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Symmetry in the Dark Sector of the Universe and Recent Astronomical Observations

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Dear Colleagues,

Despite comprising roughly 95% of the cosmic energy budget, the fundamental nature of the dark sector—encompassing both dark matter and dark energy—remains one of the most profound mysteries in modern physics. Throughout the history of physics, the concept of symmetry has served as the primary guiding principle for constructing robust theoretical frameworks. This Special Issue aims to explore how exact, broken, hidden, or emergent symmetries can shed light on these elusive cosmic components. We invite original research and comprehensive review articles that investigate theoretical and phenomenological aspects of symmetries applied to the dark sector. Topics of interest include, but are not limited to, novel dark gauge groups, discrete symmetries stabilizing dark matter, conformal symmetry in cosmology, dynamical symmetry breaking, and phase transitions in the early universe. By uniting these perspectives, we seek to advance our fundamental understanding of the universe's hidden architecture using the latest astronomical probes.

Dr. Jaime Haro Cases
Dr. Supriya Pan
Guest Editors

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22 pages, 3412 KB

Open AccessArticle

Theoretical Analysis of Barrow Holographic Dark Energy in Fractal Cosmology

by Hanshu Zhao and Weiqiang Yang

Symmetry 2026, 18(8), 1327; https://doi.org/10.3390/sym18081327 - 5 Aug 2026

Viewed by 175

Abstract

We investigate interacting Barrow holographic dark energy (BHDE) with the Hubble horizon as the infrared cutoff in a fractal cosmological background. The fractal measure modifies the Friedmann sector and leads to the nonstandard closure relation [...] Read more.

Ω_{dm} + Ω_{de} = 1 + γ

. Using Planck-inspired present-day normalization

Ω_{de 0} = 0.6847

and the benchmark parameters

(Δ, ω, β) = (0.8, 0.263, 0.123)

, we study four linear and nonlinear dark-sector interactions over

- 0.99 \leq z \leq 3

. The benchmark solutions remain within the physical background domain throughout this interval: the density fractions are non-negative, the relevant denominators remain positive, and the nonlinear

Q_{3}

and

Q_{4}

terms remain real. All four prescriptions exhibit a transition from decelerated to accelerated expansion. The non-interacting limit gives

z_{t} ≃ 0.86

, whereas for

ξ = 0.12

the transition redshifts are approximately

2.02

1.97

1.26

, and

1.87

for

Q_{1}

Q_{2}

Q_{3}

, and

Q_{4}

, respectively; in particular, the

Q_{4}

solution recovers a finite transition within the plotted redshift range. A stronger positive coupling generally shifts acceleration onset to a higher redshift, with the strongest response for

Q_{1}

and the weakest for

Q_{3}

. The statefinder quantities

S_{3}^{(1)}

S_{3}^{(2)}

, and the s–r trajectories distinguish the interaction structures through their finite-redshift evolution and present-day values. A fixed-background SN Ia comparison using 1046 selected Pantheon supernovae, diagonal FITRES uncertainties, and an analytically profiled additive nuisance parameter gives the lowest information criteria for flat

Λ

CDM. Among the BHDE benchmarks that interact,

Q_{4}

is the closest case, with

Δ AIC = Δ BIC ≃ 1.21

. This comparison is not intended as a global posterior constraint on the model parameters. Full article

(This article belongs to the Special Issue Symmetry in the Dark Sector of the Universe and Recent Astronomical Observations)

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