سمینار مشترک گروه ذرات و کیهان شناسی
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
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)
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
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)
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
Naseer Iqbal
Department of Physics University of Kashmir Srinagar, India
لطفا به زمان و مکان سمینار توجه فرمایید
Abstract: Galaxies cluster under the influence of their mutual gravitation and the characterization of this clustering problem is a subject of great interest as it somehow helps in understanding the structure formation of the universe. In the last two decades many approaches have been developed in understanding the LSS of the universe from galaxies to the cosmic web like percolation, topology, distribution functions, voids, correlation functions and fractals. All of them have been found useful in studying the clustering of galaxies problem. We have developed the theory on the basis of statistical mechanics and thermodynamics and have found it a convenient method for describing various statistical properties of galaxy clusters in an expanding universe. Statistical mechanical treatment of the cosmological many body problem in combination with the thermodynamics explains the behaviour of entropy and very recently we have postulated that how a kind of cosmological phase transition can also be described for galaxy clusters. The analytical method developed in our theory has been extended from point mass model to extended mass model of galaxy clusters. We also study various dynamical properties of galaxy clusters. In my talk, I will discuss the development of our theoretical model.
دوشنبه 8 شهریور ماه 1395، ساعت 10:00
دانشکده فیزیک، تالار پرتوی – 412
فرهنگ حبیبی
(آزمایشگاه شتابگر خطی (LAL- CNRS) – فرانسه)
لطفا به زمان و مکان سمینار توجه فرمایید
یکشنبه 7 شهریور ماه 1395، ساعت 15:00
دانشکده فیزیک، تالار پرتوی – 412
The Effect of Environment on Baryon Acoustic Oscillations (BAO)
هستی خرمی نژاد (دانشکده فیزیک دانشگاه صنعتی شریف)
استاد راهنما: شانت باغرام
دفاع از رساله کارشناسی ارشد
Abstract: Whilst often phrased in terms of accelerating expansion of the Universe or discovering the nature of the dark energy, maybe the first thing that we can think about, is measuring distances or the geometric observations. The Baryon Acoustic Oscillations (BAO) are the remaining relic of the baryon and photon decoupling epoch. And offer a powerful method of measuring cosmological distances as a standard ruler. In this thesis we first review the theoretical, observational and statistical aspects of the BAO as a statistical standard ruler and examine the impact BAO will have on our understanding of dark energy. Then we calculate this scale by LRGs data and constraints on cosmological parameters. Then we introduce the error factors like Weak Gravitational Lensing, Redshift Space Distortions, bias and non-linearities and calculate the effect of these factors on BAO scale. Additionally we combine these effects and check that what happened on the Baryon Acoustic Oscillations scale. After that we introduce the Linear Point on the two point correlation function and estimate the effect of Weak Gravitational Lensing on this point.
یکشنبه 7 شهریور ماه 1395، ساعت 10:00
دانشکده فیزیک، تالار پرتوی – 412
John Hearnshaw
Emeritus Professor of Astronomy – University of Canterbury New Zealand
President of International Astronomical Union’s Division C for Education, Outreach and Heritage
لطفا به زمان نامتعارف این سمینار توجه فرمایید
Abstract: Mt John Observatory was founded in 1965 at Lake Tekapo in the center of the S Island of New Zealand. It is owned and operated by the University of Canterbury and is NZ’s only professional research observatory for optical astronomy. It is the world’s southernmost observatory at 44 degrees S. The main topics for research have been the study of variable and binary stars, extrasolar planets, microlensing, asteroseismology and astrometry of asteroids and comets. There are four small telescopes at Mt John, the largest with an aperture of 1.8 m. High resolution stellar spectroscopy has been a major activity for the last four decades using echelle spectrographs designed and built at the University. A new high res compact high resolution fibre-fed vacuum echelle called KiwiSpec was recently developed for other observatories and is now manufactured commercially by KiwiStar Optics in NZ. In the last ten years astro-tourism has also been an important activity. About 150,000 tourists visit Mt John every year during the day or for a guided night tour. The night sky is very dark and unpolluted; we have a lighting ordinance to minimize light pollution and an International Dark Sky Reserve covering over 4000 square km centered on Mt John.
Monday 22 August 2016 – 16:30
Physics Department – Partovi auditorium
دوشنبه 1 شهریور ماه 1395، ساعت 16:30
دانشکده فیزیک، تالار پرتوی – 412
معصومه قاسمی (دانشگاه فودان، شانگهای چین)
Masoumeh Ghasemi
Fudan University, Shanghai
Abstract: Black hole (BH) shadow is a dark area over a bright background. It is unstable orbit of constant radius of photon around BH surrounded by optically thin emitting medium. The exact shape of shadow is only determined by spacetime metric. Image of shadow is strong evidence for existence of BHs and event horizon. It is widely believed Sgr A* is most likely a supermassive BH in the Galactic center but there is lack of observational evidence for existence of its event horizon. VLBI observations with Event Horizon telescope in mm/sub-mm band will image Sgr A* shadow soon. Moreover, an accurate measure of shape of shadow can constrain possible deviations from Kerr solution of General Relativity and breaks degeneracy of other observational tests of strong gravity regime. In this talk, I will present shadow of some non-Kerr BHs to constrain possible deviations from Kerr geometry.
یکشنبه 31 مرداد ماه 1395، ساعت 10:00
دانشکده فیزیک، فیزیک 4