MDPI - Publisher of Open Access Journals

20 pages, 342 KB

Open AccessArticle

Gross–Pitaevskii–Poisson Equations from a ξRϕ⁴ Non-Minimal Scalar-Curvature Coupling

by Bryan Cordero-Patino, Álvaro Duenas-Vidal and Jorge Segovia

Universe 2026, 12(3), 72; https://doi.org/10.3390/universe12030072 - 4 Mar 2026

Viewed by 422

In cosmological scenarios where the Peccei–Quinn symmetry is broken after inflation, small-scale axion field inhomogeneities can undergo gravitational collapse, leading to the formation of bound structures. The dynamics of these systems are commonly described using cosmological perturbation theory applied to the Einstein–Klein–Gordon equations. In the non-relativistic regime, this description reduces to the Gross–Pitaevskii–Poisson or Schrödinger–Poisson equations, depending on whether axion self-interactions are included. In this work, we extend the axion’s relativistic action by introducing a non-minimal scalar-curvature coupling of the form

ξ R ϕ^{4}

, which effectively induces a gravitationally mediated pairwise interaction. By performing a perturbative expansion and subsequently taking the non-relativistic limit, we derive a modified set of evolution equations governing the early stages of axion structure formation. Full article

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

8 pages, 514 KB

Open AccessProceeding Paper

The QCD Axion in Hot and Dense Matter

by Hong-Fang Gong, Qi Lu and Zhen-Yan Lu

Proceedings 2025, 123(1), 4; https://doi.org/10.3390/proceedings2025123004 - 8 Sep 2025

Viewed by 2613

Abstract

Within the framework of the two-flavor Nambu–Jona-Lasinio model, we study the quantum chromodynamics axion properties at finite temperature and chemical potential. Our analysis shows that the axion properties are strongly influenced by the critical behavior of the chiral phase transition. In particular, the axion mass follows the response of the chiral condensate to temperature and chemical potential, decreasing monotonically as either parameter increases. Moreover, we observe that at relatively low temperatures, the axion self-coupling constant exhibits a sharp peak near the critical point when the chemical potential increases, reaching more than twice its vacuum value. Beyond the critical point, the self-coupling rapidly decreases to values much smaller than in vacuum, eventually approaching zero at high chemical potential. These results suggest that the significant enhancement of the axion self-coupling in dense matter near the chiral phase transition may favor the formation or amplification of an axion Bose–Einstein condensate in compact astrophysical objects. Full article

(This article belongs to the Proceedings of The 5th International Conference on Symmetry (Symmetry 2025))

► Show Figures

Figure 1

26 pages, 526 KB

Open AccessArticle

String-Inspired Gravity through Symmetries

by José Antonio Belinchón

Universe 2016, 2(1), 3; https://doi.org/10.3390/universe2010003 - 5 Feb 2016

Cited by 1 | Viewed by 4343

Abstract

We study a string-inspired cosmological model from the symmetries point of view. We start by deducing the form that each physical quantity must take so that the field equations, in the string frame, admit self-similar solutions. In the same way, we formalize the use of power-law solutions (less restrictive than the self-similar ones) by studying the wave equation for the dilaton through the Lie group method. Furthermore, we show how to generate more solutions by using this approach. As examples, we calculate exact solutions to several cosmological models in the four-dimensional NS-NS (Neveu-Schwarz-Neveu-Schwarz) sector of low-energy effective string theory coupled to a dilaton and an axion-like H-field within the string frame background, with FRW and the Bianchi Type II metrics. We also study the existence of Noether symmetries, which allow us to determine the form of the physical quantities in the framework of FRW geometry and to find exact cosmological solutions. Full article

(This article belongs to the Special Issue Modified Gravity Cosmology: From Inflation to Dark Energy)

► Show Figures

Graphical abstract

Search Results (3)

Further Information

Guidelines

MDPI Initiatives

Follow MDPI

Saved Queries

Search Filter Reset All

Years

Feature Papers

Subjects

Journals

Article Types

Countries / Regions

Search Results (3)

Further Information

Guidelines

MDPI Initiatives

Follow MDPI