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Theoretical Physics and Cosmology: A Themed Issue in Honor of...
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Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday

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

Deadline for manuscript submissions: 31 October 2026 | Viewed by 2868

Editors

Dr. Tanmoy Paul

E-Mail Website
Guest Editor
Department of Physics, Institute of Science, Visva-Bharati University, Santiniketan, India
Interests: theoretical cosmology; quantum physics; gravity; gravitational waves
Dr. Diego Sáez-Chillón Gómez

E-Mail Website
Guest Editor
Department of Theoretical Physics, Atomic and Optics, University of Valladolid, Valladolid, Spain
Interests: theoretical cosmology; black holes; modified gravity; inflation
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Theoretical physics and cosmology are dynamic fields seeking to understand the universe's fundamental laws. To commemorate Professor Emilio Elizalde's 75th birthday and his remarkable achievements in the fields, we are pleased to present this Special Issue. 

This Special Issue aims at exploring advanced cosmological models, particularly focusing on gravitational waves, viscous cosmology, dark energy and modified gravity theories. Common themes include the investigation of quantum effects, brane cosmologies, and the implications of various gravitational frameworks on the universe's evolution. Methodologies often involve mathematical modeling and analysis of the Friedmann–Robertson–Walker equations, as well as the study of singularities and asymptotic behaviors in cosmological scenarios. The overarching goal is to deepen the understanding of cosmic acceleration, the nature of dark energy, and the potential future states of the universe, contributing to the broader field of theoretical physics and cosmology.

Dr. Tanmoy Paul
Dr. Diego Sáez-Chillón Gómez
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-anonymized peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • cosmology
  • cosmological model
  • viscous cosmology
  • dark energy
  • modified gravity

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Published Papers (4 papers)

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Research

18 pages, 1480 KB  
Article
A Scale-Invariant Fully Conformal Cosmological Model and Generalization of Schwarzschild Solution and Equation of State
by Richard Dvorsky
Universe 2026, 12(7), 191; https://doi.org/10.3390/universe12070191 - 25 Jun 2026
Viewed by 286
Abstract
This paper presents a further step in the development of scale invariant fully conformal cosmology (FCC), formulated in our previous study. Whereas the previous paper focused mainly on the global cosmological consequences of the fully conformal metric and their confrontation with selected astrophysical [...] Read more.
This paper presents a further step in the development of scale invariant fully conformal cosmology (FCC), formulated in our previous study. Whereas the previous paper focused mainly on the global cosmological consequences of the fully conformal metric and their confrontation with selected astrophysical data, here we analyze its local gravitational and background consequences. On the background of the fully conformal metric we formulate an effective generalization of the weak Schwarzschild field in the corresponding FCC global coordinates and derive from it the associated modified intensity of the Newtonian central field. We further derive the cosmological state/constitutive equation p = − ε/3 as a direct consequence of the fully conformal metric rather than as an ad hoc additional postulate. Likewise, within the fully conformal metric, spatial flatness and the critical density ρcrit are understood as direct consequences of this metric structure rather than as independently postulated inputs. From the condition of global equilibrium between negative cosmological pressure and the gravitational cohesive pressure of homogeneously distributed matter, the effective particulate fraction is obtained as β ≈ 0.45 of the total critical density ρcrit. For the relatively well-confirmed baryonic matter fraction Ω¯bar ≈0.05, this stable-equilibrium condition then leads to the corresponding particulate fraction of collisionless dark matter Ω¯FCCdm ≈0.40, which is in principle determined by the global cosmological equilibrium within this framework. Because direct identification of the entire dark fraction with standard collisionless cold dark matter would very probably be incompatible with the main structural observables, we discuss an effective phenomenological decomposition into a structuring cold dark matter component (cdm) and an almost homogeneous residual warm-dark-matter-like component (wdm). In this interpretation, the paper preserves the previously introduced global FCC framework while simultaneously providing a concrete background prediction for the matter content and a physically motivated basis for further testing of structure formation within scale invariant fully conformal cosmology. Full article
(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)
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21 pages, 398 KB  
Article
Modified Gravity as Entropic Cosmology
by Shin’ichi Nojiri, Sergei D. Odintsov, Tanmoy Paul and Soumitra SenGupta
Universe 2026, 12(5), 126; https://doi.org/10.3390/universe12050126 - 27 Apr 2026
Cited by 1 | Viewed by 624
Abstract
The present work reveals a direct correspondence between modified theories of gravity (cosmology) and entropic cosmology based on the thermodynamics of apparent horizon. It turns out that due to the total differentiable property of entropy, the usual thermodynamic law (used for Einstein gravity) [...] Read more.
The present work reveals a direct correspondence between modified theories of gravity (cosmology) and entropic cosmology based on the thermodynamics of apparent horizon. It turns out that due to the total differentiable property of entropy, the usual thermodynamic law (used for Einstein gravity) needs to be generalized for modified gravity theories having more than one thermodynamic degree of freedom (d.o.f.). For the modified theories having n number of thermodynamic d.o.f., the corresponding horizon entropy is given by Sh∼SBH+ terms containing the time derivatives of SBH up to (n−1)-th order, and moreover, the coefficient(s) of the derivative term(s) are proportional to the modification parameter of the gravity theory (compared to the Einstein gravity; SBH is the Bekenstein–Hawking entropy). By identifying the independent thermodynamic variables from the first law of thermodynamics, we show that the equivalent thermodynamic description of modified gravity naturally allows the time derivative of the Bekenstein–Hawking entropy in the horizon entropy. Full article
(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)
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23 pages, 340 KB  
Article
Linear Perturbations of an Exact Gravitational Wave in the Bianchi IV Universe
by Konstantin Osetrin
Universe 2026, 12(4), 110; https://doi.org/10.3390/universe12040110 - 9 Apr 2026
Viewed by 380
Abstract
The proper-time method for constructing perturbative dynamical gravitational fields is presented. Using the proper-time method, a perturbative analytical model of gravitational waves against the backdrop of an exact wave solution of Einstein’s equations in a Bianchi IV universe is constructed. To construct the [...] Read more.
The proper-time method for constructing perturbative dynamical gravitational fields is presented. Using the proper-time method, a perturbative analytical model of gravitational waves against the backdrop of an exact wave solution of Einstein’s equations in a Bianchi IV universe is constructed. To construct the perturbative analytical wave model a privileged wave coordinate system and a synchronous time function associated with the proper time of an observer freely moving in a gravitational wave are used. Reduction of the field equations, taking into account compatibility conditions, reduces the mathematical model of gravitational waves to a system of coupled ordinary differential equations for functions of the wave variable. Analytical solutions for the components of the gravitational wave metric have been found. The stability of the resulting perturbative solutions for the continuum domain of parameters is proven. The linear stability of the exact solution for a gravitational wave in the anisotropic Bianchi IV universe for the continuum domain of parameters is demonstrated. Full article
(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)
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36 pages, 417 KB  
Article
A Dynamical Approach to General Relativity Based on Proper Time
by Jaume de Haro
Universe 2026, 12(3), 79; https://doi.org/10.3390/universe12030079 - 12 Mar 2026
Viewed by 805
Abstract
This work places the invariant ds2 at the center of the gravitational interaction, interpreting it not as a purely geometric object but as the differential of proper time, endowed with direct physical meaning. Starting from the extension of Fermat’s principle to [...] Read more.
This work places the invariant ds2 at the center of the gravitational interaction, interpreting it not as a purely geometric object but as the differential of proper time, endowed with direct physical meaning. Starting from the extension of Fermat’s principle to massive particles—namely, the requirement that freely falling bodies follow trajectories that extremize proper time, which for timelike motion corresponds to a local maximum—and invoking the universality of Galilean free fall, we derive the form of ds2 in a static gravitational field. Lorentz invariance then provides the natural framework to extend this result to systems involving moving matter. The invariant derived through this procedure matches the weak-field limit of General Relativity formulated in the harmonic gauge. Within this linearized regime, we show that the structure of the theory already contains the seeds of its nonlinear completion: any dynamically consistent extension to strong gravitational fields necessarily involves the Ricci tensor. From this viewpoint, Einstein’s field equations appear not as a postulated geometric law but as the unique covariant closure required to ensure energy–momentum conservation and the self-consistency of the gravitational interaction. Full article
(This article belongs to the Special Issue Theoretical Physics and Cosmology: A Themed Issue in Honor of Professor Emilio Elizalde on the Occasion of His 75th Birthday)
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