arXiv subjects

Olga Avsajanishvili

Publications and source records attributed to Olga Avsajanishvili.

8 recordsLinked to original sources

arXiv2026

Background dynamics and observational constraints of flat and non-flat $Λ(t)$CDM models from $H(z)$ and DESI DR2 BAO measurements

In this work, we present comprehensive observational constraints on the time-varying vacuum Lambda(t)CDM cosmology using the latest baryon acoustic oscillation (BAO) data from Dark Energy Spectroscopic Instrument (DESI) Data Release 2 measurements in combination with cosmic chronometer H(z) data. We explicitly quantify the impact of vacuum dynamics on the expansion history, the total effective equation of state parameter of the unified cosmic fluid, the effective dark energy equation of state parameter, and the deceleration parameter in the spatially flat Lambda(t)CDM model. We perform a full Markov Chain Monte Carlo (MCMC) analysis and statistical model comparison, providing a consistent assessment of the Lambda(t)CDM model relative to the standard LambdaCDM framework. Our results demonstrate that H(z) and BAO observations strongly constrain deviations from the LambdaCDM model, driving the vacuum dynamics parameter alpha toward zero, while significantly reducing parameter degeneracies and alleviating the Hubble tension.

astro-ph.CO↗

arXiv2026

The cosmological Mass Varying Neutrino model in the late universe

The cosmological Mass Varying Neutrino (MaVaN) model is considered, where the interaction between a fermionic field and a scalar field with a Ratra-Peebles potential via a Yukawa coupling is investigated. Observational constraints on the flat and non-flat MaVaN models, as well as on the standard $Λ$CDM model, are derived from 32 $H(z)$ measurements using MCMC analysis. Comparison with the $Λ$CDM model using the criteria $Δχ^2_{\rm min}$, $AICc$, and $BIC$ shows no statistically significant improvement for MaVaN models. Deviations in the expansion history remain well below $1σ$, indicating that the $H(z)$ data alone do not provide sufficient constraining power to distinguish MaVaN models from the $Λ$CDM model. The non-flat MaVaN model reduces the tension between the $H_0$ value inferred from $H(z)$ data and the Planck CMB measurement from $\sim 2σ$ in the $Λ$CDM framework to $\sim 1.1σ$. The discrepancy with the SH0ES measurement of $H_0$ is also reduced to below $1σ$, primarily due to the large uncertainties of the $H(z)$ data.

astro-ph.CO↗

arXiv2024

Observational Constraints on Dynamical Dark Energy Models

$ϕ$CDM models provide an alternative to the standard $Λ$CDM paradigm, while being physically better motivated. These models lead to a time-dependent speed of sound for dark energy that is difficult to replicate by $w$CDM parametrizations. We review the most up-to-date status of observational evidence for the $ϕ$CDM models in this paper. We start with an overview of the motivation behind these classes of models, the basic mathematical formalism, and the different classes of models. We then present a compilation of recent results of applying different observational probes to constraining $ϕ$CDM model parameters. Over the last twenty years, the precision of observational data has increased immensely, leading to ever tighter constraints. A combination of the recent measurements favors the spatially flat $Λ$CDM model, but a large class of $ϕ$CDM models is still not ruled out.

astro-ph.CO↗

arXiv2021

Mass varying neutrinos with different quintessence potentials

The mass-varying neutrino scenario is analyzed for three trial quintessence potentials (Ferreira-Joyce, inverse exponential, and thawing oscillating). The neutrino mass is generated via Yukawa coupling to the scalar field which represents dark energy. The inverse exponential and oscillating potentials are shown to successfully generate the neutrino masses in the range $m \sim 10^{-2}-10^{-3}~$eV and to yield the current dark energy density in the regime of the late-time acceleration of the Universe. Depending on the choice of potentials, the acceleration could occur in two different regimes: (1) the regime of instability, and (2) the stable regime. The first regime of instability is after the Universe underwent a first-order transition and is rolling toward the new stable vacuum. The imaginary sound velocity $c^2_s < 0$ in this regime implies growing fluctuations of the neutrino density (clustering). In the second regime, the Universe smoothly changes its stable states via a continuous transition. Since $c^2_s > 0$, the neutrino density is stable. For all cases the predicted late-time acceleration of the Universe is asymptotically very close to that of the $Λ$CDM model. Further extensions of the theory to modify the neutrino sector of the Standard Model and to incorporate inflation are also discussed. It is also shown that in the stable regimes where the neutrino mass is given by the minimum of the thermodynamic potential, the tree-level dynamics of the scalar field is robust with respect to one-loop bosonic and fermionic corrections to the potential.

hep-ph↗

arXiv2019

Cosmological models of dark energy: theory and observations

We investigated the evolution of the background expansion and the growth rate of the matter density fluctuations in the scalar field $ϕ$CDM Ratra-Peebles model. We constrained the model parameter $α$ and the matter density parameter $Ω_{\rm m}$ using the recent measurements of the growth rate of the matter density fluctuations and baryon acoustic oscillation peak position. In addition, we studied a number of the $ϕ$CDM scalar field models in order to determine whether or not it would be possible to discriminate them from the $Λ$CDM model using predicted data for the future Dark Energy Spectroscopic Instrument (DESI) observations. For this purpose, we carried out the statistical Bayesian analysis, and computed Bayes coefficients, as well as Akaike and Bayesian information criteria. We found that projected DESI results could provide compelling when comparing most $ϕ$CDM models with the $Λ$CDM model. We also conducted the MCMC analysis and obtained the constraints on the parameters of the scalar field models, comparing the observational data for: the universe expansion rate, the angular diameter distance and the growth rate function, with the corresponding data generated for the $Λ$CDM model. We investigated how well the Chevallier-Polarsky-Linder (CPL) parametrization approximates the various scalar field models. We determined the location of scalar field model in the phase space of the CPL parameters.

astro-ph.CO↗

arXiv2018

The observational constraints on the flat $ϕ$CDM models

Most dark energy models have the $Λ$CDM as their limit, and if future observations constrain our universe to be close to $Λ$CDM Bayesian arguments about the evidence and the fine-tuning will have to be employed to discriminate between the models. Assuming a baseline $Λ$CDM model we investigate a number of quintessence and phantom dark energy models, and we study how they would perform when compared to observational data, such as the expansion rate, the angular distance, and the growth rate measurements, from the upcoming Dark Energy Spectroscopic Instrument (DESI) survey. We sample posterior likelihood surfaces of these dark energy models with Monte Carlo Markov Chains while using central values consistent with the Planck $Λ$CDM universe and covariance matrices estimated with Fisher information matrix techniques. We find that for this setup the Bayes factor provides a substantial evidence in favor of the $Λ$CDM model over most of the alternatives. We also investigated how well the CPL parametrization approximates various scalar field dark energy models, and identified the location for each dark energy model in the CPL parameter space.

astro-ph.CO↗

arXiv2015

Testing Dark Energy Models through Large Scale Structure

We explore the scalar field quintessence freezing model of dark energy with the inverse Ratra-Peebles potential. We study the cosmic expansion and the large scale structure growth rate. We use recent measurements of the growth rate and the baryon acoustic oscillation peak positions to constrain the matter density $Ω_\mathrm{m}$ parameter and the model parameter $α$ that describes the steepness of the scalar field potential. We solve jointly the equations for the background expansion and for the growth rate of matter perturbations. The obtained theoretical results are compared with the observational data. We perform the Baysian data analysis to derive constraints on the model parameters.

astro-ph.CO↗

arXiv2014

Growth Rate in the Dynamical Dark Energy Models

Dark Energy models with slowly-rolling cosmological scalar field provide a popular alternative to the standard, time-independent cosmological constant model. We study simultaneous evolution of background expansion and growth in the scalar field model with the Ratra-Peebles self-interaction potential. We use recent measurements of the linear growth rate and the baryon acoustic oscillation peak positions to constrain the model parameter $α$ that describes the steepness of the scalar field potential.

astro-ph.CO↗