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Universe
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Universe: Feature Papers 2024—'Cosmology'
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Universe: Feature Papers 2024—'Cosmology'

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

Deadline for manuscript submissions: 31 July 2025 | Viewed by 11121

Special Issue Editors

Dr. Jaime Haro Cases

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Guest Editor
Department of mathematics , Politechnic University of Catalonia, Barcelona, Spain
Interests: quintessential inflation and gravitational particle production
Special Issues, Collections and Topics in MDPI journals
Dr. Supriya Pan

E-Mail Website
Guest Editor
Department of Mathematics, Presidency University, 86/1 College Street, Kolkata 700073, India
Interests: theoretical and observational cosmology; dark energy; modified gravity theories; matter creation; massive neutrinos
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

We are pleased to announce that Cosmology is now compiling a collection of papers submitted by the Editorial Board Members (EBMs) of our section and outstanding scholars in this research field. We welcome contributions and recommendations from EBMs.

This Special Issue aims to set itself at the cutting edge of the most recent advances in the intertwined ties of cosmology with other fields at all relevant scales from mutually fertilizing theoretical, phenomenological, and experimental perspectives. We expect these papers to be widely read and highly influential within the field. Potential topics include, but are not limited to, the following: cosmological models, quantum cosmology, dark matter, dark energy, cosmological perturbation theory, cosmic microwave background (CMB), observational cosmology, cosmological tensions, etc.

We would also like to take this opportunity to ask more scholars to join the section Cosmology, so that we can work together to further develop this exciting field of research.

Dr. Jaime Haro Cases
Dr. Supriya Pan
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 100 words) can be sent to the Editorial Office for announcement on this website.

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-blind 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

  • cosmological models
  • quantum cosmology
  • dark matter and dark energy
  • cosmic acceleration
  • cosmological constants
  • cosmological perturbation theory
  • cosmic microwave background (CMB)
  • observational cosmology

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Published Papers (11 papers)

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Research

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10 pages, 246 KiB  
Article
Is Planckian Discreteness Observable in Cosmology?
by Gabriel R. Bengochea, Gabriel León and Alejandro Perez
Universe 2025, 11(5), 139; https://doi.org/10.3390/universe11050139 - 27 Apr 2025
Viewed by 129
Abstract
A Planck scale inflationary era—in a quantum gravity theory predicting discreteness of quantum geometry at the fundamental scale—produces the scale-invariant spectrum of inhomogeneities with a very small tensor-to-scalar ratio of perturbations and a hot big bang leading to a natural dark matter genesis [...] Read more.
A Planck scale inflationary era—in a quantum gravity theory predicting discreteness of quantum geometry at the fundamental scale—produces the scale-invariant spectrum of inhomogeneities with a very small tensor-to-scalar ratio of perturbations and a hot big bang leading to a natural dark matter genesis scenario. Here, we evoke the possibility that some of the major puzzles in cosmology would have an explanation rooted in quantum gravity. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
33 pages, 1207 KiB  
Article
Running Vacuum and H4 Inflation
by Joan Solà Peracaula, Cristian Moreno-Pulido and Alex González-Fuentes
Universe 2025, 11(4), 118; https://doi.org/10.3390/universe11040118 - 2 Apr 2025
Viewed by 149
Abstract
Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate Λ=Λ(H) [...] Read more.
Recent studies of QFT in cosmological spacetime indicate that the speeding up of the present universe may not just be associated with a rigid cosmological term but with a running one that evolves with the expansion rate Λ=Λ(H). This running is inherited from the cosmic evolution of the vacuum energy density (VED), ρvac, which is sensitive to quantum effects in curved spacetime that ultimately trigger that running. The VED is a function of the Hubble rate and its time derivatives ρvac=ρvac(H,H˙,H¨,…). Two nearby points of cosmic evolution during the FLRW epoch are smoothly related as δρvac∼O(H2). In the very early universe, in contrast, the higher powers of the Hubble rate take over and bring about a period of fast inflation. They originate from quantum effects on the effective action of a vacuum, which we compute. Herein, we focus on the lowest possible power for inflation to occur: H4. During the inflationary phase, H remains approximately constant and very large. Subsequently, the universe enters the usual FLRW radiation epoch. This new mechanism (‘RVM inflation’) is not based on any supplementary ‘inflaton’ field; it is fueled by pure QFT effects on the dynamical background and is different from Starobinsky’s inflation, in which H is never constant. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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16 pages, 334 KiB  
Article
A Note on Gravitational Dark Matter Production
by Jaume de Haro and Supriya Pan
Universe 2025, 11(2), 49; https://doi.org/10.3390/universe11020049 - 4 Feb 2025
Viewed by 530
Abstract
Dark matter, one of the fundamental components of the universe, has remained mysterious in modern cosmology and particle physics, and hence, this field is of utmost importance at the present moment. One of the foundational questions in this direction is the origin of [...] Read more.
Dark matter, one of the fundamental components of the universe, has remained mysterious in modern cosmology and particle physics, and hence, this field is of utmost importance at the present moment. One of the foundational questions in this direction is the origin of dark matter, which directly links to its creation. In the present article, we study the gravitational production of dark matter in two distinct contexts: firstly, when reheating occurs through gravitational particle production, and secondly, when it is driven by decay of the inflaton field. We establish a connection between the reheating temperature and the mass of dark matter, and from the reheating bounds, we determine the range of viable dark matter mass values. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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134 pages, 2234 KiB  
Article
Cosmologies with Perfect Fluids and Scalar Fields in Einstein’s Gravity: Phantom Scalars and Nonsingular Universes
by Michela Cimaglia, Massimo Gengo and Livio Pizzocchero
Universe 2024, 10(12), 467; https://doi.org/10.3390/universe10120467 - 23 Dec 2024
Viewed by 877
Abstract
In the initial part of this paper, we survey (in arbitrary spacetime dimension) the general FLRW cosmologies with non-interacting perfect fluids and with a canonical or phantom scalar field, minimally coupled to gravity and possibly self-interacting; after integrating the evolution equations for the [...] Read more.
In the initial part of this paper, we survey (in arbitrary spacetime dimension) the general FLRW cosmologies with non-interacting perfect fluids and with a canonical or phantom scalar field, minimally coupled to gravity and possibly self-interacting; after integrating the evolution equations for the fluids, any model of this kind can be described as a Lagrangian system with two degrees of freedom, where the Lagrange equations determine the evolution of the scale factor and the scalar field as functions of the cosmic time. We analyze specific solvable models, paying special attention to cases with a phantom scalar; the latter favors the emergence of nonsingular cosmologies in which the Big Bang is replaced, e.g., with a Big Bounce or a periodic behavior. As a first example, we consider the case with dust (i.e., pressureless matter), radiation, and a scalar field with a constant self-interaction potential (this is equivalent to a model with dust, radiation, a free scalar field and a cosmological constant in the Einstein equations). In the phantom subcase (say, with nonpositive spatial curvature), this yields a Big Bounce cosmology, which is a non-absurd alternative to the standard (ΛCDM) Big Bang cosmology; this Big Bounce model is analyzed in detail, even from a quantitative viewpoint. We subsequently consider a class of cosmological models with dust and a phantom scalar, whose self-potential has a special trigonometric form. The Lagrange equations for these models are decoupled passing to suitable coordinates (x,y), which can be interpreted geometrically as Cartesian coordinates in a Euclidean plane: in this description, the scale factor is a power of the radius r=x2+y2. Each one of the coordinates x,y evolves like a harmonic repulsor, a harmonic oscillator, or a free particle (depending on the signs of certain constants in the self-interaction potential of the phantom scalar). In particular, in the case of two harmonic oscillators, the curves in the plane described by the point (x,y) as a function of time are the Lissajous curves, well known in other settings but not so popular in cosmology. A general comparison is performed between the contents of the present work and the previous literature on FLRW cosmological models with scalar fields, to the best of our knowledge. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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21 pages, 899 KiB  
Article
The Shape of the Chameleon Fifth-Force on the Mass Components of Galaxy Clusters
by Lorenzo Pizzuti, Valentina Amatori, Alexandre M. Pombo and Sandeep Haridasu
Universe 2024, 10(12), 443; https://doi.org/10.3390/universe10120443 - 30 Nov 2024
Viewed by 762
Abstract
In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, [...] Read more.
In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, the mass of the Intra-Cluster Medium (ICM) and the diffuse cold dark matter (CDM). The obtained semi-analytic profile is validated against the numerical solution of the chameleon field equation and implemented in the MG-MAMPOSSt code for kinematic analyses of galaxy clusters in modified gravity scenarios. By means of mock halos, simulated both in GR and in modified gravity, we show that the combination of the velocities and positions of cluster member galaxies, along with the data of the stellar velocity dispersion profile of the BCG, can impose constraints on the parameter space of the chameleon model; for a cluster generated in GR, these constraints are at the same level as a joint lensing+kinematics analysis of a cluster modelled with a single mass profile, without the BCG data. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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28 pages, 21146 KiB  
Article
Combined Studies Approach to Rule Out Cosmological Models Which Are Based on Nonlinear Electrodynamics
by Ricardo García-Salcedo, Isidro Gómez-Vargas, Tame González, Vicent Martinez-Badenes and Israel Quiros
Universe 2024, 10(9), 353; https://doi.org/10.3390/universe10090353 - 4 Sep 2024
Cited by 2 | Viewed by 1130
Abstract
We apply a combined study in order to investigate the dynamics of cosmological models incorporating nonlinear electrodynamics (NLED). The study is based on the simultaneous investigation of such fundamental aspects as stability and causality, complemented with a dynamical systems investigation of the involved [...] Read more.
We apply a combined study in order to investigate the dynamics of cosmological models incorporating nonlinear electrodynamics (NLED). The study is based on the simultaneous investigation of such fundamental aspects as stability and causality, complemented with a dynamical systems investigation of the involved models, as well as Bayesian inference for parameter estimation. We explore two specific NLED models: the power-law and the rational Lagrangian. We present the theoretical framework of NLED coupled with general relativity, followed by an analysis of the stability and causality of the various NLED Lagrangians. We then perform a detailed dynamical analysis to identify the ranges where these models are stable and causal. Our results show that the power-law Lagrangian model transitions through various cosmological phases, evolving from a Maxwell radiation-dominated state to a matter-dominated state. For the rational Lagrangian model, including the Maxwell term, stable and causal behavior is observed within specific parameter ranges, with critical points indicating the evolutionary pathways of the universe. To validate our theoretical findings, we perform Bayesian parameter estimation using a comprehensive set of observational data, including cosmic chronometers, baryon acoustic oscillation (BAO) measurements, and supernovae type Ia (SNeIa). The estimated parameters for both models align with the expected values for the current universe, particularly the matter density Ωm and the Hubble parameter h. However, the parameters of the models are not tightly constrained within the prior ranges. Our combined studies approach rules out the mentioned models as an appropriate description of the cosmos. Our results highlight the need for further refinement and exploration of NLED-based cosmological models to fully integrate them into the standard cosmological framework. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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34 pages, 17795 KiB  
Article
Lorentzian Quantum Cosmology from Effective Spin Foams
by Bianca Dittrich and José Padua-Argüelles
Universe 2024, 10(7), 296; https://doi.org/10.3390/universe10070296 - 13 Jul 2024
Cited by 14 | Viewed by 1366
Abstract
Effective spin foams provide the most computationally efficient spin foam models yet and are therefore ideally suited for applications, e.g., to quantum cosmology. Here, we provide the first effective spin foam computations of a finite time evolution step in a Lorentzian quantum de [...] Read more.
Effective spin foams provide the most computationally efficient spin foam models yet and are therefore ideally suited for applications, e.g., to quantum cosmology. Here, we provide the first effective spin foam computations of a finite time evolution step in a Lorentzian quantum de Sitter universe. We will consider a setup that computes the no-boundary wave function and a setup describing the transition between two finite scale factors. A key property of spin foams is that they implement discrete spectra for the areas. We therefore study the effects that are induced by the discrete spectra. To perform these computations, we had to identify a technique to deal with highly oscillating and slowly converging or even diverging sums. Here, we illustrate that high-order Shanks transformation works very well and is a promising tool for the evaluation of Lorentzian (gravitational) path integrals and spin foam sums. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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22 pages, 785 KiB  
Article
Constraints on the Minimally Extended Varying Speed of Light Model Using Pantheon+ Dataset
by Seokcheon Lee
Universe 2024, 10(6), 268; https://doi.org/10.3390/universe10060268 - 19 Jun 2024
Cited by 7 | Viewed by 1589
Abstract
In the context of the minimally extended varying speed of light (meVSL) model, both the absolute magnitude and the luminosity distance of type Ia supernovae (SNe Ia) deviate from those predicted by general relativity (GR). Using data from the Pantheon+ survey, we assess [...] Read more.
In the context of the minimally extended varying speed of light (meVSL) model, both the absolute magnitude and the luminosity distance of type Ia supernovae (SNe Ia) deviate from those predicted by general relativity (GR). Using data from the Pantheon+ survey, we assess the plausibility of various dark energy models within the framework of meVSL. Both the constant equation of state (EoS) of the dark energy model (ωCDM) and the Chevallier–Polarski–Linder (CPL) parameterization model (ω=ω0+ωa(1−a)) indicate potential variations in the cosmic speed of light at the 1−σ confidence level. For Ωm0=0.30,0.31, and 0.32 with (ω0,ωa)=(−1,0), the 1−σ range of c˙0/c0(10−13yr−1) is (−8.76, −0.89), (−11.8, 3.93), and (−14.8, −6.98), respectively. Meanwhile, the 1−σ range of c˙0/c0(10−12yr−1) for CPL dark energy models with −1.05≤ω0≤−0.95 and 0.28≤Ωm0≤0.32 is (−6.31, −2.98). The value of c at z=3 can exceed that of the present by 0.2∼3% for ωCDM models and 5∼13% for CPL models. Additionally, for viable models except for the CPL model with Ωm0=0.28, we find −25.6≤G˙0/G0(10−12yr−1)≤−0.36. For this particular model, we obtain an increasing rate of the gravitational constant within the range 1.65≤G˙0/G0(10−12yr−1)≤3.79. We obtain some models that do not require dark matter energy density through statistical interpretation. However, this is merely an effect of the degeneracy between model parameters and energy density and does not imply that dark matter is unnecessary. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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14 pages, 1433 KiB  
Article
Fast Radio Burst Energy Function in the Presence of DMhost Variation
by Ji-Guo Zhang, Yichao Li, Jia-Ming Zou, Ze-Wei Zhao, Jing-Fei Zhang and Xin Zhang
Universe 2024, 10(5), 207; https://doi.org/10.3390/universe10050207 - 6 May 2024
Cited by 7 | Viewed by 1278
Abstract
Fast radio bursts (FRBs) have been found in great numbers, but the physical mechanism of these sources is still a mystery. The redshift evolutions of the FRB energy distribution function and the volumetric rate shed light on the origin of FRBs. However, such [...] Read more.
Fast radio bursts (FRBs) have been found in great numbers, but the physical mechanism of these sources is still a mystery. The redshift evolutions of the FRB energy distribution function and the volumetric rate shed light on the origin of FRBs. However, such estimations rely on the dispersion measurement (DM)–redshift (z) relationship. A few FRBs that have been detected recently show large excess DMs beyond the expectation from the cosmological and Milky Way contributions, which indicates large spread of DMs from their host galaxies. In this work, we adopt two lognormal-distributed DMhost models and estimate the energy function using the non-repeating FRBs selected from the Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB Catalog 1. By comparing the lognormal-distributed DMhost models to a constant DMhost model, the FRB energy function results are consistent within the measurement uncertainty. We also estimate the volumetric rate of the non-repeating FRBs in three different redshift bins. The volumetric rate shows that the trend is consistent with the stellar-mass density redshift evolution. Since the lognormal-distributed DMhost model increases the measurement errors, the inference of FRBs tracking the stellar-mass density is nonetheless undermined. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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Review

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19 pages, 436 KiB  
Review
Different Aspects of Entropic Cosmology
by Shin’ichi Nojiri, Sergei D. Odintsov and Tanmoy Paul
Universe 2024, 10(9), 352; https://doi.org/10.3390/universe10090352 - 3 Sep 2024
Cited by 7 | Viewed by 1116
Abstract
We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon [...] Read more.
We provide a short review of the recent developments in entropic cosmology based on two thermodynamic laws of the apparent horizon, namely the first and the second laws of thermodynamics. The first law essentially provides the change in entropy of the apparent horizon during the cosmic evolution of the universe; in particular, it is expressed by TdS=−d(ρV)+WdV (where W is the work density and other quantities have their usual meanings). In this way, the first law actually links various theories of gravity with the entropy of the apparent horizon. This leads to a natural question—“What is the form of the horizon entropy corresponding to a general modified theory of gravity?”. The second law of horizon thermodynamics states that the change in total entropy (the sum of horizon entropy + matter fields’ entropy) with respect to cosmic time must be positive, where the matter fields behave like an open system characterised by a non-zero chemical potential. The second law of horizon thermodynamics importantly provides model-independent constraints on entropic parameters. Finally, we discuss the standpoint of entropic cosmology on inflation (or bounce), reheating and primordial gravitational waves from the perspective of a generalised entropy function. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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26 pages, 388 KiB  
Review
Introduction to the Number of e-Folds in Slow-Roll Inflation
by Alessandro Di Marco, Emanuele Orazi and Gianfranco Pradisi
Universe 2024, 10(7), 284; https://doi.org/10.3390/universe10070284 - 29 Jun 2024
Cited by 4 | Viewed by 1374
Abstract
In this review, a pedagogical introduction to the concepts of slow-roll inflationary universe and number of e-folds is provided. In particular, the differences between the basic notion of number of e-folds (Ne), total number of e-folds ( [...] Read more.
In this review, a pedagogical introduction to the concepts of slow-roll inflationary universe and number of e-folds is provided. In particular, the differences between the basic notion of number of e-folds (Ne), total number of e-folds (NT) and number of e-folds before the end of inflation (N) are outlined. The proper application of the number of e-folds before the end of inflation is discussed both as a time-like variable for the scalar field evolution and as a key parameter for computing inflationary predictions. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')
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