دفاع از رساله کارشناسی ارشد (I)

 تشکیل ساختار غیرخطی در کیهان شناسی و مسئله اولین گذار در پدیده های تصادفی غیرمارکوف

Nonlinear structure formation in cosmology and first up-crossing in non-Markov processes

نیما رونقی (دانشکده فیزیک دانشگاه صنعتی شریف )

استاد راهنما: شانت باغرام، سامان مقیمی

 

شنبه ۲۹ دی ۱۳۹۷، ساعت ۱۳:۰۰

    دانشکده فیزیک، تالار پرتوی – طبقه ۴

داستان یک قاتل سریالی!

The story of a serial killer!

فربد حسنی

 Department of Theoretical Physics of the University of Geneva

Abstract: I’m going to introduce a new instability in partial differential equations, which also can be present in some dark energy and cosmological models. We have studied k-essence scalar field and have developed an N-body code, k-evolution, in which we consider non-linearities. We have proved that the k-essence non-linearities suffer from the new instability and blowup in finite time; I’ll show the solution and blowup time for different initial conditions. Moreover, to understand the new instability, I will solve the traffic flow as a similar dynamical system and show how shock waves are produced in the traffic flow.

یکشنبه ۱۶ دی ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳

 Halo bias in the excursion set approach

 Halo bias in the excursion set approach

فرنیک نیک اختر

Department of Physics and Astronomy at the University of Pennsylvania

Abstract: Nonlinear structure formation is an important area in the modern cosmology, where deep questions can be addressed by large-scale structure data, especially the questions which are related to the dark universe – the physics of dark matter and dark energy. In this arena, the abundance and clustering of dark matter halos probe important features of the standard model of cosmology. The excursion set approach is a framework for estimating how the statistical properties of nonlinear structures in the cosmos depend on the history of the universe and the nature of gravity. In this theory, one associates a random walk with each position in the initial density field, and the barrier to be crossed is determined by the collapse model. The main problem here is the first passage time problem and the first up-crossing distribution is the quantity which is related to the abundance of nonlinear objects. My recent work is about approximations of the first up-crossing distribution (for non-Markovian walks) and modeling the abundance of halos, and in this talk, I would like to show the associated implications for halo bias.

یکشنبه ۹ دی ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳

One Day Workshop

An introduction to GAIA data

فرنیک نیک اختر

 Department of Physics and Astronomy at the  University of Pennsylvania

“Gaia is an ambitious mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.”  From   http://sci.esa.int/gaia/

For registration: Send an email to baghram@sharif.edu   till Sunday December 30, 2018 with the title GAIA-Workshop.

The workshop is limited to 15 participants.

چهارشنبه ۱۲ دی ۱۳۹۷، ساعت ۱۱:۰۰ الی ۱۶:۰۰

دانشکده فیزیک، طبقه سوم – سایت کامپیوتر

Lensing of 21cm intensity mapping

 Lensing of 21cm intensity mapping

مونا جلیلوند

 Department of Theoretical Physics of the University of Geneva

 

Abstract: My talk has two parts. First, I talk about second order lensing of 21cm intensity mapping (IM). Like the CMB, 21cm IM temperature fluctuations have second and higher order lensing and no first order lensing. We find a new (third order) lensing term that is neglected in the CMB lensing but is important for 21cm IM. We study the detectability of 21cm IM lensing with a Fisher matrix approach for the redshift range of z=2 to z=6 and find that with optimistic assumptions, we obtain a signal-to-noise of ~10 for futuristic surveys like SKA2.
In the second part, I talk about our current project on estimating the first order lensing from cross correlation between 21cm IM and galaxy surveys. We introduce two new lensing estimators that have higher signal-to-noise compared to the estimator that is currently used for lensing detection.

یکشنبه ۲ دی ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳

Research WEEK in Physics Department of Sharif University

Upcoming Sunday 16 December 2018 /  ۲۵ Azar 1397, there is a one day symposium of research in  Physics Department of Sharif University.
In the attached file please find the program of the event.
Accordingly we will not have our weekly cosmology seminar.
However, it is worth to mention that  Miss Elaheh Khaloei will present her work on Photometry and Polarization of Microlensing events.
 

 Circularly polarized EM radiation from GW binary sources

 Circularly polarized EM radiation from GW binary sources

علیرضا اله یاری (پژوهشکده نجوم پژوهشگاه دانش های بنیادی – IPM )

 

Abstract: We consider the polarization characteristics of the electromagnetic (EM) counterpart of the gravitational wave (GW) created by coalescence of the binary sources. Here, we explore the impact of the photon-graviton interaction on the polarization evolution of X-ray emission of Gamma-Ray Bursts (GRBs). We show that significant circular polarization can be generated due to the gravitational wave from the binary merger. The circular polarization besides photon energy depends on parameters of GW source such as the chirp mass of the binary,  frequency of the GWs and radial distance from the source. Our predicted signal can be used as an indirect probe for GW events and also the nature of photon-graviton interaction. We argue that this polarization signal might be in sensitivity range of upcoming X-ray polarimetry missions.

یکشنبه ۱۱ آذر ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳

سمینار هفتگی گروه کیهانشناسی

Cosmological Filaments in the Light of Excursion Set of Saddle Points

سینا تاملی (دانشکده فیزیک دانشگاه صنعتی شریف )

Abstract: Filaments are one of the components of the cosmic web which can be seen in observations and simulations. They are also considered as a potential solution to missing baryon problem as the host of the baryons which does not reside in dark matter halos. The statistics of the filament could also be used as a probe for cosmology. To benefit from these applications, we need a precise definition for the filaments. There are some models, proposed by the community in order to explain these objects. In this work, we provide a route to find out the statistics of the filaments, regarding them as the saddle points of the density field, through the excursion set approach and ellipsoidal collapse model.  In this presentation, I am going to provide an introduction to Excursion Set of Peaks and saddle points, ellipsoidal collapse model, and the prediction of this framework about the length, number density and mass fraction of the filaments.

یکشنبه ۲۷ آبان ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳

A new perspective on the effective field theory of gravity and its applications

A new perspective on the effective field theory of gravity and its applications

 اصلان صیفی (دانشکده فیزیک دانشگاه صنعتی شریف )

 

Abstract: Scattering amplitude is the only observable in the quantum field theory. Since the last two decades, it has been a huge project to derive the S-matrix efficiently without needing the notion of Lagrangians of effective field theory (EFT)! On the other hand, calculating the gravitational and astrophysical results in the context of EFT is not new. In this lecture, first, I introduce the basic of the modern calculation in the S-matrix theory. Then, by applying this method, I’ll derive some results in the general relativity such as the bending of light, Schwarzschild metric and etc. Additionally, I’ll explain how the S-matrix can determine the constant coefficients in the EFTs. For example, we calculate some coefficients in EFT of extended objects. Finally, I’ll talk about an exciting topic, color-kinematic duality in Yang-Mills theory and gravity. These duality’s applications are being developed in the gravitational radiations.

یکشنبه ۲۰ آبان ۱۳۹۷، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه اول کلاس فیزیک ۳