En
  • دکتری (1360)

    کیهانشناسی

    ام-ای-تی، گمبریج، امریکا

  • کارشناسی (1353)

    فیزیک

    دانشگاه صنعتی شریف،

  • کیهان شناسی
  • گرانش
  • فیزیک نجومی

    داده ای یافت نشد

    ارتباط

    رزومه

    Gravitational waves in a closed spacetime via deviation equation

    J Khodagholizadeh, AH Abbassi, A Vahedi, K Babaei
    Journal Papers , , {Pages }

    Abstract

    Signatures of Spatial Curvature on Growth of Structures

    Mohammad H Abbassi, Amir H Abbassi
    Journal PapersarXiv preprint arXiv:2006.06347 , 2020 June 11, {Pages }

    Abstract

    We write down Boltzmann equation for massive particles in a spatially curved FRW universe and solve the approximate line-of-sight solution for evolution of matter density, including the effects of spatial curvature to the first order of approximation. It is shown that memory of early time gravitational potential is affected by presence of spatial curvature. Then we revisit Boltzmann equation for photons in the general FRW background. Using it, we show that how the frequency of oscillations and damping factor (known as Silk damping) changed in presence of spatial curvature. At last, using this modified damping factor in hydrodynamic regime of cosmological perturbations, we find our analytic solution which shows the effects of spatial curvatu

    Curvature and topology dependency of the cosmological spectra

    Ali A Asgari, Amir H Abbassi, Jafar Khodagholizadeh
    Journal PapersResults in Physics , Volume 12 , 2019 March 1, {Pages 2225-2233 }

    Abstract

    In this article we investigate dependency of two important cosmological random fields defined on spatial slices of the FLRW universe and their spectra on the geometry and topology of the background universe. Our discussion includes the post-inflationary universe ie radiation-dust mixture era. For this purpose, we first extract an explicit equation describing evolution of the comoving curvature perturbation in the FLRW universe with arbitrary spatial sectional curvature. We may realize when K≠ 0, curvature scale may be as significant as the perturbations scales to recognize the behavior of the spectral indices. We also focus on the entropy perturbation in order to extract behavior of the isocurvature spectral index in terms of the curvatur

    An analytical approach to the CMB polarization in a spatially closed background

    Pedram Niazy, Amir H Abbassi
    Journal PapersPhysics of the dark universe , Volume 19 , 2018 March 1, {Pages 36-45 }

    Abstract

    The scalar mode polarization of the cosmic microwave background is derived in a spatially closed universe from the Boltzmann equation using the line of sight integral method. The EE and TE multipole coefficients have been extracted analytically by considering some tolerable approximations such as considering the evolution of perturbation hydrodynamically and sudden transition from opacity to transparency at the time of last scattering. As the major advantage of analytic expressions, C E E, ℓ S and C T E, ℓ S explicitly show the dependencies on baryon density Ω B, matter density Ω M, curvature Ω K, primordial spectral index n s, primordial power spectrum amplitude A s, Optical depth τ r e i o n, recombination width σ t and recombina

    An analytical approach to the CMB anisotropies in a spatially closed background

    Pedram Niazy, Amir H Abbassi
    Journal PapersAstroparticle Physics , Volume 94 , 2017 September 1, {Pages 44-55 }

    Abstract

    The scalar mode temperature fluctuations of the cosmic microwave background has been derived in a spatially closed universe from two different methods. First, by following the photon trajectory after the last scattering and then from the Boltzmann equation in a closed background and the line of sight integral method. An analytic expression for the temperature multipole coefficient has been extracted at the hydrodynamical limit, where we have considered some tolerable approximations. By considering a realistic set of cosmological parameters taken from a fit to data from Planck, the TT power spectrum in the scalar mode for the closed universe has been compared with numerical one by using the CAMB code and also latest observational data. The a

    Imprints of intrinsic and exterior curvatures in cosmology

    Ali A Asgari, Amir H Abbassi
    Journal PapersarXiv preprint arXiv:1712.09358 , 2017 December 26, {Pages }

    Abstract

    Nowadays, according to the observational evidences the curvature parameter of the universe is neglected and spatially flat FLRW model is on the top of interest for cosmologists. However, due to some discrepancies between -CDM model anticipations and observations, one may think out the curvature parameter as the solution even though it may be very small. So, in this article we investigate the eras in which the curvature influence was or is more significant. In addition, a geometrical interpretation of Hubble parameter is dedicated. We find that the Hubble parameter is more appropriate to be defined as the mean curvature of the spatial section of the universe because the concept of the scale factor in a non-homogeneous universe is not precise

    Damping of gravitational waves in matter and Λ− dominated era

    Jafar Khodagholizadeh, Amir H Abbassi, Ali A Asgari
    Journal PapersAstroparticle Physics , Volume 95 , 2017 October 1, {Pages 05-Jan }

    Abstract

    The goal of this paper is studying the integro-differential equation of gravitational waves (GWs) in matter-dominated era while the presence of free streaming neutrinos is taken into account via stress tensor with high accuracy. Our focus is on GWs that enter the horizon in the matter dominated era and transmitted to the Λ− dominated era. The results show that the squared amplitude is reduced by∼ 3%(61%) in the matter dominated era and this reduction is less after transition for the flat (closed) background. Finally, we compared with the radiation-dominated era for both flat and closed background as well. A stable global topological effect has been inferred from comparison of results for two cases.

    Tensor Modes Damping in Matter and Vacuum Dominated Era

    Jafar Khodagholizadeh, Amir H Abbassi, Ali A Asgari
    Journal PapersarXiv preprint arXiv:1608.01472 , 2016 August 4, {Pages }

    Abstract

    The present paper has developed an integro-differential equation to propagate cosmological gravitation waves in matter-dominated era in accounting for the presence of free streaming neutrinos as a traceless transverse tensor part of the anisotropic stress tensor. Its focus is on short and long wavelengths of GWs that enter the horizon in matter-dominated era. Results show that the anisotropic stress reduces the squared amplitude by for wavelengths, entering the horizon during matter-dominated phase. This reduction is less for those wavelengths that enter the horizon at dominated era in flat spacetime. All of the calculations have been done in closed spacetime and the results have been compared with the radiation-dominated case for both fl

    Slow-roll inflationary scenario in the maximally extended background

    Ali A Asgari, Amir H Abbassi
    Journal PapersThe European Physical Journal C , Volume 75 , Issue 11, 2015 November 1, {Pages 544 }

    Abstract

    During the inflationary epoch, the geometry of the universe may be described by a quasi-de Sitter space. On the other hand, the maximally extended de Sitter metric in the comoving coordinates accords with a special FLRW model with positive spatial curvature; therefore, the focus of the present paper is on the positively curved inflationary paradigm, for which we first of all derive the power spectra of comoving curvature perturbation and primordial gravitational waves in a positively curved FLRW universe according to the slowly rolling inflationary scenario. It can be shown that the curvature spectral index in this model automatically has a small negative running parameter, compatible with observational measurements. Afterwar

    Scalar and tensor power spectra in a universe with positive curvature index and maximally symmetric spatial section

    Ali A Asgari, Amir H Abbassi
    Journal PapersPhysics of the Dark Universe , Volume 7 , 2015 March 1, {Pages 05-Jan }

    Abstract

    Recently, some departures from the Λ CDM model have been reported by WMAP and Planck satellites such as suppression of the lowest CMB multipoles, and the hemispherical asymmetry in CMB anisotropy plan. It motivates us to consider models with nonzero curvature index and study the imprints of curvature and topology on the observed anomalies. So in this article we study the positively curved inflationary paradigm. For this purpose, first, we derive the power spectra of comoving curvature perturbation and primordial gravitational waves in the FLRW universes with K=+ 1 and maximally symmetry spatial sections according to the slowly rolling inflationary scenario. Our study includes two spherical and elliptical models with S 3 and R P 3 as the sp

    Physics of the Dark Universe

    Ali A Asgari, Amir H Abbassi
    Journal Papers , 2015 March 9, {Pages }

    Abstract

    Inflationary cosmology which was proposed in the early 1980s, extends the standard Big-Bang model by postulating an early epoch of nearly exponential expansion in order to resolve a number of puzzles of the Big-Bang cosmology such as flatness, horizon and monopole problems [1–3]. Inflation also explains the origin of the CMB anisotropies and large scale structure of the cosmos, indeed quantum vacuum fluctuations of the inflation field (s) magnified to the cosmic sized classical perturbations after the horizon exit time and became the seeds for the growth of the structure and CMB anisotropies in the universe [4–7]. Before the advent of inflation the initial perturbations were postulated and their spectrum was supposed to be scalar-invari

    Damping of tensor mode in spatially closed cosmology

    Jafar Khodagholizadeh, Amir H Abbassi, Ali A Asgari
    Journal PapersPhysical Review D , Volume 90 , Issue 6, 2014 September 22, {Pages 063520 }

    Abstract

    We derive an integro-differential equation for propagation of cosmological gravitation waves in spatially closed cosmology whereas the traceless transverse tensor part of the anisotropic stress tensor is free-streaming neutrinos (including antineutrinos), which have been traveling essentially without collision since temperature dropped below about 1 0 10 K. We studied the short wavelengths and long wavelengths of gravitational waves (GWs) that enter the horizon in closed spacetime. The solution shows that the anisotropic stress reduces the squared amplitude by 76% for wavelengths that enter the horizon during radiation-dominated phase and this reduction is less for the wavelength that enters the horizon at later times. At the end we compare

    Evolution of the spectral index after inflation

    Ali A Asgari, Amir H Abbassi
    Journal PapersJournal of Cosmology and Astroparticle Physics , Volume 2014 , Issue 09, 2014 September 24, {Pages 042 }

    Abstract

    In this article we investigate the time evolution of the adiabatic (curvature) and isocurvature (entropy) spectral indices after inflation era for all cosmological scales with two different initial conditions. For this purpose, we first extract an explicit equation for the time evolution of the comoving curvature perturbation (which may be known as the generalized Mukhanov-Sasaki equation). It would be cleared that the evolution of adiabatic spectral index severely depends on the initial conditions moreover, as expected it is constant only for the super-Hubble scales and adiabatic initial conditions. Additionally, the adiabatic spectral index after recombination approaches a constant value for the isocurvature perturbations. Finally, we re-

    On the perturbation theory in spatially closed background

    Ali A Asgari, Amir H Abbassi, Jafar Khodagholizadeh
    Journal PapersThe European Physical Journal C , Volume 74 , Issue 6, 2014 June 1, {Pages 2917 }

    Abstract

    In this article, we investigate some features of the perturbation theory in a spatially closed universe. We will show that the perturbative field equations in a spatially closed universe always have two independent adiabatic solutions provided that the wavelengths of perturbation modes are very much longer than the Hubble horizon. It will be revealed that these adiabatic solutions do not depend on the curvature directly. We also propose a new interpretation for the curvature perturbation in terms of the unperturbed background geometry.

    Gravitational waves in a spatially closed de Sitter spacetime

    Amir H Abbassi, Jafar Khodagholizadeh, Amir M Abbassi
    Journal PapersThe European Physical Journal C , Volume 73 , Issue 10, 2013 October 1, {Pages 2592 }

    Abstract

    Perturbation of gravitational fields may be decomposed into scalar, vector and tensor components. In this paper we concern with the evolution of tensor mode perturbations in a spatially closed de Sitter background of Robertson–Walker form. It may be thought as gravitational waves in a classical description. The chosen background has the advantage of to be maximally extended and symmetric. Spatially flat models commonly emerge from inflationary scenarios are not completely extended. We first derive the general weak field equations. Then the form of the field equations in two special cases, planar and spherical waves, are obtained and their solutions are presented. The radiation field from an isolated source is calculated. We

    Density-metric unimodular gravity: Vacuum maximal symmetry

    Amir H Abbassi, Amir M Abbassi
    Journal PapersAnnals of Physics , Volume 326 , Issue 5, 2011 May 1, {Pages 1161-1173 }

    Abstract

    We have investigated the vacuum maximally symmetric solutions of recently proposed density-metric unimodular gravity theory. The results are widely different from inflationary scenario. The exponential dependence on time in deSitter space is substituted by a power law. Open space–times with non-zero cosmological constant are excluded.

    ON KERR–DE SITTER METRIC

    Amir H Abbassi, Sh Khosravi, Amir M Abbassi
    Journal PapersInternational Journal of Modern Physics A , Volume 25 , Issue 04, 2010 February 10, {Pages 837-859 }

    Abstract

    We present our derivations for Kerr–de Sitter metric in a proper comoving coordinate system. It asymptotically approaches to the de Sitter metric in Robertson–Walker form. This has been done by considering the stationary axially-symmetric space–time in which motion of particle is integrable. That is the Hamilton–Jacobi and Klein–Gordon equations are separable. In this form it is asymptotically consistent with comoving frame.

    Energy-momentum distribution in static and nonstatic cosmic string space-times

    Amir M Abbassi, Saeed Mirshekari, Amir H Abbassi
    Journal PapersPhysical Review D , Volume 78 , Issue 6, 2008 September 18, {Pages 064053 }

    Abstract

    We elaborate the problem of energy-momentum in general relativity by energy-momentum prescriptions theory. In this regard, we calculate M?ller, Landau-Lifshitz, Papapetrou, Einstein, Bergmann, Tolman, and Weinberg’s energy-momentum complexes in static and nonstatic cosmic string space-times. We obtain strong coincidences between the results. These coincidences can be considered an extension of Virbhadra’s viewpoint that different energy-momentum prescriptions may provide some basis to define a unique quantity. In addition, our results disagree with Lessner’s belief about M?ller’s prescription and support the Virbhadra’s conclusion about the power of Einstein’s prescription.

    Density-metric unimodular gravity: vacuum spherical symmetry

    Amir H Abbassi, Amir M Abbassi
    Journal PapersClassical and Quantum Gravity , Volume 25 , Issue 17, 2008 August 19, {Pages 175018 }

    Abstract

    We analyze an alternative theory of gravity characterized by metrics that are tensor density of rank (0, 2) and weight-\frac12. The metric compatibility condition is supposed to hold. The simplest expression for the action of gravitational field is used. Taking the metric and trace of connections as dynamical variables, the field equations in the absence of matter and other kinds of sources are derived. The solutions of these equations are obtained for the case of vacuum static spherical symmetric spacetime. The null geodesics and advance of perihelion of ellipses are discussed. We confirm a subclass of solutions is regular for r> 0 and there is no event horizon while it is singular at r= 0.

    ANTI-PREPRINTS IN PARTICLES AND FIELDS

    Don N Page, Bernard J Carr, Matthew J Francis, Geraint F Lewis, Eric V Linder, A Bershadskii, Matthias Stute, Davoud Kamani, Christian Wagner, Volker Muller, Matthias Steinmetz, Stanley Deser, Thomas H Greif, Jarrett L Johnson, Volker Bromm, Balazs Szendr
    Journal PapersPhys. Rev , Volume 78 , Issue 064044, 2008 January , {Pages }

    Abstract

    This list of high-energy physics anti-preprints displayed last week in the SLAC Library. See also the current list. For more info see the bottom of this page. Anti-preprints are arranged by institutions (as represented by report numbers). Send comments or questions to library@ slac. stanford. edu.

    /pro/academic_staff/ahabbasi/publication

    دروس نیمسال جاری

      داده ای یافت نشد

    دروس نیمسال قبل

      داده ای یافت نشد
      داده ای یافت نشد
      داده ای یافت نشد

    مهم

    جدید

      اطلاعیه ای درج نشده است