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.
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.
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.
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.
در آستانه روز جهانی ماده تاریک، موجودی که مانند تاری در هم تنیده کل کیهان را فراگرفته و حدود ۸۵٪ از جرم گمشده ی کیهان را به آن نسبت می دهند، گردهمایی ترتیب دادیم تا با سخنرانی دکتر شانت باغرام (عضو هیئت علمی دانشکده ی فیزیک دانشگاه صنعتی شریف) ، دکتر سارا خطیبی (عضو هیئت علمی دانشکده فیزیک دانشگاه تهران) و دکتر ابوالحسن واعظی (پژوهشگر آزمایشگاه مواد پیشرفته Gabelle دانشگاه استنفورد) با ایده های پیرامون این موجود از دیدگاه کیهان شناسی، ذرات و ماده چگال آشنا شویم.
شرکت در این برنامه برای عموم آزاد است. میهمانان خارج از دانشگاه شریف جهت هماهنگی با انتظامات دانشگاه جهت ورود می‌بایست فرم ثبت‌نام انتهای صفحه زیر را تا جمعه تکمیل و ارسال نمایند.
Abstract: Dark matter heating of gas clouds hundreds of parsecs from the Milky Way Galactic center provides us a powerful new test of dark matter interactions. To illustrate, the leading bound on nucleon scattering for 10-100 MeV mass dark matter can be derived using this method. Furthermore, millicharged dark matter models, including those proposed to match the recent EDGES 21 cm absorption anomaly can be constrained from gas cloud’s cooling rate. For Galactic center gas clouds, galactic fields’ magnetic deflection of electromagnetically charged dark matter is mitigated, because the magnetic fields around the Galactic center are poloidal, as opposed to being aligned parallel to the Milky Way disk.
Discovery of massive star formation quenching by nonthermal effects in the center of NGC 1097
فاطمه طباطبایی (پژوهشکده نجوم پژوهشگاه دانش های بنیادی IPM )
Abstract: Observations show that massive star formation quenches first at centers of galaxies. To understand quenching mechanisms, we investigate the thermal and nonthermal energy balance in the central kpc of NGC1097- a prototypical galaxy undergoing quenching- and present a systematic study of the nuclear star formation efficiency and its dependencies. This region is dominated by the nonthermal pressure from the magnetic field, cosmic rays, and turbulence. A comparison of the mass-to-magnetic flux ratio of the molecular clouds shows that most of them are magnetically critical or supported against gravitational collapse needed to form cores of massive stars. Moreover, the star formation efficiency of the clouds drops with the magnetic field strength. Such an anti-correlation holds with neither the turbulent nor the thermal pressure. Hence, a progressive built up of the magnetic field results in high-mass stars forming inefficiently, and it may be the cause of the low-mass stellar population in the bulges of galaxies.
فرشاد کمالی نژاد (دانشکده فیزیک دانشگاه صنعتی شریف)
Abstract: Large scale structure surveys are the major probes of cosmology in the next decade. Understanding the formation of structures in an expanding universe is the key to solve many open problems in cosmology. Linear perturbation theory is successful to explain the temperature fluctuations of the CMB when the perturbations are small. These fluctuations grow larger, perturbation theory at linear order breaks and the effects of nonlinearities become dominant. These nonlinearities are well described by applying the standard perturbation theory. In this talk, I study the effects of nonlinear growth of structure on the total matter power spectrum, and in the end, I prove that we can define a “linear point” which is robust against these nonlinear effects.
