همه‌ی نوشته‌های Mojahed Parsi Mood

Magnetic Penrose Process

50 years of energy extraction from rotating black hole: Magnetic Penrose Process

Naresh Dadhich

(Inter-University Center for Astronomy and Astrophysics (IUCAA), M.A. Ansari Chair in Theoretical Physics at Centre for Theoretical Physics, Jamia Millia Islamia, Delhi)

لطفا به مکان جدید سمینار توجه فرمایید!

Abstract: It is 50 years since Penrose proposed in 1969 the purely geometric process of extracting rotational energy from a rotating black hole. It was indeed a very novel and remarkable process but it was soon realized that it cannot be efficient enough for astrophysical application in powering high energy objects like quasars and AGNs. The difficulty was in the requirement that relative velocity between the two fragments must be greater half the velocity of light, which was astrophyically untenable. This difficulty was beautifully overcome by the speaker and his coworkers in 1985 when they considered magnetic version of the process and had shown that efficiency of energy extraction could even exceed 100%. Now the fully relativistic magnetohydrodynamics simulations wonderfully bear out this prediction and MPP is being considered the most favored process for powering the central engine of high energy objects. The speaker will review the fifty years journey.

یکشنبه ۱۹ آبان ۱۳۹۸، ساعت ۱۵:۰۰

دانشکده فیزیک، طبقه چهارم – تالار پرتوی

سمینار هفتگی گروه کیهانشناسی

On Primordial Gravitational Waves Evolution

مریم آقایی (دانشکده فیزیک دانشگاه صنعتی شریف )

 

Abstract:  Gravitational waves are exciting messengers for the history of the universe as they travel through the space with the speed of light, and have almost no interaction. These valuable messengers can be originated in the early universe or in emerging heavy objects like black holes. Primordial gravitational waves are the direct results of primordial tensor perturbation, applied to the metric which describes our universe. While traveling toward us, these waves experience different cosmological epochs. As the cosmological epochs (namely Inflation, reheating, radiation  dominated and matter dominated) are the background for the primordial gravitational waves, they have significant effects on the GWs evolution. In this talk I will speak about the primordial gravitational waves equation of motion and its solutions in different epochs. We will keep this question in our mind that: are they detectable?

یکشنبه 12 آبان 1398، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 3

Hunting the unknowns!

Hunting the unknowns!

علیرضا وفایی صدر (پژوهشکده فیزیک پژوهشگاه دانش های بنیادی – IPM )

Abstract:  Anomaly/outlier detection is one challenging area in machine learning, especially for large datasets in high dimensions. How can one build a trap for a new type of animal if one knows nothing at all about that animal?

We have explored a general anomaly detection framework based on dimensionality reduction and unsupervised clustering (DRAMA). This approach identifies the primary prototypes in the data with anomalies detected by their large distances from the prototypes, either in the latent space or in the original, high-dimensional space. In this talk we will know more about the anomaly detection techniques and compare them to DRAMA in a wide variety of challenges. The talk is based on: https://arxiv.org/abs/1909.04060

 

یکشنبه 28 مهر  1398، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 3

سمینار هفتگی گروه کیهانشناسی

Quasinormal Modes of a Black Hole with Quadrupole Moment

علیرضا الهیاری (پژوهشکده نجوم پژوهشگاه دانش های بنیادی – IPM )

Abstract:  We determine the quasinormal mode (QNM) frequencies of a black hole with quadrupole moment in the eikonal limit using the light-ring method. The generalized black holes that are discussed in this work possess arbitrary quadrupole and higher mass moments in addition to mass and angular momentum. Static collapsed configurations with mass and quadrupole moment are treated in detail and the QNM frequencies associated with two such configurations are evaluated to linear order in the quadrupole moment.  For a rotating system we consider the Hartle-Thorne spacetime. This collapsed system as well as its QNMs are characterized by mass M, quadrupole moment Q and angular momentum J. When the quadrupole moment is set equal to the relativistic quadrupole moment of the corresponding Kerr black hole, the Hartle-Thorne QNMs reduce to those of the Kerr black hole to second order in angular momentum J.

یکشنبه 21 مهر  1398، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 3

 سمینار مشترک گروه ذرات و کیهانشناسی

Trans-Planckian Censorship and the Swampland

آلک بدرویا

Alek Bedroya

Jefferson Physical Laboratory, Harvard University

 

Abstract: First, I am going to briefly review some of the Swampland conjectures which aim to find the common properties shared among all field theories consistent with quantum gravity. After that, I will talk about the motivations for, and consequences of a newly proposed conjecture, the Trans-Planckian Censorship Conjecture, and how it fits to the existing net of the Swampland conjectures. Finally, I am going to discuss the cosmological implications of this conjecture to inflationary models. This talk is based on arXiv preprints 1909.11063 and 1909.11106.

سه شنبه 16 مهر  1398، ساعت 15:00

دانشکده فیزیک

 سمینار هفتگی گروه کیهان شناسی

Three-dimensional map of the hot Local Bubble using diffuse interstellar bands

نقشه سه بعدی حباب محلی با استفاده از باندهای پخشی میان ستاره ای

امین فرهنگ (پژوهشکده نجوم پژوهشگاه دانش های بنیادی – IPM )

 

Abstract: Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10^6 K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers (5780A and 5797A), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of 5797A\5780A inside the Bubble compared to the outside. This finding proves that the carrier of the 5780A DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the 5797A DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the 5780A DIB must be larger than that of the 5797A DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.

یکشنبه 7 مهر  1398، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 3

دفاع از رساله کارشناسی ارشد (۳)

نسیبت عددی: فرمول بندی 3+1، استخراج عددی امواج گرانشی

Numerical Relativity: 3+1 Formalism, Numerical Extraction of Gravitational Waves

حسن خلوتی (دانشکده فیزیک دانشگاه صنعتی شریف )

استاد راهنما: سهراب راهوار

شنبه 30 شهریور 1398، ساعت 15:00

دانشکده فیزیک، تالار جناب (آمفی تئاتر دانشکده فیزیک)

دفاع از رساله کارشناسی ارشد (۲)

بررسی داده های حاصل از گرفت تصویر پاریته منفی در ریزهمگرایی گرانشی ستارههای غول

Analysing microlensing data for Detecting signal from the eclipsing negative-parity Image by a giant lens star

نوید میرپوریان (دانشکده فیزیک دانشگاه صنعتی شریف )

استاد راهنما: سهراب راهوار

چهارشنبه 27 شهریور 1398، ساعت 16:00

    دانشکده فیزیک، تالار پرتوی – طبقه 4

دفاع از رساله کارشناسی ارشد (۱)

مدل تورم گرم در نظریه موثراکسیون مونودرومی

 Warm inflation model in effective field theory of axion monodromy

 رضا قمی (دانشکده فیزیک دانشگاه صنعتی شریف )

استاد راهنما: علی اکبر ابوالحسنی

 چهارشنبه 27 شهریور 1398، ساعت 14:00

دانشکده فیزیک، تالار پرتوی – طبقه 4

سمینار هفتگی گروه کیهانشناسی

سمینار 1: Footprints of gravitational grating in a strongly lensed quasar

فراز خالقی (دانشکده فیزیک دانشگاه صنعتی شریف )

سمینار 2 : Studying Spherical Collapse by N-Body Simulation On GPU

محمدجوادشاکرارانی ( دانشکده فیزیک دانشگاه صنعتی شریف )

درفصل تابستان سمینارها 10 صبح برگزار خواهد شد!

Abstract of Seminar 1: Interaction of the electromagnetic wave(EW), from a distant quasar with the gravitational wave, sourced by the binary stars can be a candidate for indirect detection of gravitational waves. While in the regime of geometric optics, the light bending due to this interaction is negligible, the phase shifting on the wavefront of an EW, playing the role of gravitational grating, can produce diffraction patterns on the observer plane. In this talk I will briefly explain the mechanics of phase shifting, due to gravitational grating. Then I will compare theory with the data from a doubly lensed system which is my candidate for detecting gravitational grating effect

چکیده سمینار 2: رمبش کروی (Spherical Collapse) یکی از ساده‌ترین مدل‌های تحلیلی برای مطالعه تشکیل هاله‌ها، به عنوان یکی از اجزای اصلی ساختارهای کیهانی است. برای بررسی بهتر و دقیق‌تر رمبش کروی و تشکیل هاله‌ها مطالعه تحلیلی کافی نیست و به ناچار باید سراغ روش‌های عددی و شبیه‌سازی رفت. با توجه به امکان پیاده‌سازی الگوریتم‌های پردازش موازی برای مسأله برهم‌کنش گرانشی بین ذرات استفاده از واحدهای پردازنده گرافیکی (GPU) می‌تواند هزینه و زمان این شبیه‌سازی‌ها را به طور قابل ملاحظه‌ای کاهش دهد.

 

یکشنبه 16 تیر  1398، ساعت 10:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 3