Universe

18 pages, 1585 KB

Open AccessReview

Blazars as Probes for Fundamental Physics

by Giorgio Galanti

Universe 2025, 11(10), 327; https://doi.org/10.3390/universe11100327 (registering DOI) - 27 Sep 2025

Blazars are a class of active galactic nuclei characterized by having one of their relativistic jets oriented close to our line of sight. Their broad emission spectrum makes them exceptional laboratories for probing fundamental physics. In this review, we explore the potential impact on blazar observations of three scenarios beyond the standard paradigm: (i) the hadron beam model, (ii) the interaction of photons with axion-like particles (ALPs), and (iii) Lorentz invariance violation. We focus on the very-high-energy spectral features these scenarios induce in the blazars Markarian 501 and 1ES 0229+200, making them ideal targets for testing such effects. Additionally, we examine ALP-induced effects on the polarization of UV-X-ray and high-energy photons from the blazar OJ 287. The unique signatures produced by these models are accessible to current and upcoming instruments—such as the ASTRI Mini Array, CTAO, LHAASO, IXPE, COSI, and AMEGO—offering new opportunities to probe and constrain fundamental physics through blazar observations. Full article

(This article belongs to the Special Issue Multi-wavelength Properties of Active Galactic Nuclei)

11 pages, 210 KB

Open AccessCommunication

Toward a Reliability Scale for Assessing Reports of Unidentified Anomalous Phenomena (UAP)

by Dirk Schulze-Makuch and Tony Reichhardt

Universe 2025, 11(10), 326; https://doi.org/10.3390/universe11100326 - 25 Sep 2025

Abstract

Unidentified Anomalous Phenomena (UAP) observations have been reported from ancient times to today, but their true nature remains uncertain. In this paper we propose a rating scale designed to separate “signal” from “noise” in assessing UAP sighting reports. Our intention is that this will help professionals and laypeople alike distinguish cases that warrant further investigation from easily explainable false alarms. We categorize UAP sighting reports according to the quality of their evidence, considering such factors as number of observers, amount and quality of supporting evidence, especially physical evidence, and perhaps most importantly, whether UAP witnesses have made some effort to find an ordinary explanation for what they saw or experienced and whether the evidence has been subject to expert analysis. Full article

35 pages, 580 KB

Open AccessArticle

Quadrupole Perturbations of Slowly Spinning Ellis–Bronnikov Wormholes

by Bahareh Azad, Jose Luis Blázquez-Salcedo, Fech Scen Khoo, Jutta Kunz and Francisco Navarro-Lérida

Universe 2025, 11(10), 325; https://doi.org/10.3390/universe11100325 - 24 Sep 2025

Abstract

We study the axial and polar perturbations of slowly rotating Ellis–Bronnikov wormholes in General Relativity, applying a perturbative double expansion. In particular, we derive the equations for

l = 2

M_{z} = 2

perturbations of these objects, which are parametrized by [...] Read more.

We study the axial and polar perturbations of slowly rotating Ellis–Bronnikov wormholes in General Relativity, applying a perturbative double expansion. In particular, we derive the equations for

l = 2

M_{z} = 2

perturbations of these objects, which are parametrized by an asymmetry parameter. The equations constitute an astrophysically interesting sector of the perturbations that contribute dominantly to the gravitational wave radiation. Moreover, calculation of these modes may exhibit potential instabilities in the quadrupole sector. Full article

(This article belongs to the Special Issue Experimental and Observational Constraints on Wormhole Models)

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15 pages, 356 KB

Open AccessArticle

Energy–Momentum Squared Gravity Attached with Perfect Fluid Admitting Conformal Ricci Solitons

by Mohd Danish Siddiqi and Ibrahim Al-Dayel

Universe 2025, 11(10), 324; https://doi.org/10.3390/universe11100324 - 24 Sep 2025

Viewed by 25

Abstract

In the present research note, we explore the nature of the conformal Ricci solitons on the energy–momentum squared gravity model

F (R, T^{2})

that is a modification of general relativity. Furthermore, we deal with a subcase of the [...] Read more.

In the present research note, we explore the nature of the conformal Ricci solitons on the energy–momentum squared gravity model

F (R, T^{2})

that is a modification of general relativity. Furthermore, we deal with a subcase of the

F (R, T^{2}) = R + λ T^{2}

-gravity model coupled with a perfect fluid, which admits conformal Ricci solitons with a time-like concircular vector field. Using the steady conformal Ricci soliton, we derive the equation of state for the perfect fluid in the

F (R, T^{2})

-gravity model. In this series, we convey an indication of the pressure and density in the phantom barrier period and the stiff matter era, respectively. Finally, using a conformal Ricci soliton with a concircular vector field, we study the various energy constraints, black holes, and singularity circumstances for a perfect fluid coupled to

F (R, T^{2})

-gravity. Lastly, employing conformal Ricci solitons, we formulate the first law of thermodynamics, enthalpy, and the particle production rate in

F (R, T^{2})

-gravity and orthodox gravity. Full article

(This article belongs to the Section Gravitation)

32 pages, 1122 KB

Open AccessArticle

Distribution of Heavy-Element Abundances Generated by Decay from a Quasi-Equilibrium State

by Gerd Röpke, David Blaschke and Friedrich K. Röpke

Universe 2025, 11(10), 323; https://doi.org/10.3390/universe11100323 - 23 Sep 2025

Viewed by 52

Abstract

We present a freeze-out approach for describing the formation of heavy elements in expanding nuclear matter. Applying concepts used in modeling heavy-ion collisions or ternary fission, we determine the abundances of heavy elements taking into account in-medium effects such as Pauli blocking and the Mott effect, which describes the dissolution of nuclei at high densities of nuclear matter. With this approach, we search for a universal initial distribution in a quasi-equilibrium state from which the coarse-grained pattern of the solar abundances of heavy elements freezes out and evolves by radioactive decay of the excited states. The universal initial state is characterized by the Lagrange parameters, which are related to temperature and chemical potentials of neutrons and protons. We show that such a state exists and determine a temperature of 5.266 MeV, a neutron chemical potential of 940.317 MeV and a proton chemical potential of 845.069 MeV, with a baryon number density of 0.013 fm⁻³ and a proton fraction of

0.13

. Heavy neutron-rich nuclei such as the hypothetical double-magic nucleus ³⁵⁸Sn appear in the initial distribution and contribute to the observed abundances after fission. We discuss astrophysical scenarios for the realization of this universal initial distribution for heavy-element nucleosynthesis, including supernova explosions, neutron star mergers and the inhomogeneous Big Bang. The latter scenario may be of interest in the light of early massive objects observed with the James Webb Space Telescope and opens new perspectives on the universality of the observed r-process patterns and the lack of observations of population III stars. Full article

(This article belongs to the Section High Energy Nuclear and Particle Physics)

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Universe, Volume 11, Issue 10 (October 2025) – 5 articles

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