arXiv subjects

Evangelos Achilleas Paraskevas

Publications and source records attributed to Evangelos Achilleas Paraskevas.

6 recordsLinked to original sources

arXiv2026

Static Dark Fluid Thin Shells in Schwarzschild-de Sitter Spacetimes: Stability and Black Hole Shadows

We study the existence and radial stability of static, spherically symmetric thin shells joining two Schwarzschild-de Sitter (SdS) spacetimes $(m_\pm,Λ_\pm)$. Near the equilibrium radius $R_0$ the shell's surface density $σ$ and pressure $p$ obey the linearized barotropic law $p=p_0+c_s^2(σ-σ_0)$, with sound speed $c_s^2=λc^2$. Since $c_s^2$ is independent of the equilibrium ratio $w_0\equiv p_0/(σ_0 c^2)$, tension shells ($w_0<0$) stay radially stable with real $c_s$. Fixing $Λ_+$ so that its vacuum energy density equals the critical density (Planck~2018), and taking $m_-$ representative of astrophysical black holes, we systematically map the stable equilibria $(R_0,σ_0)$ over $(m_\pm,Λ_\pm,λ,w_0)$ and find that stable shells with $σ_0>0$ and $0<λ\leq 1$ exist only for $m_+/m_->1$, at three scales---the photon sphere, the SdS static radius, and the cosmological horizon. At $λ=1$ the numerical windows, checked against the analytic test-shell bounds, are $(1-\sqrt{13})/6\lesssim w_0\lesssim 1/2$ ($Λ_+=Λ_-$), $-2/3\lesssim w_0\lesssim 1/2$ ($Λ_+>Λ_-$), and $0\lesssim w_0$ ($Λ_+<Λ_-$). Positive-pressure shells ($0\lesssim w_0\lesssim 1/2$) may sit near the photon sphere for any ordering of $Λ_+/Λ_-$. Additionally, when $Λ_+<Λ_-$ with $w_0\gtrsim0$, they may also sit at the static-radius scale. Tension shells are absent for $Λ_+<Λ_-$; for $Λ_+=Λ_-$, they reach the cosmological horizon scale; for $Λ_+>Λ_-$, they are confined to the static-radius scale. Finally, we compute the dark fluid shell's imprint on the SdS black-hole shadow seen by a static observer at varying radial distance.

gr-qc↗

arXiv2026

The friction-era VOS amplitude of a $\mathbb{Z}_2$ string network from nematic disclination data

A tangle of line defects coarsens as the mean spacing $L$ between neighbouring lines grows. In a viscous medium the motion is overdamped, and the velocity-dependent one-scale (VOS) model predicts a late-time attractor $L^{2}=\mathcal{A}\,\ell_d\,t$, with $\ell_d=T/Γ$ the ratio of line tension to drag and $\mathcal{A}$ a dimensionless amplitude. The growth law $L\propto t^{1/2}$ holds for every value of the three model parameters---the momentum parameter $k\le1$, the sink coefficient $\tilde{c}$, and the curvature ratio $λ\equiv R/L$---since all three enter only through $\mathcal{A}=κ(κ+\tilde{c})$, $κ\equiv k/λ$. Thus, the exponent constrains none of them, and the amplitude is the only quantity a density history can deliver. We measure it from the disclination data of Chuang, Turok and Yurke on a nematic liquid crystal, whose companion measurement of loop collapse fixes $\ell_d$ on the same samples, canceling the 5CB material constants. Treating the unmatched per-quench $\ell_d$ as a nuisance parameter with a Gaussian prior and marginalizing it analytically, we obtain $\mathcal{A}=10.0^{+1.3}_{-1.1}$ from the three quenches in the $234~μ$m cell; the fourth, the only one in the thinner $158~μ$m cell, gives $\mathcal{A}=3.0^{+0.8}_{-0.6}$ and is fitted separately. Converting either into $\tilde{c}$ requires $λ$, which these data do not determine, so the result is a curve, $\tilde{c}(λ)=\mathcal{A}λ/k-k/λ$, not a number. At $λ=1$ with $k\le1$ they give $\tilde{c}\ge9.0$ and $\tilde{c}\ge2.0$, against $\tilde{c}=0.23$--$0.57$ from relativistic $U(1)$ simulations, values reached only at $λ\simeq0.33$--$0.35$ and $0.62$--$0.68$. We know of no VOS calibration for a cosmological $\mathbb{Z}_2$ network, so we cannot attribute the excess to topology, and we list what a repeat experiment must measure.

cond-mat.soft↗

arXiv2026

Addressing the DESI DR2 Phantom-Crossing Anomaly and Enhanced $H_0$ Tension with Reconstructed Scalar-Tensor Gravity

Recent cosmological data, including DESI DR2, highlight significant tensions within the $Λ$CDM paradigm. When analyzed in the context of General Relativity (GR), the latest DESI data favor a dynamical dark energy (DDE) equation of state, $w(z)$, that crosses the phantom divide line $w=-1$. However, this framework prefers a lower Hubble constant, $H_0$, than Planck 2018, thereby worsening the tension with local measurements. This phantom crossing is a key feature that cannot be achieved by minimally coupled scalar fields (quintessence) within GR. This suggests the need for a new degree of freedom that can simultaneously: (A) increase the best-fit value of $H_0$ in the context of the DESI DR2 data, and (B) allow the crossing of the $w=-1$ line within a new theoretical approach. We argue that both of these goals may be achieved in the context of Modified Gravity (MG), and in particular, Scalar-Tensor (ST) theories, where phantom crossing is a natural and viable feature. We demonstrate these facts by analyzing a joint dataset including DESI DR2, Pantheon+, CMB, and growth-rate (RSD) data in the context of simple parametrizations for the effective gravitational constant, $μ_G(z) \equiv G_{eff}/G_N$, and the DDE equation of state, $w(z)$. This MG framework significantly alleviates the tension, leading to a higher inferred value of $H_0 = 70.6 \pm 1.4 \, \text{km s}^{-1} \text{Mpc}^{-1}$. We also present a systematic, data-driven reconstruction of the required underlying ST Lagrangian and provide simple, generic analytical expressions for both the non-minimal coupling $F(Φ) = 1+ξΦ^{2}e^{nΦ}$ and the scalar potential $U(Φ) = U_{0}+ae^{bΦ^{2}}$, which well-describe the reconstructed functions.

astro-ph.CO↗

arXiv2025

Probing the Universe's Topology through a Quantum System?

The global topology of the Universe could, in principle, affect quantum systems through boundary condition constraints. We investigate this connection by analyzing how compact, flat, cosmologically inspired topologies, specifically the $3-$Torus ($E_1$) and half turn space ($E_2$), influence the energy eigenvalues of a quantum particle in the bound state of a 3D Dirac delta potential. Using rigorous renormalization techniques, we derive the equations satisfied by the energy eigenvalues in each topology and develop a systematic method to compute spectral shifts. Our results reveal that each topology induces characteristic deviations in the energy spectrum. In the large$-L$ limit ($L >> g_R$), to leading order, the energy eigenvalues for both the $E_1$ and $E_2$ spaces can be written in the unified form $E\simeq -\frac{\hbar^2}{2mg_R^2}(1 + C_Γ\,\frac{2g_R}{L}\,e^{-L/g_R})$, where the topology dependent coefficient is $C_Γ= 6$ for the $E_1$ space and $C_Γ= 4$ for the $E_2$ space, $g_R$ is the characteristic length scale of the quantum system, and $L$ is the side physical length of the fundamental cubic region. Using the three dimensional Dirac potential as a toy model, we show that at the current cosmic epoch ($a=1$), these topological effects are exponentially suppressed, rendering direct observation infeasible. However, such effects may become measurable in the early Universe, when the physical size of the particle horizon is comparable to the characteristic scale of the quantum system. While immediate experimental verification remains impractical, our work offers theoretical insight into how global cosmic topology might manifest in quantum bound states and may inform future studies of early Universe quantum phenomena.

quant-ph↗

arXiv2025

Gravitational Wave Signatures Induced by Dark Fluid Accretion in Binary Systems

We investigate the impact of dark fluid accretion on gravitational waveforms emitted by a compact binary system consisting of a supermassive black hole and a stellar-mass black hole. Using a Lagrangian framework with 1~PN and 2.5~PN corrections, we analyze the effects of the spherically symmetric accretion of a fluid with steady-state flow, including those characterized by an equation of state parameter resembling dark energy, on the binary's dynamics. We validate our approach by comparing it with previous studies in the common region of validity and extend the analysis to include both local effects, such as dynamical friction, and global gravitational interactions with the stellar-mass black hole, focusing on their dependence on the fluid's properties. Our analysis reveals that these interactions induce de-phasing in gravitational waveforms, with the phase shift influenced by the fluid's equation of state and energy density. We also extend the study to sudden cosmological singularities, finding that, although they can deform the binary's orbit from initially circular to elliptical, their effect on de-phasing is negligible for cosmologically relevant energy densities. By incorporating both the local and global gravitational interactions of a fluid on a two-body system into the equations of motion, this preliminary study provides a framework for understanding the interplay between fluid dynamics and gravitational wave emissions in astrophysical systems. It further reinforces the potential for probing the properties of astrophysically relevant fluids through gravitational wave observations.

gr-qc↗

arXiv2023

Effects of a Late Gravitational Transition on Gravitational Waves and Anticipated Constraints

We investigate the evolution of gravitational waves through discontinuous evolution (transition) of the Hubble expansion rate $H(z)$ at a sudden cosmological singularity, which may be due to a transition of the value of the gravitational constant. We find the evolution of the scale factor and the gravitational wave waveform through the singularity by imposing the proper boundary conditions. We also use existing cosmological data and mock data of future gravitational wave experiments (the ET) to impose current and anticipated constraints on the magnitude of such a transition. We show that mock data of the Einstein Telescope can reduce the uncertainties by up to a factor of three depending on the cosmological parameter considered.

astro-ph.CO↗