arXiv subjects

Shahram Khosravi

Publications and source records attributed to Shahram Khosravi.

13 recordsLinked to original sources

arXiv2024

Testing No slip model with pulsar timing arrays: NANOGrav and IPTA

We perform an observational study of modified gravity considering a potential inflationary interpretation of pulsar timing arrays (PTA). We use a motivated model known as no slip in which the gravitational wave propagation is modified. Specifically, by using two different parametrizations for the model, we find the approximate transfer functions for tensor perturbations. In this way, we obtain the spectral energy density of gravitational waves and use NANOGrav and IPTA second data release to constrain parameters of the model. We find that there is degeneracy between the model parameters $ξ$ and $c_M$. For $c_M$, we only get an upper bound on the parameter. Thus, it is difficult to constrain them with percent level accuracy with the current PTA data.

gr-qc↗

arXiv2020

Perturbation level interacting dark energy model and its consequence on late-time cosmological parameters

In the present paper, we study the capability of interacting dark energy model with pure momentum transfer in the dark sector to reconcile tensions between low redshift observations and cosmic microwave background (CMB) results. This class of interacting model with pure momentum exchange introduces modifications to the standard model in the level of perturbation. We investigate the model by comparing to observational data, including integrated Sachs-Wolfe-galaxy cross-correlation, galaxy power spectrum, $f σ_8$, and CMB data. It is shown that this model can alleviate the observed tension between local and global measurements of $σ_8$. According to our results, the best fit value of $σ_8$ for interacting model is $0.700$, which is lower than the one for $Λ$CDM model and also is consistent with low redshift observations. Furthermore, we perform a forecast analysis to find the constraints on parameters of the interacting model from future experiments.

astro-ph.CO↗

arXiv2019

On structure formation from a small-scales-interacting dark sector

We consider a cosmological model with an interaction between dark matter and dark energy which leaves the background cosmology unaffected and only affects the evolution of the perturbations. This is achieved by introducing a coupling given in terms of the relative velocities of dark matter and dark energy. This interaction has the distinctive feature of appearing predominantly on small scales, where peculiar velocities can become important. We confront the predictions of the model to cosmological observations and find a potential alleviation of the known tension in the amplitude of density perturbations as measured by low redshift galaxy surveys and the Planck data. The model also predicts a shift in the turnover of the matter power spectrum which does not depend on the horizon at equality (fixed by the background cosmology and, thus, unaffected by the perturbations) and is entirely due to the interaction between dark matter and dark energy. A bias in the peculiar velocity between baryons and dark matter is also shown to be a unique feature of this type of interactions in the dark sector.

astro-ph.CO↗

arXiv2019

21-cm power spectrum and ionization bias as a probe of long-mode modulated non Gaussian sky

The observed hemispherical power asymmetry in cosmic microwave background radiation can be explained by long wavelength mode (long-mode) modulation. In this work we study the prospect of the detection of this effect in the angular power spectrum of 21-cm brightness temperature. For this task, we study the effect of the neutral Hydrogen distribution on the angular power spectrum. This is done by formulating the bias parameter of ionized fraction to the underlying matter distribution. We also discuss the possibility that the long mode modulation is companied with a primordial non-Gaussianity of local type. In this case, we obtain the angular power spectrum with two effects of primordial non-Gaussianity and long mode modulation. Finally, we show that the primordial non-Gaussianity enhances the long mode modulated power of 21-cm signal via the non-Gaussian scale-dependent bias up to four orders of magnitude. {Accordingly, the observation of 21-cm signal with upcoming surveys such as the Square Kilometer Array (SKA) is probably capable of detecting hemispherical power asymmetry in the context of the long mode modulation.

astro-ph.CO↗

arXiv2016

ISW-Galaxy Cross Correlation:A probe of Dark Energy clustering and distribution of Dark Matter tracers

Cross correlation of the Integrated Sachs-Wolfe signal (ISW) with the galaxy distribution in late time is a promising tool for constraining the dark energy properties. Here, we study the effect of dark energy clustering on the ISW-galaxy cross correlation and demonstrate the fact that the bias parameter between the distribution of the galaxies and the underlying dark matter introduces a degeneracy and complications. We argue that as the galaxy's host halo formation time is different from the observation time, we have to consider the evolution of the halo bias parameter. It will be shown that any deviation from $Λ$CDM model will change the evolution of the bias as well. Therefore, it is deduced that the halo bias depends strongly on the sub-sample of galaxies which is chosen for cross correlation and that the joint kernel of ISW effect and the galaxy distribution has a dominant effect on the observed signal. In this work, comparison is made specifically between the clustered dark energy models using two samples of galaxies. The first one is a sub-sample of galaxies from Sloan Digital Sky Survey, chosen with the r-band magnitude $18 < r < 21$ and the dark matter halo host of mass $M \sim10^{12}M_{\odot}$ and formation redshift of $z_{f}\sim 2.5$. The second one is the sub-sample of Luminous Red galaxies with the dark matter halo hosts of mass $M \sim 10^{13}M_{\odot}$ and formation redshift of $z_{f}\sim 2.0$. Using the evolved bias we improve the $χ^2$ for the $Λ$CDM which reconciles the $\sim$1$σ$-2$σ$ tension of the ISW-galaxy signal with $Λ$CDM prediction.[abridged]

astro-ph.CO↗

arXiv2014

Simultaneous effect of Modified Gravity and Primordial Non-Gaussianity in Large Scale Structure Observations

In this work we study the simultaneous effect of primordial non-Gaussianity and the modification of the gravity in $f(R)$ framework on large scale structure observations. We show that non-Gaussianity and modified gravity introduce a scale dependent bias and growth rate functions. The deviation from $Λ$CDM in the case of primordial non-Gaussian models is in large scales, while the growth rate deviates from $Λ$CDM in small scales for modified gravity theories. We show that the redshift space distortion can be used to distinguish positive and negative $f_{NL}$ in standard background, while in $f(R)$ theories they are not easily distinguishable. The galaxy power spectrum is generally enhanced in presence of non-Gaussianity and modified gravity. We also obtain the scale dependence of this enhancement. Finally we define galaxy growth rate and galaxy growth rate bias as new observational parameters to constrain cosmology.

astro-ph.CO↗

arXiv2012

Local Features with Large Spiky non-Gaussianities during Inflation

We provide a dynamical mechanism to generate localized features during inflation. The local feature is due to a sharp waterfall phase transition which is coupled to the inflaton field. The key effect is the contributions of waterfall quantum fluctuations which induce a sharp peak on the curvature perturbation which can be as large as the background curvature perturbation from inflaton field. Due to non-Gaussian nature of waterfall quantum fluctuations a large spike non-Gaussianity is produced which is narrowly peaked at modes which leave the Hubble radius at the time of phase transition. The large localized peaks in power spectrum and bispectrum can have interesting consequences on CMB anisotropies.

astro-ph.CO↗

arXiv2011

Level Crossing Analysis of Cosmic Microwave Background Radiation: A method for detecting cosmic strings

In this paper we study the footprint of cosmic string as the topological defects in the very early universe on the cosmic microwave background radiation. We develop the method of level crossing analysis in the context of the well-known Kaiser-Stebbins phenomenon for exploring the signature of cosmic strings. We simulate a Gaussian map by using the best fit parameter given by WMAP-7 and then superimpose cosmic strings effects on it as an incoherent and active fluctuations. In order to investigate the capability of our method to detect the cosmic strings for the various values of tension, $Gμ$, a simulated pure Gaussian map is compared with that of including cosmic strings. Based on the level crossing analysis, the superimposed cosmic string with $Gμ\gtrsim 4\times 10^{-9}$ in the simulated map without instrumental noise and the resolution $R=1'$ could be detected. In the presence of anticipated instrumental noise the lower bound increases just up to $Gμ\gtrsim 5.8\times 10^{-9}$.

astro-ph.CO↗

arXiv2010

Cosmic Strings Collision in Cosmological Backgrounds

The collisions of cosmic strings loops and the dynamics of junctions formations in expanding backgrounds are studied. The key parameter controlling the dynamics of junctions formation, the cosmic strings zipping and unzipping is the relative size of the loops compared to the Hubble expansion rate at the time of collision. We study analytically and numerically these processes for large super-horizon size loops, for small sub-horizon size loops as well as for loops with the radii comparable to the Hubble expansion rate at the time of collision.

hep-th↗

arXiv2009

Zipping and Unzipping of Cosmic String Loops in Collision

In this paper the collision of two cosmic string loops is studied. After collision junctions are formed and the loops are entangled. We show that after their formation the junctions start to unzip and the loops disentangle. This analysis provides a theoretical understanding of the unzipping effect observed in numerical simulations of a network of cosmic strings with more than one type of cosmic strings. The unzipping phenomena have important effects in the evolution of cosmic string networks when junctions are formed upon collision, such as in a network of cosmic superstrings.

hep-th↗

arXiv2009

Modified Averaging Processes in Cosmology and the Structured FRW model

We study the volume averaging of inhomogeneous metrics within GR and discuss its shortcomings such as gauge dependence, singular behavior as a result of caustics, and causality violations. To remedy these shortcomings, we suggest some modifications to this method. As a case study we focus on the inhomogeneous structured FRW model based on a flat LTB metric. The effect of averaging is then studied in terms of an effective backreaction fluid. It is shown that, contrary to the claims in the literature, the backreaction fluid behaves like a dark matter component, instead of dark energy, having a density of the order of 10^{-5} times the matter density, and most importantly, it is gauge dependent.

astro-ph↗

arXiv2009

Gravitational Radiation by Cosmic Strings in a Junction

The formalism for computing the gravitational power radiation from excitations on cosmic strings forming a junction is presented and applied to the simple case of co-planar strings at a junction when the excitations are generated along one string leg. The effects of polarization of the excitations and of the back-reaction of the gravitational radiation on the small scale structure of the strings are studied.

hep-th↗

arXiv2006

Evolution of Thick Walls in Curved Spacetimes

We generalize our previous thick shell formalism to incorporate any codimension-1 thick wall with a peculiar velocity and proper thickness bounded by arbitrary spacetimes. Within this new formulation we obtain the equation of motion of a spherically symmetric dust thick shell immersed in vacuum as well as in Friedmann-Robertson-Walker spacetimes.

gr-qc↗