arXiv subjects

A. Mehrabi

Publications and source records attributed to A. Mehrabi.

11 recordsLinked to original sources

arXiv2020

The growth of DM and DE perturbations in DBI non-canonical scalar field scenario

We study the effect of varying sound speed on clustering dark energy in the Dirac-Born-Infeld (DBI) scenario. The DBI action is included in the class of $k$-essence models, and it has an important role in describing the effective degrees of freedom of D-branes in the string theory. In the DBI setup, we take the anti-de Sitter (AdS) warp factor $f(ϕ)=f_0\, ϕ^{-4}$, and investigate the self-interacting quartic potential $V(ϕ)=λϕ^{4}/4$. We calculate the full expression of the effective sound speed for our model, and show that it can evolve with time during the cosmological evolution. Besides, the adiabatic sound speed evolves with time here, and this influences the background dynamics to some extent. We show that the effective sound speed is very close to the adiabatic sound speed. We examine the effect of the variable sound speed on growth of the perturbations in both the linear and non-linear regimes. In the linear regime, we apply the Pseudo-Newtonian formalism, and show that dark energy suppresses the growth of perturbations at low redshifts. From study the Integrated Sachs-Wolf (ISW) effect in our setup, we see that the model manifests some deviation from the concordance $Λ$CDM model. In the non-linear regime, we follow the approach of spherical collapse model, and calculate the linear overdensity, the virial overdensity, overdensity at the turn around and the rate of expansion of collapsed region. We further compute relative number density of halo objects above a given mass in our setting, and show that the number of structures with respect to the $Λ$CDM model is reduced more in the high mass tail at high redshifts.

gr-qc↗

arXiv2020

A Bayesian comparison between $Λ$CDM and phenomenologically emergent dark energy models

In this work we examine the recently proposed phenomenological emergent dark energy (PEDE) model by \cite{Li:2019yem}, using the latest observational data in both expansion and perturbation levels. Applying the statistical Bayesian evidence as well as the AIC and BIC information criteria, we compare the PEDE model with the concordance $Λ$CDM model in both flat and non-flat universes. We combine the observational datasets as (i) expansion data (except CMB), (ii) expansion data (including CMB) and (iii) expansion data jointed to the growth rate dataset. Our statistical results show that the flat- $Λ$CDM model is still the best model. In the case of expansion data (including CMB), we observe that the flat- PEDE model is well consistent with observations as well as the concordance $Λ$CDM universe. While in the cases of (i) and (iii), the PEDE models in both of the flat and non-flat geometries are not favored. In particular, we see that in the perturbation level the PEDE model can not fit the observations as equally as standard $Λ$CDM cosmology. As the ability of the model, we show that the PEDE models can alleviate the tension of Hubble constant value appearing between the local observations and Planck inferred estimation in standard cosmology.

astro-ph.CO↗

arXiv2018

Dark energy reconstruction based on the PADE approximation; an expansion around the $Λ$CDM

We study the dynamical properties of dark energy based on a large family of PADE parameterizations for which the dark energy density evolves as a ratio between two polynomials in the scale factor of the universe. Using the latest cosmological data we perform a standard likelihood analysis in order to place constraints on the main cosmological parameters of different PADE models. We find that the basic cosmological parameters, namely $(Ω_{m0},h,σ_{8})$ are practically the same for all PADE parametrizations explored here. Concerning the free parameters which are related to dark energy we show that the best fit values indicate that the equation of state parameter at the present time is in the phantom regime ($w<-1$), however we can not exclude the possibility of $w>-1$ at $1σ$ level. Finally, for the current family of PADE parametrizations we test their ability, via AIC and Jeffreys' scale, to deviate from $Λ$CDM cosmology. Among the current PADE parametrizations, the model which contains two dark energy parameters is the one for which a small but non-zero deviation from $Λ$CDM cosmology is slightly allowed by AIC test. Moreover, based on Jeffreys' scale we show that a deviation from $Λ$CDM cosmology is also allowed and thus the possibility of having a dynamical dark energy in the form of PADE parametrization cannot be excluded.

astro-ph.CO↗

arXiv2018

Gaugessence: a dark energy model with early time radiation-like equation of state

In this work, we study a new quintessence model associated with non-Abelian gauge fields, minimally coupled to Einstein gravity. This gauge theory has been recently introduced and studied as an inflationary model, called gauge-flation. Here, however, we are interested in the late time cosmology of the model in the presence of matter and radiation to explain the present time accelerating Universe. During the radiation and matter eras, the gauge field tracks radiation and basically acts like a dark radiation sector. As we approach lower redshifts, the dark component takes the form of a dark energy source which eventually becomes the dominate part of the energy budget of the Universe. Due to the tracking feature of our model, solutions with different initial values are attracted to a common trajectory. The existence of early dark radiation is a robust prediction of our model which contributes to the effective number of relativistic species, $N_{\rm eff}$ and has its own interesting observational features.

astro-ph.CO↗

arXiv2018

Growth of perturbations in dark energy parametrization scenarios

In this paper, we study the evolution of dark matter perturbations in the linear regime by considering the possibility of dark energy perturbations. To do this, two popular parameterizations, CPL and BA with same number of free parameters and different redshift dependency have been considered. We integrate the full relativistic equations to obtain the growth of matter fluctuations for both clustering and smooth versions of CPL and BA dark energy. The growth rate is larger (smaller) than the $Λ$CDM in the smooth cases when $w<-1$ ($w>-1$) but the dark energy clustering gives a larger (smaller) growth index when $w>-1$ ($w<-1$). We measure the relative difference of the growth rate with respect to concordance $Λ$CDM and study how it changes depending on the free parameters. Furthermore, it is found that the difference of growth rates between smooth CPL and BA is negligible, less than $0.5\%$, while for clustering case, the difference is considerable and might be as large as 2$\%$. Eventually, using the latest geometrical and growth rate observational data, we perform an overall likelihood analysis and show that both smooth and clustering cases of CPL and BA parameterizations are consistent with observations. In particular, we find the dark energy FoM $\sim70$ for the BA and $\sim30$ for the CPL which indicates BA model constraints relatively better than CPL one.

astro-ph.CO↗

arXiv2018

Primordial black hole detection through diffractive microlensing

Recent observations of gravitational waves motivate investigations for the existence of Primordial Black Holes (PBHs). We propose the observation of gravitational microlensing of distant quasars for the range of infrared to the submillimeter wavelengths by sub-lunar PBHs as lenses. The advantage of observations in the longer wavelengths, comparable to the Schwarzschild radius of the lens (i.e. $R_{\rm sch}\simeq λ$) is the detection of the wave optics features of the gravitational microlensing. The observation of diffraction pattern in the microlensing light curve of a quasar can break the degeneracy between the lens parameters and determine directly the lens mass as well as the distance of the lens from the observer. We estimate the wave optics optical-depth, also calculate the rate of $\sim 0.1$ to $\sim 0.3$ event per year per a quasar, assuming that hundred percent of dark matter is made of sub-lunar PBHs. Also, we propose a long-term survey of quasars with the cadence of almost one hour to few days to resolve the wave optics features of the light curves to discover PBHs and determine the fraction of dark matter made of sub-lunar PBHs as well as their mass function.

astro-ph.CO↗

arXiv2017

Consistency of nonlinear interacting ghost dark energy with recent observations

In this paper we investigate ghost dark energy model in the presence of non-linear interaction between dark energy and dark matter. We also extend the analysis to the so called generalized ghost dark energy (GGDE) which $ρ_D=αH+βH^2$. The model contains three free parameters as $Ω_D, ζ(=\frac{8πG β}{3})$ and $b^2$ (the coupling coefficient of interactions). We propose three kinds of non-linear interaction terms and discuss the behavior of equation of state, deceleration and dark energy density parameters of the model. We also find the squared sound speed and search for signs of stability of the model. To compare the interacting GGDE model with observational data sets, we use more recent observational outcomes, namely SNIa from JLA catalog, Hubble parameter, baryonic acoustic oscillation and the most relevant CMB parameters including, the position of acoustic peaks, shift parameters and redshift to recombination. For GGDE with the first non-linear interaction, the joint analysis indicates that $Ω_D=0.7192\pm0.0062$, $b^2=0.146^{+0.030}_{-0.026}$ and $ζ=0.104\pm0.047$ at 1 optimal variance error. For the second interaction, the best fit values at $1σ$ confidence are $Ω_D=0.72091\pm0.0065$, $b^2=0.0395\pm0.0080$ and $ζ\le0.0173$. According to combination of all observational data sets considered in this paper the best fit values for third non-linearly interacting model are $Ω_D=0.7287\pm0.0062$, $b^2=0.0109\pm0.0023$ and $ζ\le0.00764$ at $1σ$ confidence interval. Finally we found that the presence of interaction is compatible in mentioned models via current observational data sets.

astro-ph.CO↗

arXiv2016

Magnetic activity analysis for a sample of G-type main sequence \emph{Kepler} targets

The variation of a stellar light curve owing to the rotational modulation by the magnetic features (starspots and faculae) on the star's surface can be used to investigate the magnetic properties of the host star. In this paper, we use the periodicity and magnitude of the light-curve variation, as two proxies, suggested by (He et al. 2015), to study the stellar magnetic properties for a large sample of G-type main sequence \emph{Kepler} targets, for which the rotation periods recently determined by (McQuillan et al. 2014). By analyzing the correlation between the two magnetic proxies, it is found that: (1) The two proxies are positively correlated for most of the stars in our sample, and the percentages of negative, zero, and positive correlation are $4.27\%$, $6.81\%$, and $88.91\%$, respectively; (2) Negative correlation stars cannot have large magnitude of light-curve variation; (3) with the increase of rotation period, the relative number of positive correlation stars decreases and the negative correlation one increases. These results indicate that the stars with shorter rotation period tend to have positive correlation between the two proxies, and a good portion of the positive correlation stars have larger magnitude of light-curve variation (and hence more intense magnetic activities) than the negative correlation stars.

astro-ph.SR↗

arXiv2016

Agegraphic dark energy: growth index and cosmological implications

We study the main cosmological properties of the agegraphic dark energy model at the expansion and perturbation levels. Initially, using the latest cosmological data we implement a joint likelihood analysis in order to constrain the cosmological parameters. Then we test the performance of the agegraphic dark energy model at the perturbation level and we define its difference from the usual $Λ$CDM model. Within this context, we verify that the growth index of matter fluctuations depends on the choice of the considered agegraphic dark energy (homogeneous or clustered). In particular, assuming a homogeneous agegraphic dark energy we find, for the first time, that the asymptotic value of the growth index is $γ\approx 5/9$, which is close to that of the usual $Λ$ cosmology, $γ^{(Λ)} \approx 6/11$. Finally, if the distribution of dark energy is clustered then we obtain $γ\approx 1/2$ which is $\sim 8\%$ smaller than that of the $Λ$CDM model.

astro-ph.CO↗

arXiv2015

How clustering dark energy affects matter perturbations

The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed $c^{2}_{\rm eff}$ and for $c_{\rm eff}=0$ dark energy clusters in a similar fashion to dark matter while for $c_{\rm eff}=1$ it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, $w_{\rm d}=const$ and $w_{\rm d}=w_0+w_1\left(\frac{z}{1+z}\right)$ with $c_{\rm eff}$ as a free parameter and we try to constrain the dark energy effective sound speed using current available data including SnIa, Baryon Acoustic Oscillation, CMB shift parameter ({\em Planck} and {\em WMAP}), Hubble parameter, Big Bang Nucleosynthesis and the growth rate of structures $fσ_{8}(z)$. At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that $c_{\rm eff}=const$. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at $c_{\rm eff}=0$, however the dark energy sound speed is degenerate with respect to the cosmological parameters, namely $Ω_{\rm m}$ and $w_{\rm d}$.

astro-ph.CO↗

arXiv2010

Compact object detection in self-lensing binary systems with a main-sequence star

Detecting compact objects by means of their gravitational lensing effect on an observed companion in a binary system has already been suggested almost four decades ago. However, these predictions were made even before the first observations of gravitational lensing, whereas nowadays gravitational microlensing surveys towards the Galactic bulge yield almost 1000 events per year where one star magnifies the light of a more distant one. With a specific view on those experiments, we therefore carry out simulations to assess the prospects for detection of the transient periodic magnification of the companion star, which lasts typically only a few hours binaries involving a main-sequence star. We find that detectability is given by the achievability of dense monitoring with the required photometric accuracy. In sharp contrast to earlier expectations by other authors, we find that main-sequence stars are not substantially less favourable targets to observe this effect than white dwarfs. The requirement of an almost edge-on orbit leads to a probability of the order of $3 \times 10^{-4}$ for spotting the signature of an existing compact object in a binary system with this technique. Assuming an abundance of such systems about 0.4 per cent, a high-cadence monitoring every 15~min with 5 per cent photometric accuracy would deliver a signal rate per target star of $γ\sim 4 \times 10^{-7}~\mbox{yr}^{-1}$ at a recurrence period of about 6 months. With microlensing surveys having demonstrated the capability to monitor about $2 \times 10^{8}$ stars, one is therefore provided with the chance to detect roughly semi-annually recurring self-lensing signals from several compact compacts in a binary system. If the photometric accuracy was pushed down to 0.3 per cent, 10 times as many signals would become detectable.

astro-ph.SR↗