, and suggested the emergence of classical space-time geometry via spontaneous localisation. In our theory, which we call non-commutative matter gravity, a black hole arises from the spontaneous localisation of an entangled state of a large number of `atoms of space-time-matter [STM]'. Prior to localisation, the non-commutative curvature of an STM atom is described by the spectral action of non-commutative geometry. By using the techniques of statistical thermodynamics from trace dynamics, we show that the gravitational entropy of a Schwarzschild black hole results from the microstates of the entangled STM atoms and is given (subject to certain assumptions) by the classical Euclidean gravitational action. This action, in turn, equals the Bekenstein-Hawking entropy (Area/$4{L_P}^2$) of the black hole. We argue that spontaneous localisation is related to black-hole evaporation through the fluctuation-dissipation theorem.

Tomáš Fabšič, V. Hromada, Paul Stankovski et al.

K. Sundar, Harsha Nagarajan, Sidhant Misra et al.

This paper develops a novel strengthened convex quadratic convex (QC) relaxation of the AC Optimal Power Flow (AC-OPF) problem and presents an optimization-based bound-tightening (OBBT) algorithm to compute tight, feasible bounds on the voltage magnitude variables for each bus and the phase angle difference variables for each branch in the network. Theoretical properties of the strengthened QC relaxation, that show its dominance over the other variants of the QC relaxation studied in the literature, are also derived. The effectiveness of the strengthened QC relaxation is corroborated via extensive numerical results on benchmark AC-OPF test networks. In particular, the results demonstrate that the proposed relaxation consistently provides the tightest variable bounds and optimality gaps with negligible impacts on runtime performance.