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Symmetry
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Symmetry and Cosmology
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Symmetry and Cosmology

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: 30 June 2026 | Viewed by 3401

Special Issue Editors

Prof. Dr. Rudnei O. Ramos

E-Mail Website
Guest Editor
Departamento de Fisica Teorica, Universidade do Estado do Rio de Janeiro, Rio de Janeiro 20550-013, Brazil
Interests: cosmic inflation; field dynamics; finite temperature quantum field theory; cosmological phase transitions; early universe physics
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Leila Lobato Graef

E-Mail Website
Guest Editor
Instituto de Física, Universidade Federal Fluminense, Niteroi 24210-240, Brazil
Interests: early universe; quantum gravity; stochastic gravitational waves; cosmological tensions; dark energy

Special Issue Information

Dear Colleagues,

This Special Issue, titled “Symmetry and Cosmology”, aims to provide an overview of the latest developments concerning aspects of symmetry in the field of cosmology. Symmetry plays an important role in most approaches to cosmology, ranging from the early- to late-time Universe. They are essential ingredients in the analysis of the large-scale structure of the current universe, in the scalar field description of cosmological components, in gauge symmetries, in the effective field theory for cosmological perturbations, in quantum gravity proposals, in gravitational wave predictions from the interface between cosmology and astrophysics, cosmological phase transitions, among many other topics. Altogether, this Special Issue aims to provide an overview of the current developments in such areas, discussing new concepts which could guide and stimulate efforts in understanding important aspects of symmetries in cosmology.

Prof. Dr. Rudnei O. Ramos
Prof. Dr. Leila Lobato Graef
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-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Symmetry is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). 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

  • symmetry
  • cosmology
  • early universe
  • large-scale structure
  • quantum gravity
  • effective field theory
  • cosmological phase transitions
  • cosmic microwave background
  • cosmological perturbations

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Published Papers (3 papers)

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Research

14 pages, 405 KB  
Article
Choice of Quantum Vacuum for Inflation Observables
by Melo Wood-Saanaoui, Rudnei O. Ramos and Arjun Berera
Symmetry 2026, 18(3), 399; https://doi.org/10.3390/sym18030399 - 25 Feb 2026
Viewed by 527
Abstract
We investigate the modifications to inflationary observables that arise when adopting an α-vacuum instead of the standard Bunch–Davies vacuum for quantum fluctuations during inflation. Within the Starobinsky inflationary model, we compute and compare the scalar spectral index, its running, and the running [...] Read more.
We investigate the modifications to inflationary observables that arise when adopting an α-vacuum instead of the standard Bunch–Davies vacuum for quantum fluctuations during inflation. Within the Starobinsky inflationary model, we compute and compare the scalar spectral index, its running, and the running of the running arising from different choices of the initial vacuum state. We further examine the energy scales associated with α-vacua and argue that, for any number of extra spatial dimensions, the relevant scale can be truncated at the Hubble scale, ∼O(1013)GeV, without conflict with current Cavendish-type experimental bounds on sub-millimeter gravity (∼250μm). Our analysis demonstrates that the α-vacuum is subject to stringent constraints as a viable de Sitter-invariant alternative to the Euclidean (Bunch–Davies) vacuum, with the corrections that it induces in the inflationary observables being strongly limited by the latest Planck data. Full article
(This article belongs to the Special Issue Symmetry and Cosmology)
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18 pages, 601 KB  
Article
Back-Reaction of Super-Hubble Fluctuations, Late Time Tracking, and Recent Observational Results
by Marco A. Alvarez, Leila L. Graef and Robert Brandenberger
Symmetry 2025, 17(9), 1507; https://doi.org/10.3390/sym17091507 - 10 Sep 2025
Cited by 1 | Viewed by 1090
Abstract
Previous studies have suggested that the back-reaction of super-Hubble cosmological fluctuations on a symmetric background space-time, with respect to being homogeneous and isotropic, could behave like a dynamical relaxation of the cosmological constant. Moreover, this mechanism appears to be self-regulatory, potentially leading to [...] Read more.
Previous studies have suggested that the back-reaction of super-Hubble cosmological fluctuations on a symmetric background space-time, with respect to being homogeneous and isotropic, could behave like a dynamical relaxation of the cosmological constant. Moreover, this mechanism appears to be self-regulatory, potentially leading to oscillatory behavior in the effective DE. Such an effect would occur in any cosmological model with super-Hubble matter fluctuations, including the standard ΛCDM model. Apart from that, recent DESI data, which indicate that DE may be dynamical, have renewed interest in exploring scenarios leading to such an oscillatory behavior. In this study, we propose a parameterization to account for the impact of super-Hubble fluctuations on the background energy density of the Universe. We model the total effective cosmological constant as the sum of a constant and an oscillating contribution. We performed a preliminary comparison of the background dynamics of this model with recent radial BAO data from DESI. We also discuss the status of the H0 tension problem in this model. Full article
(This article belongs to the Special Issue Symmetry and Cosmology)
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12 pages, 330 KB  
Article
On the Stability of Non-Singular Solutions in Effective Theory from Kaluza–Klein Unimodular Gravity
by Júlio C. Fabris and Richard Kerner
Symmetry 2025, 17(3), 419; https://doi.org/10.3390/sym17030419 - 11 Mar 2025
Cited by 1 | Viewed by 1020
Abstract
Unimodular theory incorporating the Kaluza–Klein construction in five dimensions leads, after reduction to four dimensions, to a new class of scalar–tensor theory. The vacuum cosmological solutions display a bounce with non-singular behavior the effective lower dimension model: from the four-dimensional point of view, [...] Read more.
Unimodular theory incorporating the Kaluza–Klein construction in five dimensions leads, after reduction to four dimensions, to a new class of scalar–tensor theory. The vacuum cosmological solutions display a bounce with non-singular behavior the effective lower dimension model: from the four-dimensional point of view, the solutions are completely regular. However, the propagation of gravitational waves in this geometry displays the presence of instabilities which reflect singular features of the original five-dimensional structure connected to a degenerate metric at the bounce. A four-dimensional quantum model with cosmological constant, which has a similar background behavior, is discussed and revealed to be stable. Full article
(This article belongs to the Special Issue Symmetry and Cosmology)
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