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Astronomy, Volume 3, Issue 1 (March 2024) – 5 articles

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25 pages, 7046 KiB  
Article
A Critical Examination of the Standard Cosmological Model: Toward a Modified Framework for Explaining Cosmic Structure Formation and Evolution
by Robert Nyakundi Nyagisera, Dismas Wamalwa, Bernard Rapando, Celline Awino and Maxwell Mageto
Astronomy 2024, 3(1), 43-67; https://doi.org/10.3390/astronomy3010005 - 12 Mar 2024
Viewed by 1019
Abstract
This paper explores the fundamental cosmological principle, with a specific focus on the homogeneity and isotropy assumptions inherent in the Friedmann model that underpins the standard model. We propose a modified redshift model that is based on the spatial distribution of luminous matter, [...] Read more.
This paper explores the fundamental cosmological principle, with a specific focus on the homogeneity and isotropy assumptions inherent in the Friedmann model that underpins the standard model. We propose a modified redshift model that is based on the spatial distribution of luminous matter, examining three key astronomical quantities: light intensity, number density, and the redshift of galaxies. Our analysis suggests that the model can account for cosmic accelerated expansion without the need for dark energy in the equations. Both simulations and analytical solutions reveal a unique pattern in the formation and evolution of cosmic structures, particularly in galaxy formation. This pattern shows a significant burst of activity between redshifts 0 < z < 0.4, which then progresses rapidly until approximately z ≈ 0.9, indicating that the majority of cosmic structures were formed during this period. Subsequently, the process slows down considerably, reaching a nearly constant rate until around z ≈ 1.6, after which a gradual decline begins. We also observe a distinctive redshift transition around z ≈ 0.9 before the onset of dark-matter-induced accelerated expansion. This transition is directly related to the matter density and is dependent on the geometry of the universe. The model’s ability to explain cosmic acceleration without requiring fine tuning of the cosmological constant highlights its novelty, providing a fresh perspective on the dynamic evolution of the universe. Full article
(This article belongs to the Special Issue Current Trends in Cosmology)
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14 pages, 270 KiB  
Article
Gravity on a Large Scale—Does It Necessarily Look like It Does on a Small Scale?
by Jerzy Kijowski
Astronomy 2024, 3(1), 29-42; https://doi.org/10.3390/astronomy3010004 - 1 Mar 2024
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Abstract
The notion of a local inertial reference frame is thoroughly analyzed. Dynamics of a field of such frames is derived from the variational principle. It is shown that the resulting theory splits naturally into three sectors, one of which is purely gravitational. Field [...] Read more.
The notion of a local inertial reference frame is thoroughly analyzed. Dynamics of a field of such frames is derived from the variational principle. It is shown that the resulting theory splits naturally into three sectors, one of which is purely gravitational. Field dynamics in this sector, equivalent to Einstein’s vacuum equations, is obtained unambiguously and admits no ad hoc corrections. The cosmological constant is an essential element of this construction and cannot be removed. It has been shown that the second sector of this theory corresponds to electrodynamics, while the last sector could possibly describe dark matter. Full article
8 pages, 917 KiB  
Brief Report
Possible Tests of Fundamental Physics with GINGER
by Giuseppe Di Somma, Carlo Altucci, Francesco Bajardi, Andrea Basti, Nicolò Beverini, Salvatore Capozziello, Giorgio Carelli, Simone Castellano, Donatella Ciampini, Gaetano De Luca, Angela D. V. Di Virgilio, Francesco Fuso, Francesco Giovinetti, Enrico Maccioni, Paolo Marsili, Antonello Ortolan, Alberto Porzio, Matteo Luca Ruggiero and Raffaele Velotta
Astronomy 2024, 3(1), 21-28; https://doi.org/10.3390/astronomy3010003 - 29 Feb 2024
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Abstract
The GINGER (gyroscopes in general relativity) project foresees the construction of an array of large frame ring laser gyroscopes, rigidly connected to the Earth. Large frame ring laser gyroscopes are high-sensitivity instruments used to measure angular velocity with respect to the local inertial [...] Read more.
The GINGER (gyroscopes in general relativity) project foresees the construction of an array of large frame ring laser gyroscopes, rigidly connected to the Earth. Large frame ring laser gyroscopes are high-sensitivity instruments used to measure angular velocity with respect to the local inertial frame. In particular, they can provide sub-daily variations in the Earth rotation rate, a measurement relevant for geodesy and for fundamental physics at the same time. Sensitivity is the key point in determining the relevance of this instrument for fundamental science. The most recent progress in sensitivity evaluation, obtained on a ring laser prototype, indicates that GINGER should reach the level of 1 part in 1011 of the Earth’s rotation rate. The impact on fundamental physics of this kind of apparatus is reviewed. Full article
(This article belongs to the Special Issue Current Trends in Cosmology)
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7 pages, 239 KiB  
Conference Report
Beyond mirkwood: Enhancing SED Modeling with Conformal Predictions
by Sankalp Gilda
Astronomy 2024, 3(1), 14-20; https://doi.org/10.3390/astronomy3010002 - 10 Feb 2024
Viewed by 365
Abstract
Traditional spectral energy distribution (SED) fitting techniques face uncertainties due to assumptions in star formation histories and dust attenuation curves. We propose an advanced machine learning-based approach that enhances flexibility and uncertainty quantification in SED fitting. Unlike the fixed NGBoost model used in [...] Read more.
Traditional spectral energy distribution (SED) fitting techniques face uncertainties due to assumptions in star formation histories and dust attenuation curves. We propose an advanced machine learning-based approach that enhances flexibility and uncertainty quantification in SED fitting. Unlike the fixed NGBoost model used in mirkwood, our approach allows for any scikit-learn-compatible model, including deterministic models. We incorporate conformalized quantile regression to convert point predictions into error bars, enhancing interpretability and reliability. Using CatBoost as the base predictor, we compare results with and without conformal prediction, demonstrating improved performance using metrics such as coverage and interval width. Our method offers a more versatile and accurate tool for deriving galaxy physical properties from observational data. Full article
13 pages, 1516 KiB  
Technical Note
Generating Stellar Spectra Using Neural Networks
by Marwan Gebran
Astronomy 2024, 3(1), 1-13; https://doi.org/10.3390/astronomy3010001 - 30 Jan 2024
Viewed by 896
Abstract
A new generative technique is presented in this paper that uses Deep Learning to reconstruct stellar spectra based on a set of stellar parameters. Two different Neural Networks were trained allowing the generation of new spectra. First, an autoencoder is trained on a [...] Read more.
A new generative technique is presented in this paper that uses Deep Learning to reconstruct stellar spectra based on a set of stellar parameters. Two different Neural Networks were trained allowing the generation of new spectra. First, an autoencoder is trained on a set of BAFGK synthetic data calculated using ATLAS9 model atmospheres and SYNSPEC radiative transfer code. These spectra are calculated in the wavelength range of Gaia RVS between 8400 and 8800 Å. Second, we trained a Fully Dense Neural Network to relate the stellar parameters to the Latent Space of the autoencoder. Finally, we linked the Fully Dense Neural Network to the decoder part of the autoencoder and we built a model that uses as input any combination of Teff, logg, vesini, [M/H], and ξt and output a normalized spectrum. The generated spectra are shown to represent all the line profiles and flux values as the ones calculated using the classical radiative transfer code. The accuracy of our technique is tested using a stellar parameter determination procedure and the results show that the generated spectra have the same characteristics as the synthetic ones. Full article
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