arXiv subjects

A. Emrah Yükselci

Publications and source records attributed to A. Emrah Yükselci.

16 recordsLinked to original sources

arXiv2026

A Friendly Phantom: Late-time AdS-to-dS transition and cosmological tensions

We present Ph-$Λ_{\rm s}$CDM, a phantom-scalar realization within General Relativity of the sign-switching cosmological-constant idea, $Λ_{\rm s}$CDM, in which a phantom scalar evolving on a bounded hyperbolic-tangent potential induces a smooth mirror AdS-to-dS transition in the late-time dark-energy density. The wrong-sign kinetic term, usually viewed as pathological, becomes the mechanism lifting the field from a negative- to a positive-energy vacuum-like regime. The construction also shows that the field can become repulsive while its energy density is still negative. The cosmology nevertheless remains controlled: total energy stays positive, the late-time attractor is de Sitter rather than a Big Rip, and the dynamics remain safely infrared. Ph-$Λ_{\rm s}$CDM thus offers a concrete late-time mechanism with the potential to address multiple cosmological tensions.

gr-qc↗

arXiv2026

Nonlinear Matter Power Spectrum from relativistic $N$-body Simulations: $Λ_{\rm s}$CDM versus $Λ$CDM

We present relativistic $N$-body simulations of a $Λ_{\rm s}$CDM - sign-switching cosmological constant (CC) - scenario under general relativity and compare its nonlinear matter power spectrum to $Λ$CDM at ${z = 15,\,2,\,1,\,0}$, using best-fit parameters from Planck-only and a combined ''full'' dataset. During the AdS-like CC ($Λ_{\rm s}<0$) phase, prior to the transition redshift $z_\dagger$, reduced Hubble friction dynamically enhances the growth of perturbations; after the switch, with dS-like CC ($Λ_{\rm s}>0$), the larger late-time expansion rate partly suppresses, but does not erase, the earlier amplification. Consequently, the ratio $P_{Λ_{\rm s}\rm CDM}/P_{Λ\rm CDM}$ exhibits a pronounced, redshift-dependent shape feature: a crest peaking at ${\sim 20-25\%}$ around ${k \simeq 1-3\,h\,\mathrm{Mpc}^{-1}}$ near the transition, which then migrates to larger physical scales and persists to ${z = 0}$ as a robust ${\sim 15-20\%}$ uplift at ${k \simeq 0.6-1.0\,h\,\mathrm{Mpc}^{-1}}$. These wavenumbers correspond to group or poor-cluster environments and lie within the sensitivity range of weak lensing, galaxy-galaxy lensing, cluster counts, and tSZ power, providing a concrete, falsifiable target that cannot be mimicked by a scale-independent change in $σ_8$ or $S_8$. The timing (earlier for Planck-only, later for the full dataset) and the amplitude of the crest align with the ''cosmic noon'' epoch (${z \simeq 1-2}$), offering a gravitational prior for the observed peak in the cosmic star-formation rate.

astro-ph.CO↗

arXiv2025

Dynamical dark energy with AdS-dS transitions vs. Baryon Acoustic Oscillations at $z =$ 2.3-2.4

In this paper, written in memory of Alexei Starobinsky, we discuss the observational viability of the Ph-$Λ_{\rm s}$CDM model - a dynamical dark energy scenario based on a phantom scalar field undergoing an anti-de Sitter (AdS) to de Sitter (dS) transition - and revisit the Sahni-Shtanov braneworld model in light of updated BAO Ly-$α$ data at $z \sim 2.3$. Both models are able to remain consistent with Planck CMB data while offering potential resolutions to the $H_0$ tension. In both cases, the expansion rate $H(z)$ is suppressed relative to Planck-$Λ$CDM at high redshift and enhanced at low redshift, while remaining consistent with the comoving distance to recombination as estimated by Planck-$Λ$CDM. Comparing model predictions with BAO-inferred values of $H(z)$, we find that SDSS Ly-$α$ data at $z \approx 2.33$ mildly favor such dynamical models, whereas the recent DESI Ly-$α$ measurements agree more closely with $Λ$CDM. Although current high-redshift BAO data do not decisively favor one model over another, our findings illustrate how frameworks originally developed to address earlier anomalies - such as the braneworld scenario - may gain renewed relevance in confronting today's cosmological tensions.

astro-ph.CO↗

arXiv2025

Dynamical dark energy with AdS-to-dS and dS-to-dS transitions: Implications for the $H_0$ tension

We investigate the dynamics and cosmological implications of dark energy (DE), modeled as a scalar field with a hyperbolic tangent potential that induces a smooth shift in the effective cosmological constant (CC), encompassing transitions such as AdS-dS, 0-dS, and dS-dS, with the mirror AdS-dS as a particular case aligned with the $Λ_{\rm s}$CDM scenario. In our construction, a phantom scalar field with a negative kinetic term drives a bottom-up transition from an AdS-like vacuum at high redshifts to a dS-like vacuum at low redshifts, thereby providing a physical underpinning for the $Λ_{\rm s}$CDM scenario. Despite the negative kinetic term, the step-like form of the potential prevents pathologies such as unbounded energy growth, Big Rip, and violations of the WEC. Our numerical integration of the equations of motion shows that the model is consistent with both CMB data and the SH0ES determination of $H_0$, thereby addressing the $H_0$ tension, with all key kinematical parameters-$H(z)$, $\dot{H}(z)$, and $q(z)$-evolving smoothly. The total energy density of the phantom and matter system remains positive at all times, and the effective EoS stays above -1, ensuring that the WEC is satisfied. While the phantom field's energy density and pressure remain finite throughout, its EoS exhibits a safe singularity as its energy density smoothly crosses zero. We perform analysis of the transition period, demonstrating that the evolution of the DE density from a negative CC-like regime to a positive one does not exactly mirror the behavior of the potential-e.g., it lasts longer-as it also involves the kinetic term. We also show that analogous quintessence models featuring dS-dS transitions predict an $H_0$ value lower than $Λ$CDM, thereby failing to address the $H_0$ tension. Our results establish a robust theoretical foundation for the $Λ_{\rm s}$CDM scenario.

astro-ph.CO↗

arXiv2024

Suppression of matter density growth at scales exceeding the cosmic screening length

One of the main objectives of modern cosmology is to explain the origin and evolution of cosmic structures at different scales. The principal force responsible for the formation of such structures is gravity. In a general relativistic framework, we have shown that matter density contrasts do not grow over time at scales exceeding the cosmic screening length, which corresponds to a cosmological scale of the order of two to three gigaparsecs at the present time, at which gravitational interactions exhibit an exponential cut-off. This is a purely relativistic effect. To demonstrate the suppression of density growth, we have performed N-body simulations in a box with a comoving size of $5.632\,{\rm Gpc}/h$ and obtained the power spectrum of the mass density contrast. We have shown that it becomes independent of time for scales beyond the cosmic screening length as a clear manifestation of the cosmic screening effect.

gr-qc↗

arXiv2024

Mass density vs. energy density at cosmological scales

In the presence of the gravitational field, the energy density of matter no longer coincides with its mass density. A discrepancy exists, of course, also between the associated power spectra. Within the $Λ$CDM model, we derive a formula that relates the power spectrum of the energy density to that of the mass density and test it with the help of N-body simulations run in comoving boxes of 2.816 Gpc/$h$. The results confirm the validity of the derived formula and simultaneously show that the power spectra diverge significantly from one another at large cosmological scales.

gr-qc↗

arXiv2024

A quantitative analysis of the effect of box size in N-body simulations of the matter power spectrum

We study the effect of box size on the matter power spectrum obtained via cosmological N-body simulations. Within the framework of the cosmic screening approach, we show that the relative deviation between the spectra for our largest comoving box with L = 5632 Mpc/h and those for L = 280, 560, 1680, 4480, 5120 Mpc/h boxes consistently increases with decreasing box size in the latter set in the redshift range $0\leq z\leq 80$ for the considered values. As an additional demonstrative example, at redshift zero, we determine the values $k_{1\%}$ corresponding to the modes at which relative deviations reach 1\%.

gr-qc↗

arXiv2023

Cosmological First-Order Vacuum Phase Transitions in an Expanding Anisotropic Universe

We examine the anisotropy originated from a first-order vacuum phase transitions through three-dimensional numerical simulations. We apply Bianchi Type-I metric to our model that has one scalar field minimally coupled to the gravity. We calculate the time evolution of the energy density for the shear scalar and the directional Hubble parameters as well as the power spectra for the scalar field and the gravitational radiation although there are a number of caveats for the tensor perturbations in Bianchi Type-I universe. We run simulations with different mass scales of the scalar field, therefore, in addition to investigation of anisotropy via the shear scalar, we also determine at which mass scale the phase transition completes successfully, hence, neglecting the expansion of the Universe does not significantly affect the results. Finally, we showed that such an event may contribute to the total anisotropy depending on the mass scale of the scalar field and the initial population of nucleated bubbles.

gr-qc↗

arXiv2023

Numerical Approach to the Exterior Solution of Spherically Symmetric and Static Configuration in Scalar-Tensor Theories

We numerically examine the exterior solution of spherically symmetric and static configuration in scalar-tensor theories by using the nonminimally coupled scalar field with zero potential as our sample model. Our main purpose in this work is to fit the resulting data of the numerical solutions in the interested region by seeking for approximate analytical expressions which are weakly dependent of the parameters of a model, such as the nonminimal coupling constant in the present case. To this end, we determine the main forms of the mass and the metric functions in terms of the scalar field and their surface values. Then, we provide a function for the scalar field that contains only the mass and the radius of the configuration together with the surface and the asymptotic values of the scalar field. Therefore, we show that the exterior solution can be expressed in a form which does not depend on the parameters of a chosen model up to an order of accuracy around $10^{-5}$.

gr-qc↗

arXiv2023

The Effect of Non-minimally Coupled Scalar Field on Gravitational Waves from First-order Vacuum Phase Transitions

We investigate first-order vacuum phase transitions in the presence of a non-minimally coupled scalar field starting with the coupling effect on the initial dynamics of phase transitions by defining an effective potential for the scalar field and then performing three dimensional numerical simulations to observe any possible distinction in gravitational wave power spectrum. Although we give a description of the model with the expanding background, in this particular paper, we exclude the scale factor contribution since we primarily focus on the immediate impact of the non-minimal coupling at initial phases of the transition. Even in this case we found that this modification has discernible effect on the power spectrum of the gravitational wave energy density.

gr-qc↗

arXiv2020

Effect of peculiar velocities on the gravitational potential in cosmological models with perfect fluids

We consider a universe filled with perfect fluid with the constant equation of state parameter $ω$. In the theory of scalar perturbations, we study the effect of peculiar velocities on the gravitational potential. For radiation with $ω=1/3$, we obtain the expression for the gravitational potential in the integral form. Numerical calculation clearly demonstrates the modulation of the gravitational potential by acoustic oscillations due to the presence of peculiar velocities. We also show that peculiar velocities affect the gravitational potential in the case of the frustrated network of cosmic strings with $ω=-1/3$.

gr-qc↗

arXiv2020

Formalizing Slow-roll Inflation in Scalar-Tensor Theories of Gravitation

The viability of slow-roll approximation is examined by considering the structure of phase spaces in scalar-tensor theories of gravitation and the analysis is exemplified with a nonminimally coupled scalar field to the spacetime curvature. The slow-roll field equations are obtained in the Jordan frame in two ways: first using the direct generalization of the slow-roll conditions in the minimal coupling case to nonminimal one, and second, conformal transforming the slow-roll field equations in the Einstein frame to the Jordan frame and then applying the generalized slow-roll conditions. Two inflationary models governed by the potentials $V(ϕ) \propto ϕ^2$ and $V(ϕ) \propto ϕ^4$ are considered to compare the outcomes of two methods based on the analysis of $n_s$ and $r$ values in the light of recent observational data.

gr-qc↗

arXiv2020

Dynamical System Analysis of Quintessence Models with Exponential Potential -- Revisited

Dynamical system analysis of a universe model which contains matter, radiation, and quintessence with exponential potential, $V \!(ϕ)=V_{\!o} \, exp(-ακϕ) \,$, is studied in the light of recent observations and the tensions between different datasets. The three-dimensional phase space is constructed by the energy density parameters and all the critical points of the model with their physical meanings are investigated. This approach provides an easy way of comparing the model directly with the observations. We consider a solution that is compatible with observations and is continuous in the phase space in both directions of time, past and future. Although in many studies of late-time acceleration the radiation is neglected, here we consider all components together and this makes the calculated effective equation of state parameter more realistic. Additionally, a relation between potential parameter, $α$, and the value of quintessence equation of state parameter, $ω_ϕ(t_o)$, today is found by using numerical analysis. We conclude that $α$ has to be small in order to explain the current accelerated phase of the universe and this result can be seen directly from the relation we obtain. Finally, we compare the usual dynamical system approach with the approach that we follow in this paper.

gr-qc↗

arXiv2020

Dynamical System Analysis of a Five-Dimensional Cosmological Model

A five-dimensional cosmological model including a single perfect fluid is studied in the framework of dynamical system analysis. All the critical points of the system with their stability properties are listed and some representative phase diagrams are explicitly shown. It is found that the stabilization of the extra dimension is possible and the observed flatness of the three-dimensional space is provided for certain ranges of the equation of state parameter of the fluid during the evolution of the universe. The model suggested here can be considered as a simplified model for examining the possible effects of the extra dimensions in the early universe.

gr-qc↗

arXiv2020

A Neutron Star with a Strange Quark Star-like Mass-Radius Relation

We study the structure of spherically symmetric and static objects in the presence of a nonminimally coupled scalar field having a potential of the form $V(ϕ)=-μ^2ϕ^2/2+λϕ^4/4$. We numerically solve equations of the system using two different realistic equations of state for neutron matter and give the mass-radius relations as well as the radial profiles of the scalar field, densities and the mass function for a sample configuration. We show that a specific solution type encountered in such systems causes characteristic neutron star mass-radius relations to turn into the ones belonging to strange quark stars.

gr-qc↗

arXiv2019

Neutron Star Structure in the Presence of Nonminimally Coupled Scalar Fields

We study the structure of neutron stars in scalar-tensor theories for the nonminimal coupling of the form $(1+κξϕ^{2})\cal R$. We solve the hydrostatic equilibrium equations for two different types of scalar field potentials and three different equations of state representative of different degrees of stiffness. We obtain the mass-radius relations of the configurations and determine the allowed ranges for the term $ξϕ^2$ at the center of the star and spatial infinity based on the measured maximum value of the mass for neutron stars and the recent constraints on the radius coming from gravitational wave observations. Thus we manage to limit the deviation of the model from general relativity. We examine the possible constraints on the parameters of the model and compare the obtained restrictions with the ones inferred from other cosmological probes that give the allowed ranges for the coupling constant only. In the case of the Higgs-like potential, we also find that the central value for the scalar field cannot be chosen arbitrarily but it depends on the vacuum expectation value of the field. Finally, we discuss the effect of the scalar field potential on the mass and the radius of the star by comparing the results obtained for the cases considered here.

gr-qc↗