arXiv subjects

Oleksii Sokoliuk

Publications and source records attributed to Oleksii Sokoliuk.

18 recordsLinked to original sources

arXiv2026

The cosmic web's Lyman-$α$ glow at $z \approx 2.5$; hydrodynamic models, dust, and wide-field, narrow-band detection

The diffuse Lyman-$α$ glow of the cosmic web has long been predicted but has so far eluded direct detection over cosmologically significant volumes. We construct synthetic Lyman-$α$ surface-brightness maps using five state-of-the-art hydrodynamic simulations (\texttt{IllustrisTNG, EAGLE, CROCODILE, SIMBA, and Sherwood}), modeling recombination, collisional excitation, star formation, and localized dust attenuation. Our study focuses on the redshift range $2.0<z<2.7$, motivated by the numerous detailed studies of the COSMOS region. Significant variations are seen in the results obtained from these independent simulations. Using the Anderson-Darling statistic to probe these statistical differences, we demonstrate that a $5σ$ statistical detection of the total intergalactic and circumgalactic Lyman-$α$ emission is achievable with current facilities at flux thresholds brighter than $\sim 8 \times 10^{-17} \text{ erg s}^{-1}\text{ cm}^{-2}\text{ arcsec}^{-2}$. Conversely, isolating the underlying low-density component of the cosmic web requires ultra-deep sensitivity, with the most optimistic simulation (IllustrisTNG) reaching a 5$σ$ detection only for background noise levels below $σ\sim 2 \times 10^{-19} \text{ erg s}^{-1}\text{ cm}^{-2}\text{ arcsec}^{-2}$. These quantitative limits validate the feasibility of ongoing wide-field narrow-band campaigns, opening a new era of empirical intergalactic cartography.

astro-ph.CO↗

arXiv2025

Explaining JWST star formation history at $z \sim 17$ by modifying $Λ$CDM

Recent cosmological observations indicate a $5σ$ discrepancy between the values of the Hubble constant $H_0$ derived from late and early universe probes. A further possible tension at the $\sim 3σ$ level arises from different measurements of $σ_8$. These measurements suggest the existence of new physics. Here, we explore several theories of modified gravity that may help to resolve these cosmological tensions. These include a family of phenomenological modified theories, where only Newton's gravitational constant and the Einstein-Boltzmann equations are affected. We consider one particular class of these theories: cosmologies with varying growth index $γ$ and varying dark energy Equation of State (EoS) $w_Λ$. We also consider the normal branch of the Dvali-Gabadadze-Porrati (nDGP) model as well as $k$-mouflage gravity, which involves a non-trivially coupled scalar field. Our main aim is to narrow down the modified gravity landscape by constraining each model using high-redshift JWST data. Several probes are considered in this work: Stellar Mass Function (SMF), Stellar Mass Density (SMD), Star Formation Rate Density (SFRD) and Ultra-Violet Luminosity Function (UVLF) along with the Epoch of Reionization (EoR). We find that generally, the choice of $r_c\gtrsim 10^{3.5}$ Mpc is preferred for nDGP, while $β\sim0.1$, $K_0\gtrsim 0.9$ is favored for $k$-mouflage. Moreover, in the context of phenomenological gravity, phantom-like dark energy EoS $w_Λ\lesssim -1$ is preferred over the quintessence.

astro-ph.CO↗

arXiv2024

Holographic Boundary Conformal Field Theory within Horndeski Gravity

We investigate entanglement islands and the Page curve in the framework of Horndeski gravity on a Karch-Randall braneworld background. In particular, treating the holographic boundary conformal field theory analytically we find that the Horndeski parameters significantly alter the behavior of the Page curve compared to standard general relativity, a feature caused by the nontrivial geometry induced by the Horndeski scalar field. Interestingly enough, the geometry far from the AdS limit plays a more significant role compared to previous studies. This suggests that Horndeski gravity introduces important modifications to the distribution of quantum information in the holographic model. Finally, we claim that holographic consistency can be used reversely to impose constraints on Horndeski gravity itself, providing a new tool for probing the validity of modified gravity theories.

hep-th↗

arXiv2024

Early-Time Observations of SN 2023wrk: A Luminous Type Ia Supernova with Significant Unburned Carbon in the Outer Ejecta

We present extensive photometric and spectroscopic observations of the nearby Type Ia supernova (SN) 2023wrk at a distance of about 40 Mpc. The earliest detection of this SN can be traced back to a few hours after the explosion. Within the first few days the light curve shows a bump feature, while the B - V color is blue and remains nearly constant. The overall spectral evolution is similar to that of an SN 1991T/SN 1999aa-like SN Ia, while the C II $\lambda6580$ absorption line appears to be unusually strong in the first spectrum taken at $t \approx -$15.4 days after the maximum light. This carbon feature disappears quickly in subsequent evolution but it reappears at around the time of peak brightness. The complex evolution of the carbon line and the possible detection of Ni III absorption around 4700 Å and 5300 Å in the earliest spectra indicate macroscopic mixing of fuel and ash. The strong carbon lines is likely related to collision of SN ejecta with unbound carbon, consistent with the predictions of pulsational delayed-detonation or carbon-rich circumstellar-matter interaction models. Among those carbon-rich SNe Ia with strong C II $\lambda6580$ absorption at very early times, the line-strength ratio of C II to Si II and the B-V color evolution are found to exhibit large diversity, which may be attributed to different properties of unbound carbon and outward-mixing $^{56}$Ni.

astro-ph.SR↗

arXiv2023

AdS black hole thermodynamics and microstructures from $f(Q)$ gravitation

The significant properties and phase transition of charged Anti-de Sitter (AdS) black holes have been extensively studied in a variety of modified theories of gravity in the presence of numerous matter fields. The goal of our current research is to investigate the AdS black hole's thermodynamics under the impact of $f(Q)$ gravity. Additionally, this paper explores the black hole's local stability and phase structure under the relevant gravity. Besides, we use Ruppeiner geometry to look into the AdS black hole's microscopic structure. We have numerically computed the Ricci curvature scalar $R$ to explain the interactions between the AdS black hole's microscopic particles under the influence of $f(Q)$ gravity.

gr-qc↗

arXiv2023

AdS/BCFT correspondence and Horndeski gravity in the presence of gauge fields: holographic paramagnetism/ferromagnetism phase transition

This paper presents a dual gravity model for a (2+1)-dimensional system with a limit on finite charge density and temperature, which will be used to study the properties of the holographic phase transition to paramagnetism-ferromagnetism in the presence of Horndeski gravity terms. In our model, the non-zero charge density is supported by a magnetic field. As a result, the radius $ρ/B$ indicates a localized condensate, as we increase the Horndeski gravity parameter, that is represented by $γ$. Furthermore, such condensate shows quantum Hall-type behavior. This radius is also inversely related to the total action coefficients of our model. It was observed that increasing the Horndeski parameter decreases the critical temperature of the holographic model and leads to the harder formation of the magnetic moment at the bottom of the black hole. However, when removing the magnetic field, the ferromagnetic material presents a disorder of its magnetic moments, which is observed through the entropy of the system. We also found that at low temperatures, spontaneous magnetization and ferromagnetic phase transition.

hep-th↗

arXiv2023

$H_0$ Tension in Torsion-based Modified Gravity

The rising concern in the Hubble constant tension ($H_0$ tension) of the cosmological models motivates the scientific community to search for alternative cosmological scenarios that could resolve the $H_0$ tension. In this regard, we aim to work on a torsion-based modified theory of gravity which is an alternative description to the coherence model. We solve numerically for the Hubble parameter using two exponential Lagrangian functions of torsion $T$ and a trace of energy-momentum tensor $\mathcal{T}$ for the dust case. Further, we constrain the cosmological and model parameters; to do that, we use Hubble, SNe Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background samples, and Markov Chain Monte Carlo (MCMC) simulation through Bayesian statistics. We obtain the values of Hubble constant $H_0$ for our model, and the outputs align with the recent observational measurements of $H_0$. In addition, we check the deviation of our results from model-independent measurements of $H_0$ from Planck2018, S$H_0$ES, and $H_0$LiCOW experiments. In contrast, our finding partially solved the $H_0$ tension but gave a new possible direction to alleviate the $H_0$ tension.

astro-ph.CO↗

arXiv2023

On the impact of $f(Q)$ gravity on the Large Scale Structure

We investigate the exponential $f(Q)$ symmetric teleparallel gravitation, namely $f(Q)=Q+αQ_0(1-e^{-β\sqrt{Q/Q_0}})$ using \texttt{ME-GADGET} code to probe the structure formation with box sizes $L_{\mathrm{box}}=10/100$ Mpc$/h$ and middle resolution $N_p^{1/3}=512$. To reproduce viable cosmology within the aforementioned modified gravity theory, we first perform Markov Chain Monte Carlo (MCMC) sampling on OHD/BAO/Pantheon datasets and constrain a parameter space. Furthermore, we also derive theoretical values for deceleration parameter $q(z)$, statefinder pair $\{r,s\}$ and effective gravitational constant $G_{\mathrm{eff}}$, perform $Om(z)$ diagnostics. While carrying out N-body+SPH simulations, we derive CDM+baryons over density/temperature/mean molecular weight fields, matter power spectrum (both 2/3D, with/without redshift space distortions), bispectrum, two-point correlation function and halo mass function. Results for small and big simulation box sizes are therefore properly compared, halo mass function is related to the Seth-Tormen theoretical prediction and matter power spectrum to the standard \texttt{CAMB} output.

astro-ph.CO↗

arXiv2022

Holographic complexity of braneworld in Horndeski gravity

This work investigates the influence of a probe string on the complexity of braneworld according to the CA (Complexity equals action) conjecture within the Horndeski gravity. In the current study, it is considered that scalar fields that source Horndeski gravity has a spatial dependence. In addition, our system contains a particle moving on the boundary, which corresponds to the insertion of a fundamental string in the higher dimensional bulk. Such an effect is given by the Nambu-Goto term, which also incorporates the time-dependence and evolution in our system. Both warp factor, scalar field, and superpotential values are derived numerically assuming appropriate initial conditions, and the growth rate of holographic complexity is analyzed within the so-called Wheeler-De Witt (WDW) patch with null-like hypersurfaces present.

hep-th↗

arXiv2022

Compact stars admitting Finch-Skea symmetry in the presence of various matter fields

In the present article authors investigate the anisotropic stellar solutions admitting Finch-Skea symmetry (viable and non-singular metric potentials) in the presence of some exotic matter fields, such as: Bose-Einstein Condensate (BEC) dark matter, Kalb-Ramond fully anisotropic rank-2 tensor field from the low-energy string theory effective action, gauge field imposing $U(1)$ symmetry. Interior spacetime is matched with both Schwarzchild and Reissner-Nördstrom vacuum spacetimes for BEC, KB and gauge fields respectively. Further, we have studied energy conditions, Equation of State (EoS), radial derivatives of energy density and anisotropic pressures, Tolman-Oppenheimer-Volkoff equilibrium condition, relativistic adiabatic index, sound speed and surface redshift. Most of the aforementioned conditions were satisfied and therefore solutions derived in the current study lie in the physically acceptable regime.

gr-qc↗

arXiv2022

Buchdahl quark stars within $f(Q)$ theory

In the present paper, authors study the strange stars with the MIT Bag EoS admitting Buchdahl symmetry (non-singular and physically viable metric potential) in the both linear $f(Q)=aQ+b$ and non-linear $f(Q)=Q+aQ^b$ forms of modified symmetric teleparallel gravitation. In order to obtain the correct form of Buchdahl metric coefficients, they matched interior strange star spacetime with the exterior Schwarzschild vacuum spacetime. As a strange star candidate, PSRJ1416-2230 has been used with mass $M=1.69M_\odot$ and radius $R=9.69R_\odot$. For either linear and non-linear $f(Q)$ gravities, they probed Null, Dominant and Strong Energy Conditions as well as the radial, tangential Equation of State (EoS), gradients of the perfect fluid energy-momentum tensor elements, Tolman-Oppenheimer-Volkoff equilibrium condition, relativistic adiabatic index and causality conditions, surface redshift. It was found that strange stars in the linear and non-linear $f(Q)$ gravity show physically viable behavior, respect energy and causality conditions, has EoS in the bounds $0\leqω\leq1$ and surface redshift does not exceed 2, as expected.

gr-qc↗

arXiv2022

Accretion flows around exotic tidal wormholes I. Ray-tracing

This paper investigates the various spherically symmetric wormhole solutions in the presence of tidal forces and applies numerous methods, such as test particle orbital dynamics, ray-tracing and microlensing. We make the theoretical predictions on the test particle orbital motion around the tidal wormholes with the use of normalized by $\mathcal{L}^2$ effective potential. In order to obtain the ray-tracing images (of both geometrically thin and thick accretion disks, relativistic jets), we properly modify the open source $\texttt{GYOTO}$ code with python interface. We applied this techniques to probe the accretion flows nearby the Schwarzschild-like and charged Reissner-Nördstrom (RS) wormholes (we assumed both charged RS wormhole and special case with the vanishing electromagnetic charge, namely Damour-Solodukhin (DS) wormhole). It was shown that the photon sphere for Schwarzschild-like wormhole presents for both thin and thick accretion disks and even for the vanishing tidal forces. Moreover, it was observed that $r_{\mathrm{ph}}\to\infty$ as $α\to\infty$, which constraints $α$ parameter to be sufficiently small and positive in order to respect the EHT observations. On the other hand, for the case of RS wormhole, photon sphere radius shrinks as $Λ\to\infty$, as it was predicted by the effective potential. In addition to the accretion disks, we as well probe the relativistic jets around two wormhole solutions of our consideration. Finally, with the help of star bulb microlensing, we approximate the radius of the wormhole shadow and as we found out, for Schild WH, $R_{\mathrm{Sh}}\approx r_0$ for ZTF and grows linearly with $α$. On the contrary, shadow radius for charged wormholes slowly decreases with the growing trend of DS parameter $Λ$.

gr-qc↗

arXiv2022

Traversable wormholes with charge and non-commutative geometry in the $f(Q)$ gravity

We consider modified symmetric teleparallel gravity (STG), in which gravitational Lagrangian is given by the arbitrary function of non-metricity scalar $Q$ to study static and spherically symmetric charged traversable wormhole solutions with non-commutative background geometry. The matter source at the wormhole throat is acknowledged to be anisotropic, and the redshift function has a constant value (thus, our wormhole solution is non-tidal). We study the obtained field equations with the two functional forms of $f(Q)$ STG models, such as linear $f(Q)=αQ+β$ and non-linear $f(Q)=Q+mQ^n$ models under Gaussian and Lorentzian distributions. Our analysis found the exact wormhole solutions for the linear STG model only. Also, for the non-linear model, we derived numerically suitable forms of wormhole shape functions directly from the modified Einstein Field Equations (EFEs). Besides, we probed these models via Null, Dominant, and Strong energy conditions with respect to free Modified gravity (MOG) parameters $α$, $β$, $m$, and $n$. We also used Tolman-Oppenheimer-Vokloff (TOV) equation to investigate the stability of wormhole anisotropic matter in considered MOG. Finally, we plot the equation of state.

gr-qc↗

arXiv2022

AdS/BCFT correspondence and Lovelock theory in the presence of canonical scalar field

In the current study, authors present the comprehensive analysis of the Anti-de Sitter/Boundary Conformal Field Theory (AdS/BCFT) correspondence for Lovelock theory of second order in the presence of canonical (and real) scalar field non-minimally coupled to modified gravitation sector. Authors consider the special case with three dimensional BTZ (Banados-Teitelboim-Zanelli) black hole and dual 2D Conformal Field Theory with the additional boundary for higher dimensional AdS spacetime. Moreover, for the given case additional boundary profile was investigated, as well as the holographic renormalization procedure for Lovelock-scalar theory. Besides, from holographic renormalization black hole entropy was obtained numerically. Finally, from the entropy and black hole temperature dual to BCFT some thermodynamical quantities, such as heat capacity, sound speed, energy-momentum trace and Hawking-Page phase transition were probed.

hep-th↗

arXiv2022

Probing the existence of the ZTF Casimir wormholes in the framework of $f(\mathcal{R})$ gravity

For the spherically symmetric static traversable wormholes, supported by the Casimir energy in $f(\mathcal{R})=\mathcal{R}+α\mathcal{R}^2$ Quadratic, $f(\mathcal{R})=f_0 \mathcal{R}^n$ power-law Modified Gravity (MOG) theories we investigate energy conditions and dynamical stability of the wormhole solutions. Especially, we study Zero Tidal Forces (ZTF) Casimir WH's with anisotropic fluid located at the throat. By using the Casimir energy density and modified Einstein Field Equations (EFE's) we derived suitable shape functions for each modified gravity of our consideration. The stability of Casimir traversable wormholes in different modified gravity theories is also analyzed in our paper with a modified Tolman-Oppenheimer-Voklov (MTOV) equation. Besides, we have numerically solved MTOV and derived hydrodynamical, anisotropic and extra forces, that is present due to the non-conserved stress-energy tensor. Moreover, other fundamental quantities, such as Volume Integral Quantifier and total gravitational energy were derived.

physics.gen-ph↗

arXiv2022

Generalised Ellis-Bronnikov Wormholes in $f(R)$ Gravity

In this manuscript, we construct generalized Ellis-Bronnikov wormholes in the context of $f(R)$ modified theories of gravity. We consider that the matter driving the wormhole satisfies the energy conditions so that it is the effective energy-momentum tensor containing the higher-order derivatives of curvature terms that violate the null energy condition. Thus, the gravitational fluid is interpreted by the higher-order derivatives of curvature terms to represent the wormhole geometries and is fundamentally different from its counter representation in general relativity. In particular, we explore the wormhole geometries by presuming various well-known forms of Lagrangian $f(R)$. In addition, for the seek of completeness, we discuss modified Tolman-Oppenheimer-Volkov, volume integral quantifier, and total gravitational energy.

gr-qc↗

arXiv2022

Kuchowicz gravastars in the braneworld formalism

In the current letter, we study isotropic static spherically symmetric gravastars without charge under the framework of braneworld gravity (dimensionally reduced RS-II braneworld with positive brane tension) using the metric potential of Kuchowicz type (which is physically acceptable, non-singular and stable) in the Mazur-Motolla conjuncture. We derived the Kuchowicz free parameters from the junction conditions assuming Schwarzschild vacuum without cosmological constant. As well, we assume that the interior is filled with Dark Energy (DE-like) fluid, shell with ultrarelativistic stiff fluid. Adopting this conditions, we probed several physical aspects of the gravastars, such as proper length, shell energy and entropy, surface redshift and adiabatic index, interior region mass.

gr-qc↗

arXiv2022

Non-singular T-K axion stars with/without the dynamical bosonic field in the presence of negative $Λ$ term

In the present work authors derive exact solutions for the relativistic compact stars in the presence of two fields axion (Dante's Inferno model) and with/without the complex scalar field (with the quartic self-interaction) coupled to gravity. The matter source is assumed as the perfect fluid one, and we also use barotropic/MIT bag EoS to derive energy density and anisotropic/isotropic pressures from Einstein Field Equations (EFE's). For simplicity, as the metric potentials we use Tolman-Kuchowicz metric potentials, which are non-singular and physically acceptable. Unknown variables for the T-K metric potentials were derived from the junction conditions. To examine the redshift function, adiabatic index and energy, causality conditions we used such compact stars as PSR J1416-223, PSR J1903+32, 4U 1820-30, Cen X-3, EXO 1785-248, SAX J1808.4365.

gr-qc↗