سخنران: علیرضا الهیاری (پژوهشگاه دانشهای بنیادی،تهران)

تاریخ برگزاری رویداد: دوشنبه ۳۰ فروردین ماه ۱۴۰۰

Abstract:
Blackholes appear in a variety of environments and there are direct and indirect evidence hinting towards their existence. Awarding the Nobel prize to Roger Penrose has highlighted the importance of studying these objects. This has been preceded by the first detection of gravitational waves by the LIGO collaboration and observing the shadow of a blackhole by the event horizon telescope. This way, tests of general relativity in the strong field regime has become a reality. Modified theories predict deviations from the predictions of general relativity. This has implications not only for their shadow but also the gravitational waves emitted by these objects. We set to constrain these parameters and set stringent limits on the parameters of these modified theories. Regarding the blackhole shadow, we consider various well motivated metrics which represent beyond Kerr solutions and wish to set novel constraints on the parameters in each model. Regarding gravitational waves, we set to investigate the effect of the modified theories. Specifically, we first study the quasinormal modes of a blackhole with a quadrupole moment and extend our study to gravitational waves. We also consider future prospects by integrating future earth-based or space-borne detectors in this study. In particular, I report our current ongoing work where we set the limits on the parameters that encode deviations from the Kerr geometry and complement our forecasts using different population models to find the best limits on our theory.

سخنران: محمد حسین ژولیده (پژوهشگاه دانشهای بنیادی،تهران)

تاریخ برگزاری رویداد: دوشنبه ۲۳ فروردین ماه ۱۴۰۰

Abstract:
The discrepancy between expected and observed cooling rates of X-ray emitting gas has led to the cooling flow problem at the cores of clusters of galaxies. A variety of models have been proposed to model the observed X-ray spectra and resolve the cooling flow problem, which involves heating the cold gas through different mechanisms. As a result, realistic models of X-ray spectra of galaxy clusters need to involve both heating and cooling mechanisms. In this presentation, we argue that the heating time-scale is set by the MHD turbulent viscous heating for the intracluster plasma. In the first part of the talk, we introduce the cooling problem in the cluster of galaxies and X-ray spectral fitting. Then we are going to talk about the heating mechanisms proposed to solve the cooling problem. In the second part, we introduce our model, the Cooling+Heating flow model (CpH), and talk about fitting results of this model with X-ray observations. Using the CpH flow model, we show that a value of α ≃ 0.05 (Shakura-Sunyaev viscosity parameter) provides improved fits to the X-ray spectra of cooling flow, while at the same time, predicting reasonable cooling efficiency.

سخنران: امین ده یادگاری (دانشگاه شیراز)

تاریخ برگزاری رویداد: دوشنبه، ۱۶ فروردین ۱۴۰۰

Abstract:
The Ruppeiner geometry of thermodynamic fluctuation can provide a powerful tool to study the microstructure of thermodynamic system, completely from the thermodynamic point of view. In this talk, I will start by reviewing some properties of the Ruppeiner geometry based on a metric. Then, I will use the Ruppeiner scalar to investigate the microstructure of charged AdS black hole as a thermodynamic system.

سخنران: بتی هارتمن (دانشگاه سائوپولو، برزیل)

تاریخ برگزاری رویداد: دوشنبه ۱۸ اسفند ماه ۱۳۹۹

Abstract:
With direct gravitational wave detections as well as the first picture of the shadow of a black hole, there exists now complementary observational evidence that black holes - a priori a theoretical prediction of the best theory of gravity that we have to this day, General Relativity - do, indeed, (seem to) exist in the universe. These recent observations show that the corresponding black holes can be extremely well matched to the Kerr solution, a vacuum solution of the Einstein equation that describes a black hole determined only by its mass and angular momentum. Observable black holes hence seem to be very simple objects that carry no additional structure (“hair”). However, models appearing in theories that try to explain e.g. the nature of dark energy or the inflationary epoch in the primordial universe as well as recent studies within the gauge/gravity duality contain black hole solutions that often carry hair. Moreover, stars made out of scalar fields - so-called “boson stars” - provide viable alternatives to supermassive black holes. In this talk, I will discuss strongly gravitating systems that contain scalar fields and explain the role as well as the use of scalar fields in these systems.

سخنران: محسن شادمهری (دانشگاه گلستان)

تاریخ برگزاری رویداد: دوشنبه ۱۱ اسفند ماه ۱۳۹۹

Abstract:
The protoplanetary discs (PPDs) are known as sites of planet formation. Mass of a PPD mostly consists of gas, however, dust particles also exist with an abundance around one percent. While it was believed that magnetorotational instability (MRI) is responsible for the mass accretion in the PPDs, recent observational evidence and theoretical arguments indicate that probably wind-driven accretion is dominant at least in the inner disc region. Although this feature has raised intense investigations, a big challenge is how to propose a robust diagnostic to discriminate the dominant mechanism of the accretion in the PPDs. In this talk, I first review the major mechanisms that generate PPDs turbulence. The most recent observational diagnostics supporting the existence of efficient magnetically wind-driven accretion are discussed. We then explore the evolution of the PPDs in the presence of the magnetically driven winds and the resulting isochrones tracks in the accretion rate and disc mass plane are confronted with the observed PPDs. Evolutions of the disc quantities such as its total mass and its radius are studied for different levels of the magnetic wind strength. While previous studies based on the viscous disc model were able to obtain isochrones tracks more or less consistent with the observed PPDs, our more sophisticated disc model which incorporates magnetic winds can also give us isochrones tracks consistent with the observations. Our results thereby reinforce that role of the magnetic winds cannot be neglected in PPDs.

با حضور پروفسور کامران وفا (دانشگاه هاروارد)

تاریخ برگزاری رویداد: شنبه ۲ اسفند ماه ۱۳۹۹

با توجه به محدودیت ظرفیت، ثبت نام اولیه الزامی است. پذیرفته شدن ثبت نام شما از طریق ایمیل رصدخانه ابوریحان بیرونی به اطلاعتان خواهد رسید.

سخنران: شانت باغرام (دانشگاه صنعتی شریف، تهران)

تاریخ برگزاری رویداد: دوشنبه ۲۷ بهمن ماه ۱۳۹۹

Abstract:
Nowadays, precision cosmology is based on the standard model of LCDM. One of the main predictions of this model is the clustered web-like distribution of matter. This distribution is the result of the hierarchical structure formation based on dark matter halo mergers and accretions. In this talk, first I will discuss how the characteristics of the cosmic web, such as the distribution of the filaments and statistics of nearest neighbor can be used as cosmological probes. Then I will show how CMB lensing and the merger history of dark matter halo can be used as observational probes to distinguish cosmological models, specifically those who address the LCDM challenges.

سخنران: مرضیه فرهنگ (دانشگاه شهید بهشتی، تهران)

تاریخ برگزاری رویداد: دوشنبه ۲۲ دی ماه ۱۳۹۹

چکیده: موفقیت مدل لامبدا سی دی ام در توضیح گستره متنوعی از مشاهدات حیرت انگیز هست. اما تعدادی از مشاهدات اخیر این نظریه را به چالش کشیده اند. در این سخنرانی چالش های پیش روی لامبدا سی دی ام را مرور می کنیم و بعد به صورت مفصل تر به دو نمونه از این ناهنجاری ها می پردازیم:‌ لکه سرد در تابش زمینه کیهان و تنش هابل ابتدا به این می پردازیم که وجود یک لکه بزرگ سرد در نقشه تابش زمینه کیهان جه قدر قابل توجیه هست. آیا عوامل سرراهی می توانند این لکه را ایجاد کرده باشند و یا به نظریه ای فراتر از لامبدا سی دی ام نیاز هست؟ سپس به تنش هابل می پردازیم و به صورت خاص پیشنهاد می کنیم که یک گذار فاز گرانشی می تواند این تنش (و به صورت هم زمان چند تنش دیگر) را حل کند.

سخنران: محمد آخشیک (دانشگاه کنتیکت، آمریکا)

تاریخ برگزاری رویداد: دوشنبه ۱۵ دی ماه ۱۳۹۹

In this talk, I will first discuss the methodology behind the new code, 'requiem2d', a Bayesian framework that combines HST 2D grism spectroscopy with HST and Spitzer photometric data to constrain the spatially resolved stellar populations and measure age and star-formation rate gradients. This code is developed for the purpose of analyzing the REQUIEM^2D Galaxy Survey, a sample of 8 strongly lensed quiescent galaxies spanning redshifts of z = 1.6 − 2.9 and stellar masses of log M* = 10.4 − 11.5. After briefly reviewing the sample, I will discuss the results from our REQUIEM^2D pilot target, MRG-S0851, a gravitationally lensed quintuply imaged massive compact red galaxy at z=1.9. Using our recovered ages and SFHs from the joint spectrophotometric analysis, along with trajectories in the UVJ and FUVVJ color-color diagrams, I show that MRG-S0851 is consistent with an early forming, slow quenched galaxy that is going through a centrally-concentrated rejuvenation phase. I finally discuss that using MRG-S0851 as a prototype, ~1% of massive quiescent galaxies at 1
ادامه