آیا تقارن‌های همتافته راه حلی برای مشکل میکروحالت‌های سیاهچاله پیشنهاد می‌کنند؟
Symplectic vs asymptotic symmetries and a proposal for black hole microstates
رضا جوادی نژاد (دانشکده فیزیک دانشگاه صنعتی شریف)
چکیده : از سال ١٩٧٣ که سیاهچاله ها به عنوان سیستم های ترمودینامکی شناخته شدند مسئله انتروپی سیاهچاله ها به عنوان یک مسئله حل نشده باقی مانده است. در واقع سوال بزرگ این است که یک سیاهچاله به عنوان یک موجود ترمودینامیکی چه میکروحالتهایی دارد و این سوال زمانی بزرگتر می شود که قضیه بی مویی برای این جوابهای نسبیت عام را در نظر بگیریم. در راستای پاسخ به این سوال در این ارائه قصد بر این است که در ابتدا یک بازنگری دقیقتر در مسئله تقارن های پیمانه ای نسبیت عام انجام دهیم و سپس با معرفی فضای فاز یک سیستم گرانشی، اولین قدم های بررسی نظریه میدانِ دوگان به این سیستم گرانشی را برداریم. این بحث عمدتا بر روی فضای فاز که به روش فضای فاز هموردا ساخته می شود به عنوان پایه ای برای تعریف تقارن و بار پایسته است، قرار دارد.
Long period variable stars in NGC 147 and NGC 185:
Their star formation histories
رویا همدانی گلشن (پژوهشکده نجوم پژوهشگاه بنیادی – IPM )
Abstract: NGC 147 and NGC 185 are two of the most massive satellites of the Andromeda galaxy (M 31). Close together in the sky, of similar mass and morphological type dE, they possess different amounts of interstellar gas and tidal distortion. The question therefore is, how do their histories compare? Here we present the first reconstruction of the star formation histories of NGC 147 and NGC 185 using long-period variable stars. These represent the final phase of evolution of low- and intermediate-mass stars at the asymptotic giant branch, when their luminosity is related to their birth mass. Combining near-infrared photometry with stellar evolution models, we construct the mass function and hence the star formation history. For NGC 185 we found that the main epoch of star formation occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate. In the case of NGC 147, the star formation rate peaked only 6.9 Gyr ago, staying intense until ∼ 3 Gyr ago, but no star formation has occurred for at least 300 Myr. Despite their similar masses, NGC 147 has evolved more slowly than NGC 185 initially, but more dramatically in more recent times. This is corroborated by the strong tidal distortions of NGC 147 and the presence of gas in the centre of NGC 185.
فرنیک نیک اختر (دانشکده فیزیک دانشگاه صنعتی شریف )
درفصل تابستان سمینارها 10 صبح برگزار خواهد شد!
Abstract: Excursion Set Theory (EST) is an analytical framework to study the large scale structure of the Universe. EST introduces a procedure to calculate the number density of structures by relating the non-linear structure formation to cosmological linear perturbation theory.
In this work, we propose a novel approach to re-formulate the EST in matrix formalism.The idea is to discretize the two dimensional plane of variance and density contrast, where the probability of density contrast in specific window function is represented in probability “ket”. Naturally the concept of the transition matrix pops up to make the trajectories in excursion set theory. We introduce the Gaussian transition matrix and we show that several transition in EST plane preserve the Gaussianity of the profile with the mass variance related to the window function. In this work we present a procedure for the finding the expectation values in EST as well. The concept of transition probability rate in both limits of discrete and continuous limit of EST is introduced. Finally we introduce the methods to obtain the barrier crossing, number count and statistic calculations in this new formalism.
Abstract: Barred galaxies represent a considerable fraction of the entire disc galaxy population. Bars could be considered as the key drivers of the internal secular evolution in disc galaxies by redistribution of the angular momentum and triggering of star formation, or changing the morphological structure of disk galaxies. In edge-on or highly inclined systems, bars are most easily recognized by boxy and peanut-shapes, and sometimes X-shape morphology formed by the stellar material above the disc plane. BP bulges specifically and bars in general produce distinct kinematic features that can be easily detected in galaxies. I will present our recent results of the study on the stellar kinematics and populations of a sample of intermediate inclined disk galaxies, obtained with the SAURON integral-field spectrograph on the 4.2m William Herschel Telescope. I will also discuss our approach to unveil hidden bars and inner disks on the basis of their kinematics and introduce a method to quantify the cylindrical rotation, as a common kinematics feature of barred galaxies. The results of this study allow us to establish the link between the stellar kinematics and populations of bulges and bars with their surrounding thin and thick disks, and relate them to the properties observed in disk galaxies.
Department of Physics and Astronomy, the University of British Columbia
Abstract:The precision of measurements from the next generation of large scale structure surveys will reach a level where we can no longer ignore some of the interfering astrophysical effects. The failure to do so will result in bias in our cosmological inference. This sensitivity, however, can be seen as an opportunity to investigate further these degenerate phenomena. I’ll review the impact of baryonic physics on weak lensing observables, a series of measurements to study the baryons and some theoretical models to describe the observations. I’ll demonstrate the utility of these measurements in simultaneously constraining cosmology and baryon feedback.
How real detector thresholds create false standard candles
امیر شاه مرادی
پژوھشگاه مھندسی و علوم محاسباتی، دانشگاه تگزاس آستین
Abstract:In a world in which science and technological breakthroughs dominate all aspects of almost every individual human life, scientists and researchers are under an ever increasing pressure to cross and expand the borders of human knowledge. As new discoveries require higher levels of precision and reproducibility, excess workload and hyper-competitive work environments have made researchers more prone to human cognitive biases. Consequently, these biases have led to an ever increasing number of false-positive scientific discoveries in recent years. In this talk, I present and elaborate on an example of such discoveries from the field of High-Energy Astrophysics and Cosmology. More specifically, I illustrate how the real complex interplay of human cognitive biases, instrumental biases, and incomplete data have led to the creation of false cosmological standard candles from observational properties of the most powerful explosions ever witnessed in the cosmos, known as Gamma-Ray Bursts (GRBs). These cosmological standard candles, have been frequently used over the past decade, although erroneously, to expand the Hubble diagram beyond supernovae data and to constrain Dark Energy’s equation of state.
I present a Bayesian multilevel methodology and careful analysis of GRB data that can remedy or alleviate the aforementioned existing biases in the studies of GRBs as cosmological standard candles. Furthermore, a mathematically rigorous Bayesian framework is presented for comparing different cosmological models against available observational data, and choosing the most appropriate model.
Power asymmetry of the cosmic microwave background (CMB) in the presence of a massive defect
خانم دکتر طاهره رستمی
پژوهشکده نجوم پژوهشگاه دانش های بنیادی (IPM)
Abstract: In a quest to explain the observed anomalous power asymmetry of the cosmic microwave background (CMB), we show that in a model of inflation, broken homogeneity by a point mass can give a picture to understand the origin of dipole asymmetry. In more detail, I will show that the de-Sitter-Schwarzschild (dS-Sch) space-time, in which homogeneity is broken, the power spectrum of curvature perturbations are anisotropic with non-trivial scale-dependent dipole, quadruple and higher multipoles. Also, the primordial power spectrum of tensor perturbations is modified such that it contain anisotropic scale dependent terms.
Preparing for Gaia, towards the first Three-dimensional Dust Map in the Milky Way
سارا رضایی خوشبخت
(Max Plank Institute for Astronomy (MPIA), Heidelberg, Germany)
Abstract: We are developing a non-parametric model to reconstruct the three-dimensional (3D) distribution of dust in the Milky Way. Our approach uses observed line-of-sight (l.o.s) extinctions towards stars at different positions in the Galaxy. These give the integrated dust density along each l.o.s. Making weak assumptions about the correlation of the dust, we infer the most probable 3D distribution of dust which explains the observed extinctions, even at arbitrary points. Given distances and extinctions estimated from the Gaia photometry and astrometry for tens of millions of stars, we plan to build a detailed map of dust in our Galaxy.
با عرض تبریک سال نو و آرزوی سالی سرشار از شادی و تندرستی و پیروزی، به اطلاع می‌رسانم سمینار این هفته را خانم دکتر سمیه شیخ نظامی از پژوهشکده نجوم پژوهشگاه دانش‌های بنیادی (IPM) ارائه خواهند کرد. عنوان و چکیدهٔ سمینار ایشان به شرح زیر است:
Jets and outflows: Theory, Observation, Simulations
Abstract: Jets are powerful signatures of astrophysical activity and are observed over a wide range of luminosity and spatial scale. Typical jet sources are young stellar objects (YSOs), micro-quasars, and active galactic nuclei (AGNs), while there are indications of jet motion also for a few pulsars and for gamma-ray bursts. In the first part of the talk, I give a brief introduction on astrophysical jets, their formation and acceleration. In the next part of the talk, I present our recent results of the first ever three-dimensional (3D) magneto-hydrodynamic (MHD) simulations of the accretion–ejection structure. The 3D evolution of jets launched symmetrically from single stars but also jets from warped disks in binary systems have been studied. Various model setups have been applied and tested by simulating a stable and bipolar symmetric 3D structure from a single star–disk–jet system. Our simulations show typical 3D deviations from axial symmetry, such as jet bending outside the Roche lobe or spiral arms forming in the accretion disk.
این سمینار در روز یکشنبه ۱۵ فروردین ماه ۱۳۹۵، ساعت ۱۵ در اتاق ۵۱۷ در طبقهٔ پنجم دانشکدهٔ فیزیک برگزار خواهد شد.
