Search for: perturbation-theory
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    Curvature perturbation and waterfall dynamics in hybrid inflation

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2011, Issue 10 , 2011 ; 14757516 (ISSN) Abolhasani, A. A ; Firouzjahi, H ; Sasaki, M ; Sharif University of Technology
    We investigate the parameter spaces of hybrid inflation model with special attention paid to the dynamics of waterfall field and curvature perturbations induced from its quantum fluctuations. Depending on the inflaton field value at the time of phase transition and the sharpness of the phase transition inflation can have multiple extended stages. We find that for models with mild phase transition the induced curvature perturbation from the waterfall field is too large to satisfy the COBE normalization. We investigate the model parameter space where the curvature perturbations from the waterfall quantum fluctuations vary between the results of standard hybrid inflation and the results... 

    Cosmological perturbations and the Weinberg theorem

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2015, Issue 12 , 2015 ; 14757516 (ISSN) Akhshik, M ; Firouzjahi, H ; Jazayeri, S ; Sharif University of Technology
    Institute of Physics Publishing  2015
    The celebrated Weinberg theorem in cosmological perturbation theory states that there always exist two adiabatic scalar modes in which the comoving curvature perturbation is conserved on super-horizon scales. In particular, when the perturbations are generated from a single source, such as in single field models of inflation, both of the two allowed independent solutions are adiabatic and conserved on super-horizon scales. There are few known examples in literature which violate this theorem. We revisit the theorem and specify the loopholes in some technical assumptions which violate the theorem in models of non-attractor inflation, fluid inflation, solid inflation and in the model of pseudo... 

    Effective field theory of non-attractor inflation

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2015, Issue 7 , 2015 ; 14757516 (ISSN) Akhshik, M ; Firouzjahi, H ; Jazayeri, S ; Sharif University of Technology
    Institute of Physics Publishing  2015
    We present the model-independent studies of non attractor inflation in the context of effective field theory (EFT) of inflation. Within the EFT approach two independent branches of non-attractor inflation solutions are discovered in which a near scale-invariant curvature perturbation power spectrum is generated from the interplay between the variation of sound speed and the second slow roll parameter η. The first branch captures and extends the previously studied models of non-attractor inflation in which the curvature perturbation is not frozen on super-horizon scales and the single field non-Gaussianity consistency condition is violated. We present the general expression for the amplitude... 

    Clustering fossils in solid inflation

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2015, Issue 5 , May , 2015 ; 14757516 (ISSN) Akhshik, M ; Sharif University of Technology
    Institute of Physics Publishing  2015
    In solid inflation the single field non-Gaussianity consistency condition is violated. As a result, the long tenor perturbation induces observable clustering fossils in the form of quadrupole anisotropy in large scale structure power spectrum. In this work we revisit the bispectrum analysis for the scalar-scalar-scalar and tensor-scalar-scalar bispectrum for the general parameter space of solid. We consider the parameter space of the model in which the level of non-Gaussianity generated is consistent with the Planck constraints. Specializing to this allowed range of model parameter we calculate the quadrupole anisotropy induced from the long tensor perturbations on the power spectrum of the... 

    Primordial statistical anisotropies: the effective field theory approach

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2016, Issue 3 , 2016 ; 14757516 (ISSN) Abolhasani, A. A ; Akhshik, M ; Emami, R ; Firouzjahi, H ; Sharif University of Technology
    Institute of Physics Publishing  2016
    In this work we present the effective field theory of primordial statistical anisotropies generated during anisotropic inflation involving a background U(1) gauge field. Besides the usual Goldstone boson associated with the breaking of time diffeomorphism we have two additional Goldstone bosons associated with the breaking of spatial diffeomorphisms. We further identify these two new Goldstone bosons with the expected two transverse degrees of the U(1) gauge field fluctuations. Upon defining the appropriate unitary gauge, we present the most general quadratic action which respects the remnant symmetry in the unitary gauge. The interactions between various Goldstone bosons leads to... 

    Single-field consistency relation and δ N-formalism

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2018, Issue 8 , 2018 ; 14757516 (ISSN) Abolhasani, A. A ; Sasaki, M ; Sharif University of Technology
    Institute of Physics Publishing  2018
    According to the equivalence principal, the long wavelength perturbations must not have any dynamical effect on the short scale physics up to (k2 L/k2 s). Their effect can be always absorbed to a coordinate transformation locally. So any physical effect of such a perturbation appears only on scales larger than the scale of the perturbation. The bispectrum in the squeezed limit of the curvature perturbation in single-field slow-roll inflation is a good example, where the long wavelength effect is encoded in the spectral index through Maldacena's consistency relation. This implies that one should be able to derive the bispectrum in the squeezed limit without resorting to the in-in formalism in... 

    δ M formalism: a new approach to cosmological perturbation theory in anisotropic inflation

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2018, Issue 3 , 2018 ; 14757516 (ISSN) Talebian Ashkezari, A ; Ahmadi, N ; Abolhasani, A. A ; Sharif University of Technology
    Institute of Physics Publishing  2018
    We study the evolution of the metric perturbations in a Bianchi background in the long-wavelength limit. By applying the gradient expansion to the equations of motion we exhibit a generalized "Separate Universe" approach to the cosmological perturbation theory. Having found this consistent separate universe picture, we introduce the δ M formalism for calculating the evolution of the linear tensor perturbations in anisotropic inflation models in almost the same way that the so-called δ N formula is applied to the super-horizon dynamics of the curvature perturbations. Similar to her twin formula, δ N, this new method can substantially reduce the amount of calculations related to the evolution... 

    Primordial black holes in linear and non-linear regimes

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2017, Issue 6 , 2017 ; 14757516 (ISSN) Allahyari, A ; Firouzjaee, J. T ; Abolhasani, A. A ; Sharif University of Technology
    We revisit the formation of primordial black holes (PBHs) in the radiation-dominated era for both linear and non-linear regimes, elaborating on the concept of an apparent horizon. Contrary to the expectation from vacuum models, we argue that in a cosmological setting a density fluctuation with a high density does not always collapse to a black hole. To this end, we first elaborate on the perturbation theory for spherically symmetric space times in the linear regime. Thereby, we introduce two gauges. This allows to introduce a well defined gauge-invariant quantity for the expansion of null geodesics. Using this quantity, we argue that PBHs do not form in the linear regime irrespective of the... 

    Observable quantum loop effects in the sky

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2020, Issue 6 , 2020 Abolhasani, A. A ; Sheikh Jabbari, M. M ; Sharif University of Technology
    Institute of Physics Publishing  2020
    Expanding on [1], we analyze in detail the single field chaotic inflationary models plus a cosine modulation term, augmented by a light scalar field with inflaton dependent oscillatory mass term. We work out in detail the Feynman diagrams and compute one, two and in general estimate higher loop two and three point functions in the in-in formulation. We explicitly establish how the oscillatory mass term can amplify one-loop effects to dominate over the tree as well as the higher loop contributions. The power spectrum of curvature perturbations of this model is hence enhanced compared to the simple single field chaotic model. As a consequence, one can suppress the tensor to scalar ratio r and... 

    Inflationary power asymmetry from primordial domain walls

    , Article Journal of Cosmology and Astroparticle Physics ; Vol. 2014, issue. 11 , 2014 ; ISSN: 14757516 Jazayeri, S ; Akrami, Y ; Firouzjahi, H ; Solomon, A. R ; Wang, Y ; Sharif University of Technology
    We study the asymmetric primordial fluctuations in a model of inflation in which translational invariance is broken by a domain wall. We calculate the corrections to the power spectrum of curvature perturbations; they are anisotropic and contain dipole, quadrupole, and higher multipoles with non-trivial scale-dependent amplitudes. Inspired by observations of these multipole asymmetries in terms of two-point correlations and variance in real space, we demonstrate that this model can explain the observed anomalous power asymmetry of the cosmic microwave background (CMB) sky, including its characteristic feature that the dipole dominates over higher multipoles. We test the viability of the... 

    Statistical anisotropies in gravitational waves in solid inflation

    , Article Journal of Cosmology and Astroparticle Physics ; Vol. 2014,Issue. 9 , 2014 ; ISSN: 14054179 Akhshik, M ; Emami, R ; Firouzjahi, H ; Wang, Y ; Sharif University of Technology
    Solid inflation can support a long period of anisotropic inflation. We calculate the statistical anisotropies in the scalar and tensor power spectra and their cross-correlation in anisotropic solid inflation. The tensor-scalar cross-correlation can either be positive or negative, which impacts the statistical anisotropies of the TT and TB spectra in CMB map more significantly compared with the tensor self-correlation. The tensor power spectrum contains potentially comparable contributions from quadrupole and octopole angular patterns, which is different from the power spectra of scalar, the cross-correlation or the scalar bispectrum, where the quadrupole type statistical anisotropy dominates... 

    Late time sky as a probe of steps and oscillations in primordial Universe

    , Article Journal of Cosmology and Astroparticle Physics ; Volume 2018, Issue 1 , 2018 ; 14757516 (ISSN) Ansari Fard, M ; Baghram, S ; Sharif University of Technology
    The standard model of cosmology with nearly Gaussian, isotropic, scale invariant and adiabatic initial conditions describes the cosmological observations well. However, the study of any deviation from the mentioned conditions will open up a new horizon to the physics of early universe. In this work, we study the effect of the oscillatory and step-like features in potentials of inflationary models in late time large scale structure observations. Mainly we study the matter power spectrum, number density of the structures, dark matter halo bias and specifically CMB lensing. We show that the oscillatory models can introduce some degeneracy with late time effects on BAO scale. We also conclude... 

    Analytical solutions for generalized duffing equation

    , Article Nonlinear Approaches in Engineering Applications: Advanced Analysis of Vehicle Related Technologies ; 2016 , Pages 263-278 ; 9783319270555 (ISBN) Aghdam, M. M ; Fallah, A ; Sharif University of Technology
    Springer International Publishing  2016
    Structures like beams, plates, shells, and sectors have a key role in different fields of science and engineering. Reliable design of structures requires detailed understanding of their behavior in different service conditions including statics, dynamics, and vibration. For instance, dynamics and vibration characteristics of structures such as natural frequencies, mode shapes, and damping ratios are among interesting properties. Furthermore, in most severe conditions, linear theories are not enough to accurately capture real structural behavior. Thus, investigation of nonlinear and/or large amplitude vibration of structures which is normally governed by system of nonlinear differential... 

    A new analytical expression for the free energy of hard-core fluids

    , Article Journal of the Physical Society of Japan ; Volume 72, Issue 11 , 2003 , Pages 2747-2753 ; 00319015 (ISSN) Farrokhpour, H ; Parsafar, G ; Sharif University of Technology
    A new potential function, which can in a simple and reasonable manner represent the molecular interaction of many kinds of hard-core fluids by varying the value of its parameter, is proposed. For prediction of thermodynamic properties of hard-core fluids such as the square-well (SW) and hard core Lennard-Jones (HCLJ), a simple perturbed equation of state (EOS) is derived by using the new potential function along with the Barker-Henderson perturbation theory. This method yields a simple and general analytical expression for each thermodynamic property of such fluids. The most important feature of these expressions is that they have no adjustable parameter and in some regions in which there is... 

    Monte Carlo simulation of 2-ethoxyethanol in continuum configurational biased procedure: Conformational analysis and association in aqueous and non-aqueous media

    , Article Theoretical Chemistry Accounts ; Volume 107, Issue 3 , 2002 , Pages 162-172 ; 1432881X (ISSN) Tafazzoli, M ; Jalili, S ; Sharif University of Technology
    Springer New York  2002
    Monte Carlo simulations have been carried out for 2-ethoxyethanol (C 2E1) in isothermal-isobaric ensemble (NPT) at different temperatures and 1 atm pressure with a continuum configurational biased procedure in water and chloroform media. Hydrogen bond bridges were formed between adjacent oxygen atoms in C2E1 (CH 3CH2OCH2CH2OH) through water molecules. We also found that the stable conformers of C2E 1 in water and CHCl3 are different and the effect of temperature on solute-solvent interaction energies is considerable. The self-association of C2E1 in aqueous and nonaqueous media has been studied by statistical perturbation theory, and the relative free energy has been obtained at different... 

    Towards new thermodynamic regularities for dense fluids based on the effective attraction pair potential via the perturbation theory

    , Article Journal of Molecular Liquids ; Volume 220 , 2016 , Pages 623-630 ; 01677322 (ISSN) Sohrabi Mahboub, M ; Farrokhpour, H ; Parsafar, G. A ; Sharif University of Technology
    Elsevier B.V  2016
    In the present work, several new thermodynamic linear isotherm regularities for the dense fluids have been derived for the first time. For this purpose, the thermodynamic perturbation theory (TPT) employing only the attractive effective pair potential (AEPP) as u(r) = - ϵeff (σeff / r)m was used, where σeff which is the effective hard core diameter, is temperature dependent and m > 0. Based on the derived regularities, the isotherm (Z - Z(0))v2 is a linear function of ρ2, ρ and 1/ρ, depends on the values of m = 12, 9 and 6, respectively where Z - Z(0) is the difference between the experimental compressibility factor of the real fluid (Z) and that of the reference fluid (Z(0)). Also, Z - Z(0)... 

    Damage detection of offshore jacket structures using frequency domain selective measurements

    , Article Journal of Marine Science and Application ; Volume 12, Issue 2 , June , 2013 , Pages 193-199 ; 16719433 (ISSN) Kianian, M ; Golafshani, A. A ; Ghodrati, E ; Sharif University of Technology
    The development of damage detection techniques for offshore jacket structures is vital to prevent catastrophic events. This paper applies a frequency response based method for the purpose of structural healthCgreater detection efficiency. In addition, the performance of the proposed method was evaluated in relation to multiple damages. The aforementioned points are illustrated using the numerical study of a two dimensional jacket platform, and the results proved to be satisfactory utilizing the proposed methodology  

    Application of Perturbation Theory in Fuel Management Optimization of VVER-1000 Reactor

    , M.Sc. Thesis Sharif University of Technology Hosseini, Mohammad (Author) ; Vossoughi, Nasser (Supervisor)
    One of the methods to enhance the economic performance and safety factor of nuclear power plants is to determine a suitable fuel loading strategy in a reactor core. Different methods have been already elaborated to choose the proper fuel loading pattern. Selection of the best pattern for fuel loading ensures the best use of fissile material and more safety during reactor operation. Perturbation theory method is one the methods which could be used to solve this kind of problems. The effect of non-uniform perturbations on the reactor operation can be evaluated by multigroup calculations and using proper space dependent group constants. In this study, a C# based program is developed to find the... 

    Analysis & Design of Bragg Mirror-Based Optical Fibers

    , M.Sc. Thesis Sharif University of Technology Jazayeri, Amir Mohammad (Author) ; Mehrany, Khashayar (Supervisor)
    The Bragg reflection mechanism is in this thesis employed to design optical fibers with specific characteristics. First, optical Bragg fibers (BF) with radial periodicity are considered and optimized to have the minimum leakage loss by introduction of a new parameter referred to as the point-wise wave-number. In this fashion, the optical effects of geometrical factors of the to-be-designed fiber are brought together in the point-wise wave-number and the sought-after parameters of the fiber are thus analytically found. Despite a few approximations made in extraction of optimum parameters, rigorous numerical analysis of the here-proposed optical BF clearly demonstrates that it outperforms... 

    An Investigation on the Resonant, Frequency and Nonlinear Behavior of Atomic Force Microscope (AFM) Modeled on the Basis of the Nonlocal Theory

    , M.Sc. Thesis Sharif University of Technology khosravani, Ehsan (Author) ; Ahmadian, Mohammad Taghi (Supervisor) ; Mehdigholi, Hamid (Supervisor)
    Atomic force microscope (AFM) has a significant capability of imaging surface topography on an atomic scale. It is also widely used for Nanolithography in MEMS/NEMS (micro/nanoelectromechanical systems). Although the conventional AFMs have a significant capability of nano-scale surface measurements, their probe tips cannot come in close proximity to the sidewalls, no matter how sharp they are. Therefore, no accurate nano-scale measurements at sidewalls and edges can be achieved by these conventional AFMs. In addition, regarding the increasing applications of micro-structures such as micro nozzles, micro gears, micro holes and micro trenches which all of them have sidewalls and edges,...