Hasil untuk "Astronomy"

Tipei Li, Yu-Qian Ma

I. McLean, M. Iye

S. Rosseland

Ian S. McLean, Mark Casali

Ilya Mandel, Om Sharan Salafia, Andrew Levan et al.

We analytically derive, and illustrate with a population synthesis model, the maximum offset of binary neutron star mergers ejected from their host galaxies. This approximate maximum offset is 300 kpc $\times \,{({v}_{{\rm{esc}}}/500\,{\rm{km}}\,{{\rm{s}}}^{-1})}^{-7}$ , where v _esc is the escape velocity from the host galaxy. Massive hosts with high escape velocities are unlikely to yield very large offsets. This maximum offset should inform the host associations of mergers that are not coincident with galaxies. We also discuss potential correlations between offsets and system masses, and possibly the duration of the gamma-ray burst accompanying the merger.

R. Davies, M. Kasper

Adaptive Optics is a prime example of how progress in observational astronomy can be driven by technological developments. At many observatories it is now considered to be part of a standard instrumentation suite, enabling ground-based telescopes to reach the diffraction limit and thus providing spatial resolution superior to that achievable from space with current or planned satellites. In this review we consider adaptive optics from the astrophysical perspective. We show that adaptive optics has led to important advances in our understanding of a multitude of astrophysical processes, and describe how the requirements from science applications are now driving the development of the next generation of novel adaptive optics techniques.

P. D. Marinos, T. A. Porter, G. P. Rowell et al.

We use the GALPROP cosmic ray (CR) framework to model the Galactic CR distributions and associated nonthermal diffuse emissions up to PeV energies. We consider ensembles of discrete, finite lifetime CR sources, e.g., supernova remnants, for a range of creation rates and lifetimes. We find that the global properties of the CR sources are likely not directly recoverable from the current “snapshot” of the historic injection and propagation of CRs within the Galaxy that are provided by the data. We show that models for the diffuse γ rays based on the discrete/time-dependent scenarios we consider are able to explain the LHAASO very-/ultra-high-energy (VHE/UHE) γ -ray data with up to 50% contribution by unresolved leptonic sources at the highest energies. Over the models that we consider, variations in the diffuse VHE emissions can be ∼25%, which is comparable to those for the steady-state models that we investigated in an earlier work. Such variations due to the discrete/finite nature of the CR sources are an important factor that are necessary to construct accurate physical models of the diffuse emissions from the Galaxy at VHE/UHEs.

Marcin Wieśniak

The measurement problem in quantum mechanics (QM) is related to the inability to include learning about the properties of a quantum system by an agent in the formalism of quantum theory. It includes questions about the physical processes behind the measurement, uniqueness, and randomness of obtained outcomes and an ontic or epistemic role of the state. These issues have triggered various interpretations of quantum theory. They vary from refusing any connection between physical reality and a measurement process to insisting that a collapse of the wave-function is real and possibly involves consciousness. On the other hand, the actual mechanism of a measurement is not extensively discussed in these interpretations. This essay attempts to investigate the quantum measurement problem from the position of the scientific consensus. We begin with a short overview of the development of sensing in living organisms. This is performed for the purpose of stressing the relation between reality and our experience. We then briefly present different approaches to the measurement problem in chosen interpretations. We then state three philosophical assumptions for further consideration and present a decomposition of the measurement act into four stages: transformation, conversion, amplification and broadcasting, and, finally, perception. Each of these stages provides an intuition about the physical processes contributing to it. These conclusions are then used in a discussion about, e.g., objectivity, the implausibility of reversing a measurement, or the epistemic status of the wave-function. Finally, we argue that those in favor of some of the most popular interpretations can find an overlap between their beliefs and the consequences of considerations presented here.

Bruce G. Elmegreen, Daniela Calzetti, Angela Adamo et al.

Power spectra (PS) of high-resolution images of M51 (NGC 5194) taken with the Hubble Space Telescope and the James Webb Space Telescope (JWST) have been examined for evidence of disk thickness in the form of a change in slope between large scales, which map two-dimensional correlated structures, and small scales, which map three-dimensional correlated structures. Such a slope change is observed here in H α , and possibly Pa α , using average PS of azimuthal intensity scans that avoid bright peaks. The physical scale of the slope change occurs at ∼120 pc and ∼170 pc for these two transitions, respectively. A radial dependence in the shape of the H α PS also suggests that the length scale drops from ∼180 pc at 5 kpc, to ∼90 pc at 2 kpc, to ∼25 pc in the central ∼kpc. We interpret these lengths as comparable to the thicknesses of the star-forming disk traced by H ii regions. The corresponding emission measure is ∼100 times larger than what is expected from the diffuse ionized gas. The PS of JWST Mid-IR Instrument images in eight passbands have more gradual changes in slope, making it difficult to determine a specific value of the thickness for this emission.

S J Watts, L Crow

In recent years generative artificial intelligence has been used to create data to support scientific analysis. For example, generative adversarial networks (GANs) have been trained using Monte Carlo simulated input and then used to generate data for the same problem. This has the advantage that a GAN creates data in a significantly reduced computing time. $N$ training events for a GAN can result in $NG$ generated events with the gain factor $G$ being greater than one. This appears to violate the principle that one cannot get information for free. This is not the only way to amplify data so this process will be referred to as data amplification which is studied using information theoretic concepts. It is shown that a gain greater than one is possible whilst keeping the information content of the data unchanged. This leads to a mathematical bound, $2\log (\text{Generated}\ \text{Events}) \unicode{x2A7E} {\text{3log(Training Events)}}$ , which only depends on the number of generated and training events. This study determined the conditions for both the underlying and reconstructed probability distributions to ensure this bound. In particular, the resolution of variables in amplified data is not improved by the process but the increase in sample size can still improve statistical significance. The bound was confirmed using computer simulation and analysis of GAN generated data from the literature.

Sotiria Fotopoulou

This review summarizes popular unsupervised learning methods, and gives an overview of their past, current, and future uses in astronomy. Unsupervised learning aims to organise the information content of a dataset, in such a way that knowledge can be extracted. Traditionally this has been achieved through dimensionality reduction techniques that aid the ranking of a dataset, for example through principal component analysis or by using auto-encoders, or simpler visualisation of a high dimensional space, for example through the use of a self organising map. Other desirable properties of unsupervised learning include the identification of clusters, i.e. groups of similar objects, which has traditionally been achieved by the k-means algorithm and more recently through density-based clustering such as HDBSCAN. More recently, complex frameworks have emerged, that chain together dimensionality reduction and clustering methods. However, no dataset is fully unknown. Thus, nowadays a lot of research has been directed towards self-supervised and semi-supervised methods that stand to gain from both supervised and unsupervised learning.

Amedeo Balbi, Claudio Grimaldi

The probability of detecting technosignatures (i.e., evidence of technological activity beyond Earth) increases with their longevity, or the time interval over which they manifest. Therefore, the assumed distribution of longevities has some bearing on the chances of success of technosignature searches, as well as on the inferred age of technosignatures following a first contact. Here, we investigate the possibility that the longevity of technosignatures conforms to the so-called Lindy’s law, whereby, at any time, their remaining life expectancy is roughly proportional to their age. We show that, if Lindy’s law applies, the general tenet that the first detected technosignature ought to be very long lived may be overruled. We conclude by discussing the number of emitters that had to appear, over the history of the Galaxy, in order for one of them to be detectable today from Earth.

Masayuki Kano, Yusuke Tanaka, Daisuke Sato et al.

Abstract Monitoring and predicting fault slip behaviors in subduction zones is essential for understanding earthquake cycles and assessing future earthquake potential. We developed a data assimilation method for fault slip monitoring and the short-term prediction of slow slip events, and applied to the 2010 Bungo Channel slow slip event in southwest Japan. The observed geodetic data were quantitatively explained using a physics-based model with data assimilation. We investigated short-term predictability by assimilating observation data within limited periods. Without prior constraints on fault slip style, observations solely during slip acceleration predicted the occurrence of a fast slip; however, the inclusion of slip deceleration data successfully predicted a slow transient slip. With prior constraints to exclude unstable slip, the assimilation of data after slow slip event occurrence also predicted a slow transient slip. This study provides a tool using data assimilation for fault slip monitoring and prediction based on real observation data. Graphical Abstract

Luis A. Anchordoqui, Ignatios Antoniadis, Dieter Lüst

Over the last few years, low- and high-redshift observations set off tensions in the measurement of the present-day expansion rate H0 and in the determination of the amplitude of the matter clustering in the late Universe (parameterized by S8). It was recently noted that both these tensions can be resolved if the cosmological constant parametrizing the dark energy content switches its sign at a critical redshift zc∼2. However, the anti-de Sitter (AdS) swampland conjecture suggests that the postulated switch in sign of the cosmological constant at zero temperature seems unlikely because the AdS vacua are an infinite distance apart from de Sitter (dS) vacua in moduli space. We provide an explanation for the required AdS → dS crossover transition in the vacuum energy using the Casimir forces of fields inhabiting the bulk. We then use entropy arguments to claim that any AdS → dS transition between metastable vacua must be accompanied by a reduction of the species scale where gravity becomes strong. We provide a few examples supporting this AdS → dS uplift conjecture.

Chang Lai, Jiyao Xu, Zhishuang Lin et al.

Abstract For the first time, we used the machine learning method to analyze the statistical occurrence and propagation characteristics of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) from October 2011 to December 2021 observed by the all‐sky airglow imager deployed at Xinglong (40.4°N, 117.6°E, 30.5° MLAT), China. We developed a program code using the algorithms to identify and extract the propagation and morphological features of MSTIDs in 630 nm airglow images automatically. The classification model and detection model have accuracies of 96.9% and 70%–85%, respectively. We identified 611 MSTID events from 749,888 airglow images, and obtained the following statistical results: (a) the MSTIDs occurrence peaked at 2200–2300 local time in summer and 2300–2400 in winter; (b) the annual average of horizontal wavelength and velocity are 160–311 km and 98–133 m/s, respectively; (c) among 611 events, 589 MSTIDs propagated southwestward. Fifteen events are northeastward and all of them are periodic MSTIDs, most of which occurred between April and August; (d) the annual trend of relative intensity perturbation (%) shows a negative correlation with the horizontal phase speed; (e) horizontal wavelengths of MSTIDs are independent of the solar activity. Further analyses found those southwestward propagating MSTIDs are consistent with the Es‐Perkins coupling theory, while those non‐southwestward ones could be related to the atmospheric gravity waves and other possible sources. The northeastward events exhibit morphological and seasonal characteristics, which cannot be explained by the Perkins instability, more simultaneous observations (GPS‐TEC, OH airglow, etc.) are required to reveal the mechanism behind these characteristics.

Qu Cao, Liang Zhang

Abstract In this paper, we generalize the Nguyen–Spradlin–Volovich–Wen (NSVW) tree formula from the MHV sector to any helicity sector. We find a close connection between the Permutohedron and the KLT relation, and construct a non-trivial mapping between them, linking the amplitudes in the gauge and gravity theories. The gravity amplitude can also be mapped from a determinant followed from the matrix-tree theorem. Besides, we use the binary tree graphs to manifest its Lie structure. In our tree formula, there is an evident Hopf algebra of the permutation group behind the gravity amplitudes. Using the tree formula, we can directly re-derive the soft/collinear limit of the amplitudes.

Aida Kazemi Hokmabad, Seyede Elahe Khatoon Abadi Kalali, Amir Reza Kosari et al.

This paper investigates solar activities and its phenomena from the perspective of risks to the earth's environment, human health, and space weather risks to space systems. In this article, in addition to a brief explanation about the physics of the sun and space weather phenomena, the effects of these phenomena on human health have been investigated. moreover the results of international researches have been studied and analyzed to determine the relationship between heart diseases, brain diseases, cancer, birth rates, health of astronauts, and animal life with space weather phenomena. The results of this article help to predict these events during the occurrence of solar events and by taking the correct actions in addition to preserving biological health, possible damages can also be minimized.

M.M.E. Barakat, T.A. Abdel-Baset, M. Belhaj et al.

In the phase diagram of iron pnictides, superconductivity arises at the border of antiferromagnetism, which raises the question of the role of symmetry of the gap and quantum criticality. Although more than 15-years of extensive research, the microscopic origin of the pairing symmetry inside the superconducting (SC) dome and its link to quantum criticality still remains elusive. Here, we report two new findings on BaFe2−xNixAs2: (1) A sharp peak in the x-dependence of the lower and upper critical fields, the SC critical current density Jc, the size of the jump in the specific heat ΔCel/T and the Sommerfeld coefficient (γ) at the optimum composition x = 0.10, where the SC transition temperature Tc reaches a maximum. Our obtained reliable values as a function of doping of the normal-state Sommerfeld coefficient increase with doping, illustrating the strong competition between magnetism and superconductivity and attributed to closing of spin density wave gap with Ni doping. (2) We show that doping induced a sudden change of the gap structure from nodeless to nodal. Our results imply that the superconductivity in BaFe2−xNixAs2 is closely linked to the quantum criticality and is characterized by a complex order parameter.

Valentin D. Ivanov

The mysteries of the Universe are international, the skies are not crossed by borders. However, the knowledge is transmitted by language, imposing linguistic barriers that are often difficult to break through. Bulgaria is considered as an example of a country with relatively small reader base -- it has a population of about 6.5 million (2021) and the Bulgarian language has probably $\sim$7 million speakers, if the diaspora in US, Germany and elsewhere is accounted for. The smaller-scale market, in comparison with larger non-English speaking countries, poses a number of limitation to the publishing landscape: (i) the local authors are discouraged to pen both popular and scientific astronomy books, because of the limited financial incentive; (ii) the market is heavily dominated by translations (from Russian before 1989, from English nowdays), but even those are fewer than in bigger countries, because the translation overhead costs are spread over smaller print runs. The history of the astronomy publishing in Bulgaria is summarized, with some distinct periods: pre-1944, the communist era 1944-1989, the modern times post 1989. A few notable publications are reviewed. Finally, some practices to help astronomy book publishing in languages with smaller reader bases are suggested, taking advantage of the recent technological developments.

A. Zanella, C. M. Harrison, S. Lenzi et al.

Over the last ten years there has been a large increase in the number of projects using sound to represent astronomical data and concepts. Motivation for these projects includes the potential to enhance scientific discovery within complex datasets, by utilising the inherent multi-dimensionality of sound and the ability of our hearing to filter signals from noise. Other motivations include creating engaging multi-sensory resources, for education and public engagement, and making astronomy more accessible to people who are blind or have low vision, promoting their participation in science and related careers. We describe potential benefits of sound within these contexts and provide an overview of the nearly 100 sound-based astronomy projects that we identified. We discuss current limitations and challenges of the approaches taken. Finally, we suggest future directions to help realise the full potential of sound-based techniques in general and to widen their application within the astronomy community.