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M. Sadegh Movahed

Publications and source records attributed to M. Sadegh Movahed.

31 records · Page 2Linked to original sources
arXiv2008

Observational constraints on the braneworld model with brane-bulk energy exchange

We investigate the viability of the braneworld model with energy exchange between the brane and bulk, by using the most recent observational data related to the background evolution. We show that this energy exchange behaves like a source of dark energy and can alter the profile of the cosmic expansion. The new Supernova Type Ia (SNIa) Gold sample, Supernova Legacy Survey (SNLS) data, the position of the acoustic peak at the last scattering surface from the Wilkinson Microwave Anisotropy Probe (WMAP) observations and the baryon acoustic oscillation peak found in the Sloan Digital Sky Survey (SDSS) are used to constrain the free parameters of this model. To infer its consistency with the age of the Universe, we compare the age of old cosmological objects with what computed using the best fit values for the model parameters. At 68% level of confidence, the combination of Gold sample SNIa, Cosmic Microwave Background (CMB) shift parameter and SDSS databases provide $Ω_m=0.29_{-0.02}^{+0.03}$, $Ω_{A}=-0.71_{-0.03}^{+0.03}$ and $μ=-0.40_{-0.26}^{+0.28}$, hence a spatially flat Universe with $Ω_K=0.00_{-0.04}^{+0.04}$. The same combination with SNLS supernova observation give $Ω_m=0.27_{-0.02}^{+0.02}$, $Ω_{A}=-0.74_{-0.02}^{+0.04}$ and $μ=0.00_{-0.30}^{+0.30}$ consequently provides a spatially flat Universe $Ω_K=-0.01_{-0.03}^{+0.04}$. These results obviously seem to be compatible with the most recent WMAP results indicating a flat Universe.

astro-ph↗
arXiv2007

Is Thick Brane Model Consistent with the Recent Observations?

There exist many observational evidences implying the expansion of our universe is undergoing a late-time acceleration, the mechanism of this acceleration is yet unknown. In the so-called thick brane model this phenomena is attributed to the thickness of the brane along the extra dimension. In this study we mainly rely to the consistency of this model with most recent observational data related to the background evolution. The new Supernova Type Ia (SNIa) Gold sample and Supernova Legacy Survey (SNLS) data, the position of the acoustic peak at the last scattering surface from the WMAP observations and the baryon acoustic oscillation peak found in the Sloan Digital Sky Survey (SDSS) are used to constrain the free parameter of the thick codimension 1 brane model. To infer its consistency with age of our universe, we compare the age of old cosmological objects with what computed using the best fit values for the model parameters. When the universe is matter dominated, $w=0$, at 68% level of confidence, the combination of Gold sample SNIa, CMB shift parameter and SDSS databases provides $Ω_m=0.31_{-0.02}^{+0.02}$, $Ω_{\cal{C}}=0.05_{-0.01}^{+0.01}$, $w_r=-1.40_{-0.20}^{+0.20}$, hence a spatially open universe with $Ω_k=0.21_{-0.08}^{+0.08}$. The same combination with SNLS supernova observation gives $Ω_m=0.28_{-0.02}^{+0.03}$, $Ω_{\cal{C}}=0.037_{-0.004}^{+0.003}$, $w_r=-2.05_{-0.15}^{+0.15}$ consequently provides a spatially open universe $Ω_k=0.11_{-0.07}^{+0.10}$. These results obviously seem to be in contradiction with the most recent WMAP results indicating a flat universe.

astro-ph↗
arXiv2007

Uncertainty in the Fluctuations of the Price of Stocks

We report on a study of the Tehran Price Index (TEPIX) from 2001 to 2006 as an emerging market that has been affected by several political crises during the recent years, and analyze the non-Gaussian probability density function (PDF) of the log returns of the stocks' prices. We show that while the average of the index did not fall very much over the time period of the study, its day-to-day fluctuations strongly increased due to the crises. Using an approach based on multiplicative processes with a detrending procedure, we study the scale-dependence of the non-Gaussian PDFs, and show that the temporal dependence of their tails indicates a gradual and systematic increase in the probability of the appearance of large increments in the returns on approaching distinct critical time scales over which the TEPIX has exhibited maximum uncertainty.

q-fin.ST↗
arXiv2007

Consistency of $f(R)=\sqrt{R^{2}-R_{0}^2}$ Gravity with the Cosmological Observations in Palatini Formalism

In this work we study the dynamics of universe in $f(R)=\sqrt{R^2-R_{0}^2}$ modified gravity with Palatini formalism. We use data from recent observations as Supernova Type Ia (SNIa) Gold sample and Supernova Legacy Survey (SNLS) data, size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), the position of the acoustic peak from the CMB observations and large scale structure formation (LSS) from the 2dFGRS survey to put constraint on the parameters of the model. To check the consistency of this action, we compare the age of old cosmological objects with the age of universe. In the combined analysis with the all the observations, we find the parameters of model as $R_0=6.192_{-0.177}^{+0.167}\times H_0^2$ and $Ω_m=0.278_{-0.278}^{+0.273}$.

astro-ph↗
arXiv2006

Level Crossing Analysis of Burgers Equation in 1+1 Dimensions

We investigate the average frequency of positive slope $ν_α^{+}$, crossing the velocity field $u(x)- \bar u = α$ in the Burgers equation. The level crossing analysis in the inviscid limit and total number of positive crossing of velocity field before creation of singularities are given. The main goal of this paper is to show that this quantity, $ν_α^{+}$, is a good measure for the fluctuations of velocity fields in the Burgers turbulence.

cond-mat.stat-mech↗
arXiv2006

Exact Analysis of Level-Crossing Statistics for (d+1)-Dimensional Fluctuating Surfaces

We carry out an exact analysis of the average frequency $ν_{αx_i}^+$ in the direction $x_i$ of positive-slope crossing of a given level $α$ such that, $h({\bf x},t)-\bar{h}=α$, of growing surfaces in spatial dimension $d$. Here, $h({\bf x},t)$ is the surface height at time $t$, and $\bar{h}$ is its mean value. We analyze the problem when the surface growth dynamics is governed by the Kardar-Parisi-Zhang (KPZ) equation without surface tension, in the time regime prior to appearance of cusp singularities (sharp valleys), as well as in the random deposition (RD) model. The total number $N^+$ of such level-crossings with positive slope in all the directions is then shown to scale with time as $t^{d/2}$ for both the KPZ equation and the RD model.

cond-mat.stat-mech↗
arXiv2006

Characteristic Angular Scales in Cosmic Microwave Background Radiation

We investigate the stochasticity in temperature fluctuations in the cosmic microwave background (CMB) radiation data from {\it Wilkinson Microwave Anisotropy Probe}. We show that the angular fluctuations of the temperature is a Markov process with a {\it Markov angular scale}, $Θ_{\rm Markov}=1.01^{+0.09}_{-0.07}$. We characterize the complexity of the CMB fluctuations by means of a Fokker-Planck or Langevin equation and measure the associated Kramers-Moyal coefficients for the fluctuating temperature field $T(\hat n)$ and its increment, $ΔT =T(\hat n_1) - T(\hat n_2)$. Through this method we show that temperature fluctuations in the CMB has fat tails compared to a Gaussian distribution.

astro-ph↗
arXiv2006

Power-law Parameterized Quintessence Model

We introduce a power-law parameterized quintessence model for the dark energy which accelerate universe at the low redshifts while behaves as an ordinary matter for the early universe. We construct a unique scalar potential for this parameterized quintessence model. As the observational test, the Supernova Type Ia (SNIa) Gold sample data, size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), the position of the acoustic peak from the CMB observations and structure formation from the 2dFGRS survey are used to constrain the parameters of the quintessence model. The best fit parameters indicates that the equation of state of this model at the present time is less than one $(w_0<-1)$ which violates the energy condition in General Relativity. Finally we compare the age of old objects with age of universe in this model.

astro-ph↗
arXiv2006

A Parameterized Variable Dark Energy Model: Structure Formation and Observational Constraints

In this paper we investigate a simple parameterization scheme of the quintessence model given by Wetterich (2004). The crucial parameter of this model is the bending parameter $b$, which is related to the amount of dark energy in the early universe. Using the linear perturbation and the spherical infall approximations, we investigate the evolution of matter density perturbations in the variable dark energy model, and obtain an analytical expression for the growth index $f$. We show that increasing $b$ leads to less growth of the density contrast $δ$, and also decreases the growth index. Giving a fitting formula for the growth index at the present time we verify that the approximation relation $f\simeqΩ_m^α$ also holds in this model. To compare predictions of the model with observations, we use the Supernovae type Ia (SNIa) Gold Sample and the parameters of the large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS). The best fit values for the model parameters by marginalizing on the remained ones, are $Ω_m=0.21_{-0.06}^{+0.07}$, $w_0=-2.05_{-2.05}^{+0.65}$ and $b=4.05^{+7.05}_{-2.25}$ at $1σ$ confidence level. As a final test we calculate the age of universe for different choices of the free parameters in this model and compare it with the age of old stars and some high redshift objects. Then we show that the predictions of this variable dark energy model are consistent with the age observation of old star and can solve the "age crisis" problem.

astro-ph↗
arXiv2006

Observational Constraints on a Variable Dark Energy Model

We study the effect of a phenomenological parameterized quintessence model on low, intermediate and high redshift observations. At low and intermediate redshifts, we use the Gold sample of supernova Type Ia (SNIa) data and recently observed size of baryonic acoustic peak from Sloan Digital Sky Survey (SDSS), to put constraint on the parameters of the quintessence model. At the high redshift, the same fitting procedure is done using WAMP data, comparing the location of acoustic peak with that obtain from the dark energy model. As a complementary analysis in a flat universe, we combine the results from the SNIa, CMB and SDSS. The best fit values for the model parameters are $Ω_m = 0.27^{+0.02}_{-0.02}$ (the present matter content) and $w_0=-1.45^{+0.35}_{-0.60}$ (dark energy equation of state). Finally we calculate the age of universe in this model and compare it with the age of old stars and high redshift objects

astro-ph↗
arXiv2006

Multifractal Detrended Fluctuation Analysis of Sunspot Time Series

We use multifractal detrended fluctuation analysis (MF-DFA), to See query 1 study sunspot number fluctuations. The result of the MF-DFA shows that there are three crossover timescales in the fluctuation function. We discuss how the existence of the crossover timescales is related to a sinusoidal trend. Using Fourier detrended fluctuation analysis, the sinusoidal trend is eliminated. The Hurst exponent of the time series without the sinusoidal trend is $0.12\pm 0.01$. Also we find that these fluctuations have multifractal nature. Comparing the MF-DFA results for the remaining data set to those for shuffled and surrogate series, we conclude that its multifractal nature is almost entirely due to long range correlations.

physics.data-an↗
arXiv2005

The Effect of Uncertainty Principle on the Thermodynamics of Early Universe

We discuss the concept of measurement in cosmology from the relativistic and quantum mechanical points of view. The uncertainty principle within the particle horizon, excludes the momentum of particles to be less than $π\hbar H/c$. This effect modifies the standard thermodynamics of early universe for the ultra-relativistic particles such that the equation of state as well as dependence of energy density and pressure to the temperature. We show that this modification to the thermodynamics of early universe is important for energies $E>10^{17} GeV$. During the inflation, the particle horizon inflates to a huge size and makes the uncertainty in the momentum to be negligible.

astro-ph↗