Closed FLRW model in loop quantum cosmology and its effective dynamics

آسیه کرمی (پژوهشکده نجوم پژوهشگاه دانش های بنیادی – IPM)

Abstract: I will review the quantization and effective dynamics of closed FLRW model within loop quantum cosmology, paying special attention to the existence of an ambiguity in the quantization process. This ambiguity arises from different ways of defining the curvature operator. One definition is through closed paths and the other one is through open paths. For spatially flat models these two methods coincide and for spatially non-flat anisotropic models the first one is not admissible. However in the case of the closed FLRW model, one can apply both methods and they yield different quantum theories. In this talk I will show the different effective descriptions they provide of the bouncing cyclic universe. In particular, the most significant difference is that in the theory defined by the ‘open path’ method, there is not one, but two different bounces through which the cyclic universe alternates. Also, I will show that the quantum corrections of both theories can cure the unboundedness of matter density and expansion in effective phase space.

یکشنبه 9 آبان ماه 1395، ساعت 15:00

    دانشکده فیزیک، تالار پرتوی

Convective dynamo action in a spherical shell: symmetries and modulation

Raphaël Raynaud

 School of Astronomy – IPM

 Abstract:  We consider dynamo action driven by three-dimensional rotating anelastic convection in a spherical shell. Motivated by the behaviour of the solar dynamo, we examine the interaction of hydromagnetic modes with different symmetries and demonstrate how complicated interactions between convection, differential rotation and magnetic fields may lead to modulation of the basic cycle. For some parameters, Type 1 modulation occurs by the transfer of energy between modes of different symmetries with little change in the overall amplitude; for other parameters, the modulation is of Type 2, where the amplitude is significantly affected (leading to grand minima in activity) without significant changes in symmetry. Most importantly, we identify the presence of ‘supermodulation’ in the solutions, where the activity switches chaotically between Type 1 and Type 2 modulation; this is believed to be an important process in solar activity.

یکشنبه 2 آبان ماه 1395، ساعت 15:00

    دانشکده فیزیک، تالار پرتوی

پذیرش محقق پسادکتری در گروه کیهان شناسی دانشکده فیزیک دانشگاه صنعتی شریف

بدین وسیله به اطلاع می رساند که گروه کیهان شناسی دانشکده فیزیک دانشگاه صنعتی شریف، از متقاضیان برای موقعیت  محقق پسادکتری  و همکاری با گروه  کیهان شناسی به مدت یکسال دعوت به عمل می آورد.متقاضیان می توانند تقاضای خود را همراه شرح حال علمی (شامل مقالات)، پیشنهاد پژوهشی و دو  توصیه نامه محرمانه را تا تاریخ   یکم  بهمن ماه 1395 به  پست الکترونیکی دکتر سهراب راهوار  با آدرس     rahvar@sharif.edu   ارسال فرمایند. موضوعات همکاری در زمینه کیهان شناسی  و اخترفیزیک می باشد.

گروه کیهان شناسی دانشکده فیزیک دانشگاه صنعتی شریف

۲۵ مهرماه ۱۳۹۵

Postdoctoral Research position in Cosmology Group of  SUT

Cosmology group of Physics Department – Sharif University of Technology invites applicants for a postdoctoral positions in field Astrophysics  and Cosmology for one year appointment. The applicants are encouraged to send their CV including the list of publications, a statement of research interests and at least 2 confidential professional references, to Prof. Sohrab Rahvar, with the email address of   rahvar@sharif.edu

The deadline of the application is 20 January 2017.

Cosmology Group

Physics Department

Sharif University of Technology

16 October 2016

درجستجوی ناهمسانگردی با استفاده از ابرنواخترهای نوع Ia

Anisotropy search using Supernovae Type Ia

هدی قدسی (دانشکده فیزیک دانشگاه صنعتی شریف)

Abstract:  There are many successes regarding the standard model of cosmology but there are also problems with this model. A few of these relate to the existence of a preferred axis in the universe which undermines the assumption of isotropy and homogeneity of the standard model known as the cosmological principle. It is therefore of high importance to utilize probes such as Supernovae Type Ia, which are pretty useful for such an investigations owing to the practical redshift range they are found in. Several different methods have been employed in this field of research so far. In this talk I will go over our latest work in this regard.

یکشنبه 25 مهر ماه 1395، ساعت 15:00

    دانشکده فیزیک، تالار پرتوی

مشاهدات ریزهمگرایی گرانشی: از تصاویر خام تا منحنی های نوری

Microlensing Observations: from crude images to light curves

صدیقه سجادیان (دانشکده فیزیک دانشگاه صنعتی شریف)

Abstract: In a microlensing event, the light of a background source star is temporarily magnified owing to passing through the gravitational field of a massive foreground object. One of important advantages of microlensing observations is detecting cold and dark extra solar planets often located far from us. For this reason nowadays two survey and several follow up telescopes are probing ongoing microlensing events toward the Galactic centre. Consequently, many astronomers are busy to reduce the data of these events, i.e. converting crude images to light curves and then characterising the lens and source stars. In this talk, I will explain all steps for reducing the data of a microlensing event which are calibrating the images, finding a good reference images, doing difference photometry (DIA) and finally constructing a light curve. Then, I will discuss how to obtain the best fitted model to the resulted light curve through Markov Chain Monte Carlo (MCMC) simulation.

یکشنبه 18 مهر ماه 1395، ساعت 15:00

دانشکده فیزیک، اتاق 517

سمینار مشترک  گروه ذرات و  کیهان شناسی

The Evolution of the Fornax Cluster, and other science at the Kapteyn Astronomical Institute in Groningen

Reynier Peletier

 Kapteyn Astronomical  Institute

University of Groningen

لطفا به مکان و زمان سمینار توجه نمایید.

سه شنبه 13 مهر ماه 1395، ساعت 15:00

    دانشکده فیزیک، تالار پرتوی

بررسی تشکیل ساختارهای کیهانی با استفاده از فیزیک فرایندهای تصادفی

The Study of Cosmological Structure Formation by Physics of Stochastic Processes

محمدرضا آیرملو (دانشکده فیزیک دانشگاه صنعتی شریف)

استاد راهنما: شانت باغرام و محمدرضارحیمی تبار

دفاع از رساله کارشناسی ارشد

به مکان و زمان جلسه توجه فرمایید

Abstract:  One of the most important studies in cosmology is the study of Large Scale Structure of cosmos, where one of the useful and valuable tools is the Excursion Set Theory( EST). EST or Extended Press-Schechter is a theory based on stochastic processes which gives us the distribution function (mass function) of dark matter halos. In order to have smoothed density field, Excursion Set Theory uses a specific window function (filter) called sharp k-space, which is a top-hat in Fourier space. If one uses this specific window function the process of density field with respect to variance will be a Markov process, with any other window function the process won’t be a Markov process.

Excursion Set Theory uses the concept of Markov processes to define some mirror trajectories and that’s one of the basics of the theory. However practically, instead of using sharp k-space filter, physicists use more common filters such as top-hat and Gaussian. In the other side there is an important concept in stochastic processes called Einstein-Markov length, which is a length of nature, where a stochastic process becomes Markovian. In this Thesis using Dark-Sky simulation we investigate Einstein-Markov length of density contrast when it is smoothed with a top-hat filter. The results give us the best variance step which gives Markov process for smoothed density contrast with a top-hat window function.

یکشنبه 4 مهر ماه 1395، ساعت 15:00

دانشکده فیزیک، آمفی تئاتر (تالارجناب)

گرانش دورموازی و پدیدارشناسی ماده تاریک

Teleparallel Gravity and Phenomenology of Dark Matter

الهه خالویی (دانشکده فیزیک دانشگاه صنعتی شریف)

استاد راهنما: سهراب راهوار

استاد راهنما همکار: شانت باغرام 

دفاع از رساله کارشناسی ارشد

به مکان و زمان جلسه توجه فرمایید

Abstract:  One of the biggest challenges facing cosmologists is the nature of dark matter. The cosmological observations, like the rotation curve of galaxies, gravitational weak Lensing and cosmic microwave background radiation indicate that baryonic matter alone cannot explain the density of matter in the Universe. In the context of standard cosmological models, it means that the Universe is made of an unknown component (Dark Matter), which approximately  27% of total energy-momentum. The Modify Gravity and  is an alternative theory that is introduced to describe these observations. In This thesis, we introduce the issue of the measurements of accelerated observers in Minkowski space-time that goes beyond the postulate of locality. This is a step towards introducing nonlocal special theory of relativity. Inertia and gravitation are intimately connected in accordance with the equivalence principle. In order to arrive indirectly at a nonlocal gravity, to explain nature of dark matter, we work within framework of teleparallel gravity. Teleparallel gravity is a gauge theory for the translation group and is an alternative theory, fully equivalent to general relativity that renders field equations of gravity in close analogy with Maxwell’s equations. This analogy helps us to present Nonlocal gravity. The linear approximation of this theory is thoroughly examined. It is shown that there is a room for dark matter like dynamics in the linear regime. Finally we introduce the gravitational dipole moment to present phenomenology of nonlocal gravity.

یکشنبه 28 شهریور ماه 1395، ساعت 15:00

دانشکده فیزیک، تالار پرتوی – 412

کیهان شناسی دقیق با استفاده از اعوجاج فضای انتقال به سرخ

The precision cosmology with redshift space distortion

علی حسین ترکمان غلامی (دانشکده فیزیک دانشگاه صنعتی شریف)

استاد راهنما: شانت باغرام

دفاع از رساله کارشناسی ارشد

به مکان و زمان نامتعارف جلسه توجه فرمایید

Abstract:  The distance measurement is the corner stone of the modern cosmology. They are measured via “Doppler Effect”, accordingly the distances in cosmology are obtained in red-shift space. Despite the cosmological background expansion, and the peculiar velocities due to the local density contrast distort these measurements. One of the main challenges in upcoming cosmological studies is to distinguish the peculiar velocity effect from the background expansion. Supernova Type Ia as standard candles can be think of a crucial observation to address the above problem. In this thesis, we study the effect of cosmological perturbations on distance-redshift relation of the SNIa. The corrective effects are from weak gravitational lensing, integrated Sachs Wolfe effect and the peculiar velocity of the SNe Ia host galaxies. Understanding these corrections will help us to disentangle the peculiar velocity effect from the background dynamic and other corrections. We conclude that by future surveys where the number of SNIa becomes larger and the precision will be improved, we will be able to find the peculiar velocity field and also the bulk flow.

یکشنبه 21 شهریور ماه 1395، ساعت 9:00

    دانشکده فیزیک، آمفی تئاتر (تالار جناب)

Helioseismology and Helioseismic Inversion

Majid Pourabdian

Solar and Stellar Interiors Department

 Max Planck Institute for Solar System Research

لطفا به مکان سمینار توجه فرمایید

Abstract:  Helioseismology and Helioseismic Inversion Helioseismology studies the structure and dynamics of the Sun’s interior by observing oscillations on the surface. In recent years, helioseismology has made substantial progress towards the understanding of the physics of solar oscillations and the physical processes inside the Sun. In addition, a new field called local helioseismology has been developed which studies oscillation travel times and its aim is to address fundamental questions in time-distance helioseismology, such as the choice of travel-time measurements and the sensitivity kernels for inversions, to probe the complex structures like convective flows. The choice of regularization method and considering computational cost in terms of number of observations are key factors in helioseismic inversions.

یکشنبه 14 شهریور ماه 1395، ساعت 10:00

    دانشکده فیزیک، فیزیک 3