Recent zbMATH articles in MSC 85https://zbmath.org/atom/cc/852022-11-17T18:59:28.764376ZWerkzeugSolving optical tomography with deep learninghttps://zbmath.org/1496.652002022-11-17T18:59:28.764376Z"Fan, Yuwei"https://zbmath.org/authors/?q=ai:fan.yuwei"Ying, Lexing"https://zbmath.org/authors/?q=ai:ying.lexingSummary: This paper presents a neural network approach for solving two-dimensional optical tomography (OT) problems based on the radiative transfer equation. The mathematical problem of OT is to recover the optical properties of an object based on the albedo operator that is accessible from boundary measurements. Both the forward map from the optical properties to the albedo operator and the inverse map are high-dimensional and nonlinear. For the circular tomography geometry, a perturbative analysis shows that the forward map can be approximated by a vectorized convolution operator in the angular direction. Motivated by this, we propose effective neural network architectures for the forward and inverse maps based on convolution layers, with weights learned from training datasets. Numerical results demonstrate the efficiency of the proposed neural networks.Time in powers of ten. Natural phenomena and their timescales. Translated from the Dutch by Saskia Eisberg-'t Hooft. With a foreword by Steven Weinberghttps://zbmath.org/1496.810122022-11-17T18:59:28.764376Z"'t Hooft, Gerard"https://zbmath.org/authors/?q=ai:t-hooft.gerard"Vandoren, Stefan"https://zbmath.org/authors/?q=ai:vandoren.stefanPublisher's description: In this richly illustrated book, Nobel Laureate Gerard 't Hooft and theoretical physicist Stefan Vandoren describe the enormous diversity of natural phenomena that take place at different time scales.
In the tradition of the bestseller Powers of Ten, the authors zoom in and out in time, each step with a factor of ten. Starting from one second, time scales are enlarged until processes are reached that take much longer than the age of the universe. After the largest possible eternities, the reader is treated to the shortest and fastest phenomena known. Then the authors increase with powers of ten, until again the second is reached at the end of the book.
At each time scale, interesting natural phenomena occur, spread over all scientific disciplines: orbital and rotation periods of planets and stars, decay times of elementary particles and atoms, biological rhythms and evolution processes, but also the different geological time scales.Applications of the close-limit approximation: horizonless compact objects and scalar fieldshttps://zbmath.org/1496.830212022-11-17T18:59:28.764376Z"Annulli, Lorenzo"https://zbmath.org/authors/?q=ai:annulli.lorenzo"Cardoso, Vitor"https://zbmath.org/authors/?q=ai:cardoso.vitor"Gualtieri, Leonardo"https://zbmath.org/authors/?q=ai:gualtieri.leonardoTesting gravity with black hole shadow subringshttps://zbmath.org/1496.830222022-11-17T18:59:28.764376Z"Ayzenberg, Dimitry"https://zbmath.org/authors/?q=ai:ayzenberg.dimitryOptical and thermodynamic behaviors of Ayón-Beato-García black holes for 4D Einstein Gauss-Bonnet gravityhttps://zbmath.org/1496.830232022-11-17T18:59:28.764376Z"Belhaj, Adil"https://zbmath.org/authors/?q=ai:belhaj.adil"Sekhmani, Yassine"https://zbmath.org/authors/?q=ai:sekhmani.yassineSummary: We investigate a family of four dimensional \((4D)\) Ayón-Beato-García (ABG) black holes in Einstein Gauss-Bonnet (EGB) gravity. We approach and examine the associated thermodynamic and the optical aspects by varying the involved parameters. We first compute and analyze the corresponding thermodynamic quantities. Among others, we inspect the global and the local stability behaviors. Then, we study the optical behaviors. Using Hamilton-Jacobi method, we study the shadow geometrical configurations in terms of one dimensional real closed curves. Using Gauss-Bonnet theorem, we calculate and examine the deflection angle of light rays by such black holes. In specific regions of a reduced moduli space obtained by fixing the mass and varying the remaining parameters, the present work recovers certain previous findings. Finally, we provide a possible speculative connection with observations from Event Horizon Telescope by imposing certain constraints on the involved b parameters in the light of the M\(87^\ast\) image.Observational constraints on inflection point quintessence with a cubic potentialhttps://zbmath.org/1496.830482022-11-17T18:59:28.764376Z"Storm, S. David"https://zbmath.org/authors/?q=ai:storm.s-david"Scherrer, Robert J."https://zbmath.org/authors/?q=ai:scherrer.robert-jSummary: We examine the simplest inflection point quintessence model, with a potential given by \(V(\phi) = V_0 + V_3\phi^3\). This model can produce either asymptotic de Sitter expansion or transient acceleration, and we show that it does not correspond to either pure freezing or thawing behavior. We derive observational constraints on the initial value of the scalar field, \(\phi_i\), and \(V_3/V_0\) and find that small values of either \(\phi_i\) or \(V_3/V_0\) are favored. While most of the observationally-allowed parameter space yields asymptotic de Sitter evolution, there is a small region, corresponding to large \(V_3/V_0\) and small \(\phi_i\), for which the current accelerated expansion is transient. The latter behavior is potentially consistent with a cyclic universe.Quark condensate and chiral symmetry restoration in neutron starshttps://zbmath.org/1496.850012022-11-17T18:59:28.764376Z"Jin, Hao-Miao"https://zbmath.org/authors/?q=ai:jin.hao-miao"Xia, Cheng-Jun"https://zbmath.org/authors/?q=ai:xia.cheng-jun"Sun, Ting-Ting"https://zbmath.org/authors/?q=ai:sun.tingting"Peng, Guang-Xiong"https://zbmath.org/authors/?q=ai:peng.guang-xiongSummary: Based on an equivparticle model, we investigate the in-medium quark condensate in neutron stars. Carrying out a Taylor expansion of the nuclear binding energy to the order of \(\rho^3\), we obtain a series of EOSs for neutron star matter, which are confronted with the latest nuclear and astrophysical constraints. The in-medium quark condensate is then extracted from the constrained properties of neutron star matter, which decreases non-linearly with density. However, the chiral symmetry is only partially restored with non-vanishing quark condensates, which may vanish at a density that is out of reach for neutron stars.Regions without invariant tori of given class for the planar circular restricted three-body problemhttps://zbmath.org/1496.850022022-11-17T18:59:28.764376Z"Kallinikos, N."https://zbmath.org/authors/?q=ai:kallinikos.n"MacKay, R. S."https://zbmath.org/authors/?q=ai:mackay.robert-s"Syndercombe, T."https://zbmath.org/authors/?q=ai:syndercombe.tSummary: A method to establish regions of phase space through which pass no invariant tori transverse to a given direction field is applied to the planar circular restricted three-body problem. Implications for the location of stable orbits for planets around a binary star are deduced. It is expected that lessons learnt from this problem will be useful for applications of the method to other contexts such as flux surfaces for magnetic fields, guiding centre motion in magnetic fields, and classical models of chemical reaction dynamics.New light rings from multiple critical curves as observational signatures of black hole mimickershttps://zbmath.org/1496.850032022-11-17T18:59:28.764376Z"Olmo, Gonzalo J."https://zbmath.org/authors/?q=ai:olmo.gonzalo-j"Rubiera-Garcia, Diego"https://zbmath.org/authors/?q=ai:rubiera-garcia.diego"Sáez-Chillón Gómez, Diego"https://zbmath.org/authors/?q=ai:saez-chillon-gomez.diegoSummary: We argue that the appearance of additional light rings in a shadow observation -- beyond the infinite sequence of exponentially demagnified self-similar rings foreseen in the Kerr solution - would make a compelling case for the existence of black hole mimickers having multiple critical curves. We support this claim by discussing three different scenarios of spherically symmetric wormhole geometries having two such critical curves, and explicitly work out the optical appearance of one such object when surrounded by an optically and geometrically thin accretion disk.Structure of magnetized strange quark star in perturbative QCDhttps://zbmath.org/1496.850042022-11-17T18:59:28.764376Z"Sedaghat, J."https://zbmath.org/authors/?q=ai:sedaghat.j"Zebarjad, S. M."https://zbmath.org/authors/?q=ai:zebarjad.s-mohammad|zebarjad.seyed-mostafa"Bordbar, G. H."https://zbmath.org/authors/?q=ai:bordbar.g-h"Eslam Panah, B."https://zbmath.org/authors/?q=ai:eslam-panah.bSummary: We have performed the leading order perturbative calculation to obtain the equation of state (EoS) of the strange quark matter (SQM) at zero temperature under the magnetic field \(B = 10^{18}G\). The SQM comprises two massless quark flavors (up and down) and one massive quark flavor (strange). Consequently, we have used the obtained EoS to calculate the maximum gravitational mass and the corresponding radius of the magnetized strange quark star (SQS). We have employed two approaches, including the regular perturbation theory (\textbf{RPT}) and the background perturbation theory (\textbf{BPT}). In \textbf{RPT} the infrared (IR) freezing effect of the coupling constant has not been accounted for, while this effect has been included in \textbf{BPT}. We have obtained the value of the maximum gravitational mass to be more than three times the solar mass. The validity of isotropic structure calculations for SQS has also been investigated. Our results show that the threshold magnetic field from which an anisotropic approach begins to be significant lies in the interval \(2 \times 10^{18}G < B < 3 \times 10^{18}G\). Furthermore, we have computed the redshift, compactness and Buchdahl-Bondi bound of the SQS to show that this compact object cannot be a black hole.Calculation of the solar radiation field in the general circulation model of the Earth's lower and middle atmospherehttps://zbmath.org/1496.860142022-11-17T18:59:28.764376Z"Fedotova, E. A."https://zbmath.org/authors/?q=ai:fedotova.e-aSummary: The results of testing two radiation blocks used for economical calculation of radiation transfer in the general circulation model of the lower and middle atmosphere of the Earth are presented. The first block is designed to calculate the field of intrinsic radiation in the frequency range from 10 to \(3000 cm^{-1}\), and the second block is designed to calculate the field of solar radiation in the frequency range from 2000 to \(50,000 cm^{-1}\). Each of these blocks uses its own parameterization of the optical parameters of the air in the altitude range from the Earth's surface to 90 km. When constructing these parameterizations, the change in the gas composition of the atmosphere with altitude was taken into account. For the numerical solution of the radiation transfer equation, the method of discrete ordinates is used. The results of calculations performed using the radiation block of the model are compared with the results of line-by-line calculations of the solar radiation field in the lower and middle atmosphere of the Earth, performed with a very high frequency resolution. It is shown that the model block provides good accuracy of calculations both in the absence of clouds and in the presence of cloud layers with a large optical thickness.