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Astrophysics and Cosmology at High Z

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A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Cosmology".

Deadline for manuscript submissions: closed (31 December 2025) | Viewed by 4570

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Special Issue Editors

Prof. Dr. Júlio César Fabris

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Guest Editor

Núcleo Cosmo UFES, Universidade Federal do Espírito Santo (UFES), Campus Goiabeiras, 14-Goiabeiras, Vitória 29075-910, Brazil
Interests: cosmology; dark energy; black holes; quantum cosmology
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Hermano Velten

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Departamento de Física, Universidade Federal de Ouro Preto (UFOP), Ouro Preto 35400-000, Minas Gerais, Brazil
Interests: cosmology; astrophysics; dark matter; dark energy; general relativity and its extensions
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue is devoted to debating the prospect of new observational data at high redshifts, thanks to the new generation of telescopes on Earth and in Space, that will explore the cosmos at distances close to current observational limits. The impact of these new data on the fields of Astrophysics and Cosmology is one the main focuses of this Special Issue. This focus includes the consequences these impacts have on the many theoretical models proposed in the literature.

Prof. Dr. Júlio César Fabris
Prof. Dr. Hermano Velten
Guest Editors

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Keywords

astrophysics
cosmology
high-redshift surveys
structures in the universe
dark sector of the universe

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Published Papers (3 papers)

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Research

26 pages, 1367 KB

Open AccessArticle

Supermassive Dark Stars and Their Remnants as a Possible Solution to Three Recent Cosmic Dawn Puzzles

by Cosmin Ilie, Jillian Paulin, Andreea Petric and Katherine Freese

Universe 2026, 12(1), 1; https://doi.org/10.3390/universe12010001 - 19 Dec 2025

Viewed by 2762

Abstract

The James Webb Space Telescope (JWST) has begun to revolutionize our view of the Cosmos. The discovery of Blue Monsters (i.e., ultra-compact yet very bright high-z galaxies) and the Little Red Dots (i.e., very compact dustless strong Balmer break cosmic dawn sources) pose significant challenges to pre-JWST era models of the assembly of first stars and galaxies. In addition, JWST data further strengthen the problem posed by the origin of the supermassive black holes that power the most distant quasars observed. Stars powered by Dark Matter annihilation (i.e., Dark Stars) can form out of primordial gas clouds during the cosmic dawn era and subsequently might grow via accretion and become supermassive. In this paper we argue that Supermassive Dark Stars (SMDSs) offer natural solutions to the three puzzles mentioned above. Full article

(This article belongs to the Special Issue Astrophysics and Cosmology at High Z)

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17 pages, 754 KB

Open AccessArticle

Non-Linear f(Q,T) Gravity and the Late-Time Acceleration of the Universe

by Alnadhief H. A. Alfedeel

Universe 2025, 11(12), 382; https://doi.org/10.3390/universe11120382 - 21 Nov 2025

Viewed by 595

Abstract

This study examines cosmic acceleration in the framework of

f (Q, T)

gravity and compares it to the standard

Λ

CDM model. It considers a generalized nonlinear form of the nonmetricity, expressed as [...] Read more.

This study examines cosmic acceleration in the framework of

f (Q, T)

gravity and compares it to the standard

Λ

CDM model. It considers a generalized nonlinear form of the nonmetricity, expressed as

f (Q, T) = - Q + α_{0} Q^{2} / H_{0}^{2} + β_{0} T + η_{0}

, where

α_{0}, β_{0},

and

η_{0}

are constants, and

H_{0}

is the current value of the Hubble constant. In the solution process, we did not rely on any additional conditions to solve the field equations; instead, the field equations were reduced to a time-dependent closed system of nonlinear first-order coupled differential equations for H and

ρ

. Subsequently, these differential equations were converted to the redshift space for numerical integration alongside the Runge–Kutta method. Furthermore, the study demonstrates that the deceleration parameter q changes sign from being positive in an early period of time at high redshift values to a negative value, passing through a transitional redshift

z_{t} \approx [0.766, 0.769, 0.771]

and

z_{t} \approx [0.521, 0.770, 1.010]

, reaching their current values at

q_{0} = [- 0.61, - 0.60, - 0.59]

and

[- 0.455, - 0.595, - 0.694]

for different values of

β_{0}

and

α_{0}

, respectively. Similarly, the effective equation of state

w_{e f f}

shifted from the matter-dominated phase

w_{e f f} = 0

at high redshift to a quintessence-like behavior at low redshift. Moreover, a super-accelerated or phantom-like regime with

q_{0} ≃ - 1.59

and

w_{eff, 0} ≃ - 1.40

was obtained when

α_{0} = 0.55

and

β_{0} = 0.60

were employed. The model analysis reveals that the universe is presently experiencing an accelerating expansion phase, propelled by a quintessence-type and phantom-like dark energy component, as corroborated by the Om(z) diagnostic test. The results obtained were strongly consistent with the concordance

Λ

CDM model. Full article

(This article belongs to the Special Issue Astrophysics and Cosmology at High Z)

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15 pages, 909 KB

Open AccessArticle

Gravitational Lensing by Lemaître–Tolman–Bondi Wormholes in a Friedmann Universe

by Kirill A. Bronnikov, Valeria A. Ishkaeva and Sergey V. Sushkov

Universe 2025, 11(11), 374; https://doi.org/10.3390/universe11110374 - 12 Nov 2025

Viewed by 591

Abstract

The Lemaître–Tolman–Bondi (LTB) solution to the Einstein equations describes the dynamics of a self-gravitating spherically symmetric dust cloud with an arbitrary density profile and any distribution of initial velocities, encoded in three arbitrary functions

f (R)

F (R)

[...] Read more.

f (R)

F (R)

, and

τ_{0} (R)

, where R is a radial coordinate in the comoving reference frame. A particular choice of these functions corresponds to a wormhole geometry with a throat defined as a sphere of minimum radius at a fixed time instant. In this paper we explore LTB wormholes and discuss their possible observable appearance, studying in detail the effects of gravitational lensing by such objects. For this aim, we study photon motion in wormhole space-time inscribed in a closed Friedmann dust-filled universe and find the wormhole shadow as it could be seen by a distant observer. Because the LTB wormhole is a dynamic object, we analyze the dependence of its shadow size on the observation time and on the initial size of the wormhole region. We reveal that the angular size of the shadow exhibits a non-monotonic dependence on the observation time. At early times, the shadow size decreases as photons with smaller angular momentum gradually reach the observer. At later times, the expansion of the Friedmann universe becomes a dominant factor that leads to an increase in the shadow size. Full article

(This article belongs to the Special Issue Astrophysics and Cosmology at High Z)

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