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15 pages, 1420 KiB

Open AccessArticle

Spectral Dimensionality of Spacetime Around a Radiating Schwarzschild Black-Hole

by Mauricio Bellini, Juan Ignacio Musmarra, Pablo Alejandro Sánchez and Alan Sebastián Morales

Universe 2025, 11(8), 243; https://doi.org/10.3390/universe11080243 - 24 Jul 2025

In this work we study the spectral dimensionality of spacetime around a radiating Schwarzschild black hole using a recently introduced formalism of quantum gravity, where the alterations of the gravitational field produced by the radiation are represented on an extended manifold, and describe a non-commutative and nonlinear quantum algebra. The relation between classical and quantum perturbations of spacetime can be measured by the parameter

z \geq 0

. In this work we have found that when

z = (1 + \sqrt{3}) / 2 ≃ 1.3660

, a relativistic observer approaching the Schwarzschild horizon perceives a spectral dimension

N (z) = 4 [θ (z) - 1] ≃ 2.8849

, which is related to quantum gravitational interference effects in the environment of the black hole. Under these conditions, all studied Schwarzschild black holes with masses ranging from the Planck mass to

10^{46}

times the Planck mass present the same stability configuration, which suggests the existence of a universal property of these objects under those particular conditions. The difference from the spectral dimension previously obtained at cosmological scales leads to the conclusion that the spacetime dimensionality is scale-dependent. Another important result presented here is the fundamental alteration of the effective gravitational potential near the horizon due to Hawking radiation. This quantum phenomenon prevents the potential from diverging to negative infinity as the observer approaches the Schwarzschild horizon. Full article

(This article belongs to the Special Issue Extensions of General Relativity and Applications to Cosmology and Gravitation)

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11 pages, 961 KiB

Open AccessArticle

Viscous Cosmology in f(Q,L_m) Gravity: Insights from CC, BAO, and GRB Data

by Dheeraj Singh Rana, Sai Swagat Mishra, Aaqid Bhat and Pradyumn Kumar Sahoo

Universe 2025, 11(8), 242; https://doi.org/10.3390/universe11080242 - 23 Jul 2025

Abstract

In this article, we investigate the influence of viscosity on the evolution of the cosmos within the framework of the newly proposed

f (Q, L_{m})

gravity. We have considered a linear functional form [...] Read more.

In this article, we investigate the influence of viscosity on the evolution of the cosmos within the framework of the newly proposed

f (Q, L_{m})

gravity. We have considered a linear functional form

f (Q, L_{m}) = α Q + β L_{m}

with a bulk viscous coefficient

ζ = ζ_{0} + ζ_{1} H

for our analysis and obtained exact solutions to the field equations associated with a flat FLRW metric. In addition, we utilized Cosmic Chronometers (CC), CC + BAO, CC + BAO + GRB, and GRB data samples to determine the constrained values of independent parameters in the derived exact solution. The likelihood function and the Markov Chain Monte Carlo (MCMC) sampling technique are combined to yield the posterior probability using Bayesian statistical methods. Furthermore, by comparing our results with the standard cosmological model, we found that our considered model supports the acceleration of the universe in late time. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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13 pages, 793 KiB

Open AccessCommunication

Gamma-Ray Bursts Calibrated by Using Artificial Neural Networks from the Pantheon+ Sample

by Zhen Huang, Xin Luo, Bin Zhang, Jianchao Feng, Puxun Wu, Yu Liu and Nan Liang

Universe 2025, 11(8), 241; https://doi.org/10.3390/universe11080241 - 23 Jul 2025

Abstract

In this paper, we calibrate the luminosity relation of gamma−ray bursts (GRBs) by employing artificial neural networks (ANNs) to analyze the Pantheon+ sample of type Ia supernovae (SNe Ia) in a manner independent of cosmological assumptions. The A219 GRB dataset is used to [...] Read more.

E_{p}

E_{iso}

) at low redshift with the ANN framework, facilitating the construction of the Hubble diagram at higher redshifts. Cosmological models are constrained with GRBs at high redshift and the latest observational Hubble data (OHD) via the Markov chain Monte Carlo numerical approach. For the Chevallier−Polarski−Linder (CPL) model within a flat universe, we obtain

Ω_{m} = {0.321}_{- 0.069}^{+ 0.078}

h = {0.654}_{- 0.071}^{+ 0.053}

w_{0} = {- 1.02}_{- 0.50}^{+ 0.67}

, and

w_{a} = {- 0.98}_{- 0.58}^{+ 0.58}

at the 1 −

σ

confidence level, which indicates a preference for dark energy with potential redshift evolution (

w_{a} \neq 0

). These findings using ANNs align closely with those derived from GRBs calibrated using Gaussian processes (GPs). Full article

(This article belongs to the Special Issue Gamma-Ray Bursts: Unveiling the Mysteries of the Universe’s Most Powerful Explosions)

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12 pages, 549 KiB

Open AccessArticle

Disruption of Planetary System Architectures by Stellar Flybys

by Robert Przyłuski, Hans Rickman, Paweł Wajer, Tomasz Wiśniowski, Diego Turrini, Danae Polychroni, Camilla Danielski, J. M. Diederik Kruijssen, Steven Longmore and Mélanie Chevance

Universe 2025, 11(8), 240; https://doi.org/10.3390/universe11080240 - 22 Jul 2025

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Abstract

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and is systematically scaled in both planetary mass and orbital compactness to explore the parameter space of observed exoplanetary architectures. Simulations explore a range of stellar encounter scenarios drawn from four distinct cluster environments. Our results show that system survival depends critically on the interplay between planetary mass and orbital scale: compact configurations are more resistant to external perturbations, while increased planetary mass improves resilience only up to a threshold, beyond which internal instabilities dominate. No system whose planets are twice as massive as the ones in the solar system survives stellar encounters. Systems that are at least an order of magnitude more compact than the solar system remain stable under typical encounter conditions. These findings place strong constraints on the initial architectures of planetary systems that can endure stellar-dense birth environments. Full article

(This article belongs to the Section Planetary Sciences)

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Universe, Volume 11, Issue 8 (August 2025) – 4 articles

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