arXiv subjects

Martiros Khurshudyan

Publications and source records attributed to Martiros Khurshudyan.

At least 19 recordsLinked to original sources

arXiv2024

Can we learn from matter creation to solve the $H_{0}$ tension problem?

The $H_{0}$ tension problem is studied in the light of a matter creation mechanism (an effective approach to replacing dark energy), the way to define the matter creation rate being of pure phenomenological nature. Bayesian (probabilistic) Machine Learning is used to learn the constraints on the free parameters of the models, with the learning being based on the generated expansion rate, $H(z)$. Taking advantage of the method, the constraints for three redshift ranges are learned. Namely, for the two redshift ranges: $z\in [0,2]$~(cosmic chronometers) and $z\in [0,2.5]$~(cosmic chronometers + BAO), covering already available $H(z)$ data, to validate the learned results; and for a third redshift interval, $z\in[0,5]$, for forecasting purposes. It is learned that the $3αH_{0}$ term in the creation rate provides options that have the potential to solve the $H_{0}$ tension problem.

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arXiv2024

Constraints on prospective deviations from the cold dark matter model using a Gaussian Process

Recently, using Bayesian Machine Learning, a deviation from the cold dark matter model on cosmological scales has been put forward. Such model might replace a proposed non-gravitational interaction between dark energy and dark matter, and help solve the $H_{0}$ tension problem. The idea behind the learning procedure relied there on a generated expansion rate, while the real expansion rate was just used to validate the learned results. In the present work, however, the emphasis is put on a Gaussian Process (GP) with the available $H(z)$ data confirming the possible existence of the already learned deviation. Three cosmological scenarios are considered: a simple one, with equation of state parameter for dark matter $ω_{dm} = ω_{0} \neq 0$, and two other models, with corresponding parameters $ω_{dm} = ω_{0} + ω_{1} z$ and $ω_{dm} = ω_{0} + ω_{1} z/(1+z)$. The constraints obtained on the free parameters $ω_{0}$ and $ω_{1}$ hint towards a dynamical nature of the deviation. The dark energy dynamics is also reconstructed, revealing interesting aspects connected with the $H_{0}$ tension problem. It is concluded, however, that improved tools and more data are needed, in order to reach a better understanding of the reported deviation.

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arXiv2023

Swampland criteria and neutrino generation in a non-cold dark matter universe

In this paper, the implications of string Swampland criteria for a dark energy-dominated universe, where we have a deviation from the cold dark matter model, will be discussed. In particular, we have considered two models. One of them is one parameter model, while the second one has been crafted to reveal the dynamics in the deviation. The analysis has been obtained through the use of Gaussian processes (GPs) and $H(z)$ expansion rate data (a $30$-point sample deduced from a differential age method and a $10$-point sample obtained from the radial BAO method). We learned that the tension with the Swampland criteria still will survive as in the cases of the models where dark matter is cold. In the analysis besides mentioned $40$-point $H(z)$ data, we used the latest values of $H_{0}$ reported by the Planck and Hubble missions to reveal possible solutions for the $H_{0}$ tension problem. Finally, the constraints on the neutrino generation number have been obtained revealing interesting results to be discussed yet. This and various related questions have been left to be discussed in forthcoming papers.

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arXiv2023

Solving the $H_{0}$ tension in $f(T)$ Gravity through Bayesian Machine Learning

Bayesian Machine Learning~(BML) and strong lensing time delay~(SLTD) techniques are used in order to tackle the $H_{0}$ tension in $f(T)$ gravity. The power of BML relies on employing a model-based generative process which already plays an important role in different domains of cosmology and astrophysics, being the present work a further proof of this. Three viable $f(T)$ models are considered: a power law, an exponential, and a squared exponential model. The learned constraints and respective results indicate that the exponential model, $f(T)=αT_{0}\left(1-e^{-p T / T_{0}}\right)$, has the capability to solve the $H_{0}$ tension quite efficiently. The forecasting power and robustness of the method are shown by considering different redshift ranges and parameters for the lenses and sources involved. The lesson learned is that these values can strongly affect our understanding of the $H_{0}$ tension, as it does happen in the case of the model considered. The resulting constraints of the learning method are eventually validated by using the observational Hubble data(OHD).

astro-ph.CO↗

arXiv2023

Assessing the foundation and applicability of some dark energy fluid models in the Dirac-Born-Infeld framework

In this paper, we will deepen the understanding of some fluid models proposed by other authors for the description of dark energy. Specifically, we will show that the so-called (Modified) Berthelot fluid is the hydrodynamic realization of the free Dirac-Born-Infeld theory and that the Dieterici fluid admits a non-relativistic $k$-essence formulation; for the former model the evolution of the scalar field will be written in terms of some cosmographic parameters. The latter model will also be tested using Machine Learning algorithms with respect to cosmic chronometers data, and results about the dynamics at a background level will be compared with those arising when other fluids (Generalized Chaplygin Gas and Anton-Schmidt) are considered. Due to some cosmic opacity effects, the background cosmology of universes filled by these inequivalent fluids, as they arise in physically different theories, may not be enough for discriminating among them. Thus, a perturbation analysis in the long-wavelength limit is carried out revealing a rich variety of possible behaviors. It will also be shown that the free Dirac-Born-Infeld theory cannot account for flat galactic rotation curves, and therefore we derive an appropriate relationship between the scalar field potential and the brane tension for achieving this goal; this provides an estimate for the dark matter adiabatic speed of sound inside the halo consistent with other literature. A certain relationship between the Newtonian gravitational potential within the galaxy and the Lagrangian potential in the non-relativistic regime for the (Modified) Berthelot fluid will also be enlightened.

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arXiv2022

Reconstruction of the quintessence dark energy potential from a Gaussian process

The quintessence dark energy potential is reconstructed in a model-independent way. Reconstruction relies on a Gaussian process and on available expansion-rate data. Specifically, 40-point values of $H(z)$ are used, consisting of a 30-point sample deduced from a differential age method and an additional 10-point sample obtained from the radial BAO method. Results are obtained for two kernel functions and for three different values of $H_{0}$. They shed light on the $H_{0}$ tension problem for a universe described with quintessence dark energy. They are also a clear indication that the tension has to do with the physical understanding of the issue, rather than being just a numerical problem with statistics. Moreover, the model-independent reconstruction of the potential here obtained can serve as a reference to constraint available models and it can be also used as a reference frame to construct new ones. Various possibilities, including $V(ϕ) \sim e^{-λϕ}$, are compared with the reconstructions here obtained, which is notably the first truly model independent reconstruction of the quintessence dark energy potential. This allows to select new models that can be interesting for cosmology. The method can be extended to reconstruct the potential of related dark energy models, to be considered in future work.

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arXiv2021

Constraints on interacting dark energy models through cosmic chronometers and Gaussian process

In this paper, after reconstructing the redshift evolution of the Hubble function by adopting Gaussian process techniques, we estimate the best-fit parameters for some flat Friedmann cosmological models based on a Modified Chaplygin Gas interacting with dark matter. In fact, the expansion history of the Universe will be investigated because passively evolving galaxies constitute cosmic chronometers. An estimate for the present-day values of the deceleration parameter, adiabatic speed of sound within the dark energy fluid, effective dark energy, and dark matter equation of state parameters is provided. By this, we mean that the interaction term between the two dark fluids, which breaks the Bianchi symmetries, will be interpreted as an effective contribution to the dark matter pressure similarly to the framework of the "Generalized Dark Matter". We investigate whether the estimates of the Hubble constant and of the present-day abundance of dark matter are sensitive to the dark matter - dark energy coupling. We will also show that the cosmic chronometers data favor a cold dark matter and that our findings are in agreement with the Le Châtelier-Braun principle according to which dark energy should decay into dark matter.

astro-ph.CO↗

arXiv2021

An approach to cold dark matter deviation and the $H_{0}$ tension problem by using machine learning

In this work, two different models, one with cosmological constant $Λ$, and baryonic and dark matter (with $ω_{dm} \neq 0$), and the other with an $X$ dark energy (with $ω_{de} \neq -1$), and baryonic and dark matter (with $ω_{dm} \neq 0$), are investigated and compared. Using Bayesian machine learning analysis, constraints on the free parameters of both models are obtained for the three redshift ranges: $z\in [0,2]$, $z\in [0,2.5]$, and $z\in [0,5]$, respectively. For the first two redshift ranges, high-quality observations of the expansion rate $H(z)$ exist already, and they are used for validating the fitting results. Additionally, the extended range $z\in [0,5]$ provides predictions of the model parameters, verified when reliable higher-redshift $H(z)$ data are available. This learning procedure, based on the expansion rate data generated from the background dynamics of each model, shows that, at cosmological scales, there is a deviation from the cold dark matter paradigm, $ω_{dm} \neq 0$, for all three redshift ranges. The results show that this approach may qualify as a solution to the $H_{0}$ tension problem. Indeed, it hints at how this issue could be effectively solved (or at least alleviated) in cosmological models with interacting dark energy.

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arXiv2020

Constraints on Cosmic Opacity from Bayesian Machine Learning: The hidden side of the $H_{0}$ tension problem

Bayesian (Probabilistic) Machine Learning is used to probe the opacity of the Universe. It relies on a generative process where the model is the key object to generate the data involving the unknown parameters of the model, our prior beliefs, and allows us to get the posterior results. The constraints on the cosmic opacity are determined for two flat models, $Λ$CDM and XCDM (this having $ω_{de} \neq -1$), for three redshift ranges, $z\in[0,2.5]$, $z \in[0,5]$, and $z\in[0,10]$, in each case. This is to understand how the constraints on the cosmic opacity could change in the very deep Universe, and also to check to what extent there is a redshift-range dependence. The following forms for the opacity, $τ(z) = 2εz$ and $τ(z) = (1+z)^{2ε} -1$, corresponding to an observer at $z=0$ and a source at $z$, are considered. The results of our analysis show that the Universe is not fully transparent, and this may have a significant impact on the $H_{0}$ tension problem. In the generative process, the fact that, owing to cosmic opacity, the flux received by the observer is reduced has been taken into account. In the analysis, the luminosity distance associated with the cosmological model has been employed.

astro-ph.CO↗

arXiv2020

Scrutinizing Various Phenomenological Interactions In The Context Of Holographic Ricci Dark Energy Models

In this paper, we examine two types of interacting holographic dark energy model using Pantheon supernova data, BAO BOSS DR12, CMB Planck 2015, fgas (gas mass fraction) and SZ/Xray (Sunyaev-Zeldovich effect and X-ray emission) data from galaxy clusters (GC). In particular, we considered the Holographic Ricci dark energy and Extended holographic Ricci dark energy models. During this analysis we considered seven type of phenomenological interaction terms (three linear and four non-linear) $Q_1=3Hbρ_{D}$, $Q_2=3Hbρ_{m}$, $Q_3=3Hb\left(ρ_{D}+ρ_m\right)$, $Q_4=3Hb\left(ρ_{D}+\frac{ρ_{D}^2}{ρ_{D}+ρ_m}\right)$, $Q_5=3Hb\left(ρ_{m}+\frac{ρ_{m}^2}{ρ_{D}+ρ_m}\right)$, $Q_6=3Hb\left(ρ_{D}+ρ_{m}+\frac{ρ_{D}^2}{ρ_{D}+ρ_m}\right)$, $Q_7=3Hb\left(ρ_{D}+ρ_{m}+\frac{ρ_{m}^2}{ρ_{D}+ρ_m}\right)$ respectively. To find the best model we apply Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) and use the $Λ$CDM as the referring model for comparison. Using AIC and BIC models selection method we note that the $Q_1$ and $Q_4$ interaction terms are favored by observational data within the context of the holographic Ricci dark energy models. The obtained results also demonstrated that the considered types of holographic Ricci dark energy model are not favored by observational data since the $Λ$CDM is considered as the reference model. We also observed that the values of the deceleration parameter and the transition redshift for all models are compatible with the latest observational data and Planck 2015. In addition, we studied the jerk parameter for all models. Using our modified CAMB code, we observed that the interacting models suppress the CMB spectrum at low multi-poles and enhances the acoustic peaks.

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arXiv2020

Interplay between Swampland and Bayesian Machine Learning in constraining cosmological models

Constraints on a dark energy dominated Universe are obtained from an interplay between Bayesian Machine Learning and string Swampland criteria. The approach here differs from previous studies, since in the generative process Swampland criteria are used and, only later, the results of the fit are validated, by using observational data-sets. A generative process based Bayesian Learning approach is applied to two models and the results are validated by means of available $H(z)$ data. For the first model, a parametrization of the Hubble constant is considered and, for the second, a parametrization of the deceleration parameter. This study is motivated by a recent work, where constraints on string Swampland criteria have been obtained from a Gaussian Process and $H(z)$ data. However, the results obtained here are fully independent of the observational data and allow to estimate how the high-redshift behavior of the Universe will affect the low-redshift one. Moreover, both parameterizations in the generative process, for the Hubble and for the deceleration parameters, are independent of the dark energy model. The outcome, both data- and dark energy model-independent, may highlight, in the future, the borders of the Swampland for the low-redshift Universe and help to develop new string-theory motivated dark-energy models. The string Swampland criteria considered might be in tension with recent observations indicating that phantom dark energy cannot be in the Swampland. Finally, a spontaneous sign switch in the dark energy equation of state parameter is observed when the field traverses are in the $z\in[0,5]$ redshift range, a remarkable phenomenon requiring further analysis.

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arXiv2020

An analysis of the $H_{0}$ tension problem in a universe with a viscous dark fluid

In this paper, two inhomogeneous single fluid models for the Universe, which are able to naturally solve the $H_{0}$ tension problem, are discussed. The analysis is based on a Bayesian Machine Learning approach that uses a generative process. The adopted method allows to constrain the free parameters of each model by using the model itself, only. The observable is taken to be the Hubble parameter, obtained from the generative process. Using the full advantages of our method, the models are constrained for two redshift ranges. Namely, first this is done with mock $H(z)$ data over $z\in [0,2.5]$, thus covering known $H(z)$ observational data, which are most helpful to validate the fit results. Then, aiming to extend to redshift ranges to be covered by the most recent ongoing and future planned missions, the models are constrained for the range $z\in[0,5]$, too. Full validation of the results for this extended redshift range will have to wait for the near future, when higher redshift $H(z)$ data become available. This makes our models fully falsifiable. Finally, our second model here is able to explain the BOSS reported value for $H(z)$ at $z=2.34$.

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arXiv2020

Phase space analysis and singularity classification for linearly interacting dark energy models

In this paper, applying the Hartman-Grobman theorem we carry out a qualitative late-time analysis of some unified dark energy-matter Friedmann cosmological models, where the two interact through linear energy exchanges, and the dark energy fluid obeys to the dynamical equation of state of Redlich-Kwong, Modified Berthelot, and Dieterici respectively. The identification of appropriate late-time attractors allows to restrict the range of validity of the free parameters of the models under investigation. In particular, we prove that the late-time attractors which support a negative deceleration parameter correspond to a de Sitter universe. We show that the strength of deviation from an ideal fluid for the dark energy does not influence the stability of the late-time attractors, as well as the values of all the cosmological parameters at equilibrium, but for the Hubble function (which represents the age of the universe). Our analysis also shows that a singularity in the effective equation of state parameter for the dark energy fluid is not possible within this class of models.

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arXiv2020

Model-independent reconstruction of $f(T)$ gravity from Gaussian Processes

We apply Gaussian processes and Hubble function data in $f(T)$ cosmology, to reconstruct for the first time the $f(T)$ form in a model-independent way. In particular, using $H(z)$ datasets coming from cosmic chronometers as well as from the radial BAO method, alongside the latest released local value $H_{0} = 73.52 \pm 1.62$ km/s/Mpc, we reconstruct $H(z)$ and its derivatives, resulting eventually in a reconstructed region for $f(T)$, without any assumption. Although the cosmological constant lies in the central part of the reconstructed region, the obtained mean curve follows a quadratic function. Inspired by this we propose a new $f(T)$ parametrization, i.e. $f(T) = -2Λ+ξT^2$, with $ξ$ the sole free parameter that quantifies the deviation from $Λ$CDM cosmology. Additionally, we confront three viable one-parameter $f(T)$ models of the literature, which respectively are the power-law, the square-root exponential, and the exponential one, with the reconstructed $f(T)$ region, and then we extract significantly improved constraints for their model parameters, comparing to the constraints that arise from usual observational analysis. Finally, we argue that since we are using the direct Hubble measurements and the local value for $H_0$ in our analysis, with the above reconstruction of $f(T)$, the $H_0$ tension can be efficiently alleviated.

astro-ph.CO↗

arXiv2019

Implications of the possible 21-cm line excess at cosmic dawn on dynamics of interacting dark energy

In this paper we study implications of the possible excess of 21-cm line global signal at the epoch of cosmic dawn on the evolutions of a class of dynamically interacting dark energy (IDE) models. We firstly summarize two dynamical mechanisms in which different background evolutions can exert considerable effects on the 21-cm line global signal. One is the decoupling time of Compton scattering heating, the other stems from the direct change of optical depth due to the different expansion rate of the Universe. After that, we investigate the IDE models to illustrate the tension between the results of Experiment to Detect the Global Epoch of reionization Signature (EDGES) and other experiments. To apply the analyses of these two mechanisms to IDE models, we find that only the optical depth can be significantly changed. Accordingly, in order to relieve the tension by including the effects of the decoupling time of Compton scattering heating, we deduce a possible evolution form for the Hubble parameter within IDE that begins at an early stage around $z\sim 100$ and then smoothly evolves to a value at $z\sim 17$ which is smaller than that obtained in the standard paradigm. Eventually, we fulfill this scenario by adding an early dark energy dominated stage to the cosmological paradigm described by IDE models, which can alleviate the tension between EDGES and other cosmological observations but cannot completely solve it.

astro-ph.CO↗

arXiv2019

Wormhole models in $f(\textit{R}, \textit{T})$ gravity

Models of static wormholes within the $f(\textit{R}, \textit{T})$ extended theory of gravity are investigated and, in particular, the family $f(\textit{R}, \textit{T}) = R + λT$, with $T = ρ+ P_{r} + 2P_{l}$ being the trace of the energy-momentum tensor. Models corresponding to different relations for the pressure components (radial and lateral), and several equations of state (EoS), reflecting different matter content, are worked out explicitly. The solutions obtained for the shape functions of the generated wormholes obey the necessary metric conditions, as manifested in other studies in the literature. The respective energy conditions reveal the physical nature of the wormhole models thus constructed. It is found, in particular, that for each of those considered, the parameter space can be divided into different regions, in which the exact wormhole solutions fulfill the null~(NEC) and the weak energy conditions~(WEC), respectively, in terms of the lateral pressure. Moreover, the dominant energy condition~(DEC) in terms of both pressures is also valid, while $ρ+ P_{r} + 2P_{l} = 0$. A similar solution for the theory $P_{r} = ω_{1} ρ+ ω_{2} ρ^{2}$ is found numerically, where $ω_{1}$ and $ω_{2}$ are either constant or functions of $r$, leading to the result that the NEC in terms of the radial pressure is also valid. For non-constant $ω_{i}$ models, attention is focused on the behavior $ω_{i} \propto r^{m}$. To finish, the question is addressed, how $f(R) = R+αR^{2}$ will affect the wormhole solutions corresponding to fluids of the form $P_{r} = ω_{1} ρ+ ω_{2} ρ^{2}$, in the three cases mentioned above. Issues concerning the nonconservation of the matter energy-momentum tensor, the stability of the solutions obtained, and the observational possibilities for testing these models are discussed in the last section.

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arXiv2019

On An Interacting New Holographic Dark Energy Model: observational constraints

In this paper, we study the interacting DGP braneworld Holographic Dark Energy model in a spatially flat FRW universe. Mainly, in this study we concentrate our attention on both interacting and non-interacting form of the model. The study shows that the equation of state and the deceleration parameter depict an accelerated universe for all variety of interactions. On the other hand, the StateFinder analysis shows that of the interacting and non-interacting behave similar to both quintessence and phantom dark energy and for the present value obey the behavior of quintessence. Moreover, the result of $Om$-diagnostic is an emphasis on the result of the equation of state showing that the current model is in the quintessence are with Phantom-like behavior in the late time. By the use of the squared sound speed $v^2_s$ we find that the present mode has a good stability. In order to obtain the best fit values of the parameters in this work we used the latest observational data (Pantheon, Boss DR12 and Planck 2015) implementing MCMC method by the use of EMCEE python package. We also employ Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) model selection tools and compare the model with $Λ$CDM as the reference model.

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arXiv2018

Wormholes with $ρ(R,R^{\prime})$ matter in $f(\textit{R}, \textit{T})$ gravity

Models of static wormholes are investigated in the framework of $f(\textit{R}, \textit{T})$ gravity ($\textit{R}$ being the curvature scalar, and $\textit{T}$ the trace of the energy momentum tensor). An attempt to link the energy density of the matter component to the Ricci scalar is made, which for the Morris and Thorne wormhole metric, with constant redshift function, yields $R(r) = 2b^{\prime}(r)/r^{2}$. Exact wormhole solutions are obtained for three particular cases: $ρ(r) = αR(r) + βR^{\prime}(r)$, $ρ(r) = αR^{2}(r) + βR^{\prime}(r)$, and $ρ(r) = αR(r) + βR^{2}(r)$, when $f(\textit{R}, \textit{T}) = R + 2 λT$. For the two first ones, traversable wormhole models are obtained. When the matter energy density is of the 3rd type, only solutions with constant shape may correspond to traversable wormholes. Exact wormhole solutions with same properties can be constructed for $ρ= αR(r) + βR^{-2}(r)$, $ρ= αR(r) + βr R^{2}(r)$, $ρ= αR(r) + βr^{-1} R^{2}(r)$, $ρ= αR(r) + βr ^{2} R^{2}(r)$, $ρ= αR(r) + βr^{3} R^{2}(r)$ and $ρ= αr^m R(r) \log (βR(r))$, as well. On the other hand, for $f(\textit{R}, \textit{T}) = R + γR^{2} + 2 λT$ gravity, two wormhole models are constructed, assuming that the energy density of the wormhole matter is $ρ(r) = αR(r) + βR^{2}(r)$ and $ρ(r) = αR(r) + βr^{3} R^{2}(r)$, respectively. In this case, the functional form of the shape function is taken to be $b(r) = \sqrt{\hat{r}_{0} r}$ and possible existence of appropriate static traversable wormhole configurations is proven. These results can be viewed as an initial step towards using specific properties of the new exact wormhole solutions, in order to propose new functional forms for describing the matter content of the wormhole.

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