arXiv subjects

Yongjun Yun

Publications and source records attributed to Yongjun Yun.

5 recordsLinked to original sources

arXiv2026

Dynamical Resolution of the Cosmic Coincidence Problem in Non-Interacting Holographic Dark Energy via Einstein-Cartan Torsion

We investigate the cosmic coincidence problem in non-interacting holographic dark energy with the Hubble radius as the infrared cutoff in Einstein-Cartan gravity. In general relativity, this cutoff gives a dust-like equation of state in the non-interacting case, whereas interacting models require a phenomenological dark sector coupling and yield a constant density ratio. We show that the Einstein-Cartan torsion scalar $\Phi$, compatible with the cosmological principle, with the self-consistent scaling behavior $\Phi\sim a^{-3}$, makes the density ratio $r\equiv \rho_m/\rho_X$ dynamical even when the phenomenological interaction term is absent, $Q=0$. The same torsion contribution shifts the equation of state for holographic dark energy toward negative values, allowing cosmic acceleration, and realizes the observed order-unity density ratio within the weak torsion regime without tuning the holographic free parameter. Thus, Einstein-Cartan torsion provides a geometric mechanism that replaces the phenomenological dark sector interaction and offers a dynamical resolution of the cosmic coincidence problem.

gr-qc↗

arXiv2026

Holographic Dark Energy with Hubble Radius as an Infrared Cutoff in Einstein-Cartan Gravity

In this work, we investigate non-interacting holographic dark energy (HDE) with the Hubble radius as the infrared cutoff in Einstein-Cartan gravity. We derive the Einstein-Cartan equations from the action principle and obtain Friedmann-like equations by introducing a torsion scalar. Considering a Weyssenhoff spin fluid, we determine the scaling behavior of the torsion scalar as $\Phi \sim a^{-3}$ without introducing an ad hoc ansatz, resolving the ansatz problem of previous torsion scalar scenarios. In the absence of interactions between dark matter and dark energy, the torsion scalar shifts the equation of state for holographic dark energy toward negative values from the dust-like value obtained in HDE without torsion, making cosmic acceleration possible. In particular, the resulting equation of state can approach $\omega_X \simeq -1$ and cross the phantom divide within the weak torsion regime $|\Phi/H| < 1$. The model predicts a dynamical equation of state in which cosmic acceleration gradually weakens, potentially consistent with recent DESI observations. In spacetimes with torsion, the cosmic distance duality relation between the luminosity distance $d_L$ and the angular diameter distance $d_A$ is modified as $d_L = d_A (1+z)^2 (1+\eta)$. In the presence of the torsion scalar, we show that the standard relation between redshift and the scale factor is preserved, while the deviation parameter arising from torsion effects is determined as $\eta \sim \int_{t_S}^{t_O} dt a^{-3}$, where $t_S$ and $t_O$ denote the emission time at the source and the observation time at the observer, respectively. Overall, our results support the feasibility of the model and provide a theoretical framework for preparing likelihood analyses.

gr-qc↗

arXiv2025

Non-interacting holographic dark energy with Torsion via Hubble Radius

We reconstruct a holographic dark energy model within a Friedmann cosmology incorporating torsion scalar, assuming no interaction between dark energy and dark matter. Setting the Hubble radius as an infrared (IR) cut-off, we focus on a system dominated by contribution of a time-dependent torsion scalar induced by the spin of matter. In this regime, our results show that even very weak torsion causes cosmic acceleration. Specifically, we find that minima of the current equation of state for holographic dark energy, $(\omega_X^{0})_{min}$, lies in the range $-1 < (\omega_X^{0})_{min} < -0.778$ as a free parameter $d$ varies from $1$ to $0.654$. Focusing on the free parameter $d \approx 1$, we find that $(\omega_X^{0})_{min}$ exhibits slightly different behavior from the cosmological constant. Introducing torsion allows the Hubble radius to serve as a viable IR cut-off even without assuming the interaction between them. Moreover, this approach provides a non-interacting limit not found in earlier interacting models that use the Hubble radius as the IR cut-off.

gr-qc↗

arXiv2025

Electromagnetism from relativistic fluid dynamics

We reformulate classical electromagnetism within the matter-space framework of relativistic fluid dynamics. The central assumption is that the relevant degrees of freedom are encoded in differential forms on a three-dimensional matter space and mapped to spacetime by pull-back. The absence of four-forms in matter space imposes nontrivial kinematical constraints on the induced spacetime fields and restricts gauge transformations to those compatible with the flow. Because of this (matter-space) gauge symmetry, the physically relevant sector is retained, and the Aharonov-Bohm phase is naturally associated with the matter-space potential. The construction admits two electromagnetic frames. We argue that the frame identifying the spacetime field strength directly with the intrinsic matter-space two-form is geometrically preferred. In the first frame, the homogeneous sector is fixed by the matter-space structure, while the sourced equation follows from an action-based relativistic-fluid formulation in a first-order setting where the potential and field strength are varied independently and the matter-space constraints are imposed on shell. In the massless case and to quadratic order, locality and the (matter-space) gauge symmetry fix the leading field term in the action uniquely, so the resulting equations provide the minimal dynamical completion once charge carriers are included. We also clarify how duality controls the status of the Bianchi identity in the absence of magnetic charge carriers, and we briefly discuss helicity conservation and a natural nonlinear extension implied by the one-fluid constraints. In the second frame, on the other hand, the matter space 1-form is not directly related with the gauge potential.

physics.gen-ph↗

arXiv2024

Holographic Dark Energy with Torsion

We consider the holographic dark energy model with axial torsion which satisfy the cosmological principle. Subsequently, by using the torsional analogues of Friedmann equations for the new equation from Einstein-Cartan gravity theory, we obtain the equation of state for dark energy in this model. We find that the extended holographic dark energy from the particle horizon as the infrared (IR) cut-off does not give the accelerating expansion of the universe. Also, employing the future event horizon as IR cut-off still achieves the accelerating expansion of the universe. In contrast, there is a possibility that the Hubble radius as IR cut-off achieves to the accelerating expansion of the universe in superluminal region for axial torsion. More precisely, the current value of ratio for torsion to the matter density, $\gamma^{0}=0.5$ gives the equation of state of dark energy $\omega_{\Lambda}\cong-1$.

gr-qc↗