arXiv subjects

Ahmad Mehrabi

Publications and source records attributed to Ahmad Mehrabi.

At least 19 recordsLinked to original sources

arXiv2023

Model-independent estimation of the cosmography parameters using cosmic chronometers

Measurement of the universe expansion rate through the cosmic chronometers proves to be a novel approach to understanding cosmic history. Although it provides a direct determination of the Hubble parameters at different redshifts, it suffers from underlying systematic uncertainties. In this work, we analyze the recent cosmic chronometer data with and without systematic uncertainties and investigate how they affect the results. We perform our analysis in both model-dependent and independent methods to avoid any possible model bias. In the model-dependent approach, we consider the $Λ$CDM, wCDM and CPL models. On the Other hand, since the Gaussian process provides a unique tool to study data including a non-diagonal covariance matrix, our model-independent analysis is based on the Gaussian process.

astro-ph.CO↗

arXiv2023

A semi-model-independent approach to describe a cosmological database

A model-independent or non-parametric approach for modeling a database has been widely used in cosmology. In these scenarios, the data has been used directly to reconstruct an underlying function. In this work, we introduce a novel semi-model-independent method to do the task. The new approach not only removes some drawbacks of previous methods but also has some remarkable advantages. We combine the well-known Gaussian linear model with a neural network and introduce a procedure for the reconstruction of an arbitrary function. In the scenario, the neural network produces some arbitrary base functions which subsequently are fed to the Gaussian linear model. Given a prior distribution on the free parameters, the Gaussian linear model provides a close form for the posterior distribution as well as the Bayesian evidence. In addition, contrary to other methods, it is straightforward to compute the uncertainty.

astro-ph.CO↗

arXiv2022

Gaussian discriminators between $Λ$CDM and wCDM cosmologies using expansion data

The Gaussian linear model provides a unique way to obtain the posterior probability distribution as well as the Bayesian evidence analytically. Considering the expansion rate data, the Gaussian linear model can be applied for $Λ$CDM, wCDM, and a non-flat $Λ$CDM. In this paper, we simulate the expansion data with various precision and obtain the Bayesian evidence, then it has been used to discriminate the models. The data uncertainty is in the range $σ\in(0.5,10)\%$ and two different sampling rates have been considered. Our results indicate that it is possible to discriminate $w=-1.02$ (or $w=-0.98$) model from the $Λ$CDM $(w=-1)$ with $σ=0.5\%$ uncertainty in expansion rate data. Finally, we perform a parameters inference in both the MCMC and Gaussian linear model, using currently available expansion rate data, and compare the results.

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arXiv2022

Reconstruction of dark energy density by non-parametric approaches

The evolution of dark energy density is a crucial quantity in understanding the nature of dark energy. Often, the quantity is described by the so-called equation of state, that is the ratio of dark energy pressure to its density. In this scenario, the dark energy density is always positive throughout cosmic history and a negative value is not allowed. Assuming a homogeneous and isotropic universe, we reconstruct the dark energy density directly from observational data and investigate its evolution through cosmic history. We consider the latest SNIa, BAO and cosmic chronometer data and reconstruct the dark energy density in both flat and non-flat universes up to redshift $z\sim 3$. The results are well in agreement with the $Λ$CDM up to redshift $z\sim 1.5$, whereas all data and methods, in our analysis, provide a negative dark energy density at high redshifts.

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arXiv2021

Non-parametric modeling of the cosmological data, base on the $χ^2$ distribution

In the $Λ$CDM model, cosmological observations from the late and recent universe reveal a puzzling $\sim 4.5σ$ tension in the current rate of universe expansion. In addition to the various scenarios suggested to resolve the tension, non-parametric modeling may provide useful insights. In this paper, we look at three well-known non-parametric methods, the smoothing method, the genetic algorithm, and the Gaussian process. Considering these three methods, we employ the recent Hubble parameters data to reconstruct the rate of universe expansion and supernovae Pantheon sample to reconstruct the luminosity distance. In contrast to the similar studies in the literature, the chi-squared distribution has been used to construct a reliable criterion to select a reconstruction. Finally, we compute the current rate of universe expansion ($H_0$) for each method, provide some discussions regarding the performance of each approach, and compare the results.

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arXiv2021

Using newest VLT-KMOS HII Galaxies and other cosmic tracers to test the $Λ$CDM tension

We place novel constraints on the cosmokinetic parameters by using a joint analysis of the newest VLT-KMOS HII galaxies (HIIG) with the Supernovae Type Ia (SNIa) Pantheon sample. We combine the latter datasets in order to reconstruct, in a model-independent way, the Hubble diagram to as high redshifts as possible. Using a Gaussian process we derive the basic cosmokinetic parameters and compare them with those of $Λ$CDM. In the case of SNIa, we find that the extracted values of the cosmokinetic parameters are in agreement with the predictions of $Λ$CDM model. Combining SNIa with high redshift tracers of the Hubble relation, namely HIIG data, we obtain consistent results with those based on $Λ$CDM as far as the present values of the cosmokinetic parameters are concerned, but find significant deviations in the evolution of the cosmokinetic parameters with respect to the expectations of the concordance $Λ$CDM model.

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arXiv2021

Cosmographic parameters in model-independent approaches

Cosmographic approach, a Taylor expansion of the Hubble function, has been used as a model-independent method to investigate the evolution of the universe in the presence of cosmological data. Apart from possible technical problems like the radius of convergence, there is an ongoing debates about the tensions appear when one investigates some high redshift cosmological data. In this work, we consider two common data sets namely SNIa (Pantheon sample) and the Hubble data to investigate advantages and disadvantages of the cosmographic approach. To do this, we obtain the evolution of cosmographic functions using cosmographic method as well as two other well known model-independent approaches namely, the Gaussian process and the Genetic algorithm. We also assume $Λ$CDM model as concordance model to compare the results of mentioned approaches. Our results indicate that the results of cosmography comparing with the other approaches, are not exact enough. Considering the Hubble data which is less certain, the results of $q_0$ and $j_0$ obtained in cosmography, provides a tension at more than $3σ$ away from the best result of $Λ$CDM. Assuming both of data samples in different approaches we show that the cosmographic approach, because of providing some biased results, is not the best approach for reconstruction of cosmographic functions, especially at higher redshifts.

astro-ph.CO↗

arXiv2021

Flare Activity and Magnetic Feature Analysis of the Flare Stars II: Sub-Giant Branch

We present an investigation of the magnetic activity and flare characteristics of the sub-giant stars mostly from F and G spectral types and compare the results with the main-sequence (MS) stars. The light curve of 352 stars on the sub-giant branch (SGB) from the Kepler mission is analyzed in order to infer stability, relative coverage and contrast of the magnetic structures and also flare properties using three flare indexes. The results show that: (i) Relative coverage and contrast of the magnetic features along with rate, power and magnitude of flares increase on the SGB due to the deepening of the convective zone and more vigorous magnetic field production (ii) Magnetic activity of the F and G-type stars on the SGB does not show dependency to the rotation rate and does not obey the saturation regime. This is the opposite of what we saw for the main sequence, in which the G-, K- and M-type stars show clear dependency to the Rossby number; (iii) The positive relationship between the magnetic features stability and their relative coverage and contrast remains true on the SGB, though it has lower dependency coefficient in comparison with the MS; (iv) Magnetic proxies and flare indexes of the SGB stars increase with increasing the relative mass of the convective zone.

astro-ph.SR↗

arXiv2020

Does $Λ$CDM really be in tension with the Hubble diagram data?

In this article, we elaborate further on the $Λ$CDM "tension", suggested recently by the authors \cite{Lusso:2019akb,Risaliti:2018reu}. We combine Supernovae type Ia (SNIa) with quasars (QSO) and Gamma Ray Bursts (GRB) data in order to reconstruct a model independent Hubble diagram to as high redshifts as possible. Specifically, in the case of either SNIa or SNIa/QSO data, we find that current values of the cosmokinetic parameters extracted from the Gaussian process are consistent with those of $Λ$CDM. Including GRBs, in the analysis, we find a tension, which however is not as significant as that mentioned in \cite{Lusso:2019akb, Risaliti:2018reu}. Finally, we argue that the choice of the kernel function used in extracting the luminosity distance might affect the amount of tension.

astro-ph.CO↗

arXiv2019

Flare Activity and Magnetic Feature Analysis of the Flare Stars

We analyze the light curve of 1740 flare stars to study the relationship between the magnetic feature characteristics and the identified flare activity. Coverage and stability of magnetic features are inspired by rotational modulation of light curve variations and flare activity of stars are obtained using our automated flare detection algorithm. The results show that (i) Flare time occupation ratio (or flare frequency) and total power of flares increase by increasing relative magnetic feature coverage and contrast in F-M type stars (ii) Magnetic feature stability is highly correlated with the coverage and the contrast of the magnetic structures as this is the case for the Sun (iii) Stability, coverage and contrast of the magnetic features, time occupation ratio and total power of flares increases for G, K and M-type stars by decreasing Rossby number due to the excess of produced magnetic field from dynamo procedure until reaching to saturation level.

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arXiv2019

Information gains from Monte Carlo Markov Chains

In this paper, we present a novel method for computing the relative entropy as well as the expected relative entropy using an MCMC chain. The relative entropy from information theory can be used to quantify differences in posterior distributions of a pair of experiments. In cosmology, the relative entropy has been proposed as an interesting tool for model selection, experiment design, forecasting and measuring information gain from subsequent experiments. In contrast to Gaussian distributions, these quantities are not generally available analytically and one needs to use numerical methods to estimate them which are certainly computationally expensive. We propose a method and provide its python package to estimate the relative entropy as well as expected relative entropy from a posterior sample. We consider the linear Gaussian model to check the accuracy of our code. Our results indicate that the relative error is below $0.2\%$ for sample size larger than $10^5$ in the linear Gaussian model. In addition, we study the robustness of our code in estimating the expected relative entropy in this model.

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arXiv2018

Activity Analyses for Solar-type Stars Observed with Kepler. II. Magnetic Feature Versus Flare Activity

The light curves of solar-type stars present both periodic fluctuation and flare spikes. The gradual periodic fluctuation is interpreted as the rotational modulation of magnetic features on the stellar surface and is used to deduce magnetic feature activity properties. The flare spikes in light curves are used to derive flare activity properties. In this paper, we analyze the light curve data of three solar-type stars (KIC 6034120, KIC 3118883, and KIC 10528093) observed with Kepler space telescope and investigate the relationship between their magnetic feature activities and flare activities. The analysis shows that: (1) both the magnetic feature activity and the flare activity exhibit long-term variations as the Sun does; (2) unlike the Sun, the long-term variations of magnetic feature activity and flare activity are not in phase with each other; (3) the analysis of star KIC 6034120 suggests that the long-term variations of magnetic feature activity and flare activity have a similar cycle length. Our analysis and results indicate that the magnetic features that dominate rotational modulation and the flares possibly have different source regions, although they may be influenced by the magnetic field generated through a same dynamo process.

astro-ph.SR↗

arXiv2018

Tachyon warm inflation with the effects of Loop Quantum Cosmology in the light of Planck 2015

We investigate the observational signatures of quantum cosmology in the Cosmic Microwave Background data provided by Planck collaboration. We apply the warm inflationary paradigm with a tachyon scalar field to the loop quantum cosmology. In this context, we first provide the basic cosmological functions in terms of the tachyon field. We then obtain the slow-roll parameters and the power spectrum of scalar and tensor fluctuations respectively. Finally, we study the performance of various warm inflationary scenarios against the latest Planck data and we find a family of models which are in agreement with the observations.

gr-qc↗

arXiv2017

Constraints to dark energy using PADE parameterisations

We put constraints on dark energy properties using the PADE parameterisation, and compare it to the same constraints using Chevalier-Polarski-Linder (CPL) and $Λ$CDM, at both the background and the perturbation levels. The dark energy equation of state parameter of the models is derived following the mathematical treatment of PADE expansion. Unlike CPL parameterisation, the PADE approximation provides different forms of the equation of state parameter which avoid the divergence in the far future. Initially, we perform a likelihood analysis in order to put constraints on the model parameters using solely background expansion data and we find that all parameterisations are consistent with each other. Then, combining the expansion and the growth rate data we test the viability of PADE parameterisations and compare them with CPL and $Λ$CDM models respectively. Specifically, we find that the growth rate of the current PADE parameterisations is lower than $Λ$CDM model at low redshifts, while the differences among the models are negligible at high redshifts. In this context, we provide for the first time growth index of linear matter perturbations in PADE cosmologies. Considering that dark energy is homogeneous we recover the well known asymptotic value of the growth index, namely $γ_{\infty}=\frac{3(w_{\infty}-1)}{6w_{\infty}-5}$, while in the case of clustered dark energy we obtain $γ_{\infty}\simeq \frac{3w_{\infty}(3w_{\infty}-5)}{(6w_{\infty}-5)(3w_{\infty}-1)}$. Finally, we generalize the growth index analysis in the case where $γ$ is allowed to vary with redshift and we find that the form of $γ(z)$ in PADE parameterisation extends that of the CPL and $Λ$CDM cosmologies respectively.

astro-ph.CO↗

arXiv2017

Measuring the effects of Loop Quantum Cosmology in the CMB data

In this Essay we investigate the observational signatures of Loop Quantum Cosmology (LQC) in the CMB data. First, we concentrate on the dynamics of LQC and we provide the basic cosmological functions. We then obtain the power spectrum of scalar and tensor perturbations in order to study the performance of LQC against the latest CMB data. We find that LQC provides a robust prediction for the main slow-roll parameters, like the scalar spectral index and the tensor-to-scalar fluctuation ratio, which are in excellent agreement within $1σ$ with the values recently measured by the Planck collaboration. This result indicates that LQC can be seen as an alternative scenario with respect to that of standard inflation.

gr-qc↗

arXiv2016

Constraints on shear and rotation with massive galaxy clusters

A precise determination of the mass function is an important tool to verify cosmological predictions of the $Λ$CDM model and to infer more precisely the better model describing the evolution of the Universe. Galaxy clusters have been currently used to infer cosmological parameters, in particular the matter density parameter $Ω_{\rm m}$, the matter power spectrum normalization $σ_8$ and the equation of state parameter $w_{\rm de}$ of the dark energy fluid. In this work, using data on massive galaxy clusters ($M>8\times 10^{14}~h^{-1}~M_{\odot}$) in the redshift range $0.05\lesssim z\lesssim 0.83$ we put constraints on the parameter $α$ introduced within the formalism of the extended spherical collapse model to quantify deviations from sphericity due to shear and rotation. Since at the moment there is no physical model describing its functional shape, we assume it to be a logarithmic function of the cluster mass. By holding $σ_8$ fixed and restricting our analysis to a $Λ$CDM model, we find, at $1-σ$ confidence level, $Ω_{\rm m}=0.284\pm0.0064$, $h=0.678\pm0.017$ and $β=0.0019^{+0.0008}_{-0.0015}$, where $β$ represents the slope of the parameter $α$. This results translates into a $9\%$ decrement of the number of massive clusters with respect to a standard $Λ$CDM mass function, but better data are required to better constrain this quantity, since at the $2-σ$ and $3-σ$ confidence level we are only able to infer upper limits.

astro-ph.CO↗

arXiv2016

Tachyon warm-intermediate inflation in the light of Planck data

We study the main properties of the warm inflationary model based on Barrow's solution for the scale factor of the universe. Within this framework we calculate analytically the basic slow roll parameters for different versions of warm inflation. We test the performance of this inflationary scenario against the latest observational data and we verify that the predicted spectral index and the tensor-to-scalar fluctuation ratio are in excellent agreement with those of {\it Planck 2015}. Finally, we find that the current predictions are consistent with those of viable inflationary models.

gr-qc↗

arXiv2015

Growth of matter perturbations in clustered holographic dark energy cosmologies

We investigate the growth of matter fluctuations in holographic dark energy cosmologies. First we use an overall statistical analysis involving the latest observational data in order to place constraints on the cosmological parameters. Then we test the range of validity of the holographic dark energy models at the perturbation level and its variants from the concordance $Λ$ cosmology. Specifically, we provide a new analytical approach in order to derive, for the first time, the growth index of matter perturbations. Considering a homogeneous holographic dark energy we find that the growth index is $γ\approx \frac{4}{7}$ which is somewhat larger ($\sim 4.8\%$) than that of the usual $Λ$ cosmology, $γ^{(Λ)}\approx \frac{6}{11}$. Finally, if we allow clustering in the holographic dark energy models then the asymptotic value of the growth index is given in terms of the effective sound speed $c_{\rm eff}^2$, namely $γ\approx \frac{3(1-c_{\rm eff}^2)}{7}$.

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