Reza Rezaei

Department of Physics, Sharif University of Technology

Radio Telescopes

Abstract: In this seminar, I try to motivate people why we are interested in Radio Astronomy and why it has awarded several Nobel prizes. Then I focus on technical aspects of how actually a radio telescope works. Radio telescopes are different from giant antennas in telecommunications. To have an idea about sensitivity of radio signal, you can imagine that if we combine all the radio signal that a telescope gathers during half a century, we cannot illuminate a bulb lamp. Finally, I briefly review some of the leading radio telescopes across the globe. As most of the audience are familiar with optical astronomy, I try to use language of the optical telescopes to familiarize people with radio telescopes. This is the first part of three talks. The second and third talks will be about radio interferometry and detectors.

یکشنبه 20 فروردین 1402، ساعت 17:30

Sunday 9 April 2023 – 17:30 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Seminar 1: Determination of the error of measuring the direction of the extensive air showers in terms of the distance from the center of the array, energy, type of particle, Zenith angle, and azimuth angle using the CORSIKA simulation code

Javad Rahimi

Department of Physics, Sharif University of Technology

Seminar 2: Mass, Inertial Force and Mach Principle

Amirmohammad Zakeri

Department of Physics, Sharif University of Technology

Abstract 1: In this project, we explore how cosmic rays reach Earth. Rays that can have different origins such as stars, supernovae, black holes, active galactic nuclei, etc. In general, we divide these cosmic particles into three categories: 1- Nuclei (proton, carbon, iron, etc.), 2- Neutrinos, 3- Photons, which have different production mechanisms and interactions with their surroundings. After reaching the earth’s atmosphere, these particles make the first interaction with atmospheric atoms, which leads to the production of secondary particles. Secondary particles also continuously increase in number in successive collisions with atmospheric atoms, resulting in an extensive air shower that propagates in the atmosphere like a wave with a specific wave front to reach the ground. The earth’s surface can also be covered by the use of detectors that are located in different arrays, both in terms of geometric shape and in terms of covered area, to determine the direction, energy and type of the primary particle. The measurement of the direction of an extensive air shower is significant because it can be used to estimate the direction of the primary particle. The measurement of such a direction depends on the number of detectors and the geometry of the arrays. The method of calculating the direction of the incidence particle is that the arrays determine the arrival time of the particles, and the initial direction of the incidence particle can be measured from the time difference between different detectors. Therefore, if we can have a proper estimate of the direction measurement error, we can determine accurately of the primary particle direction. This will enable us to determine the direction of the cosmic origin of the particle. In this project, we want to use the CORSIKA simulation code to determine the dependence of the input particle direction measurement error on the distance from the center of the array, zenith angle, energy. and the type of primary particle.

Abstract 2: Particle physics ، ever since began to discover particles and put them in particle zoo, has been responsible for defining mass and describing a becoming-massive mechanism for every particle; but, is that really the one and only option to answer ‘what is mass?’.

In this talk, I’m going to introduce a cosmological interpretation of mass under the hood of Mach Principle, which has inspired C.Brans and R.H.Dicke in their famous modified gravity theory, Brans-Dicke theory. I’m going to show that mass, is pretty entangled with inertial force, the force that classical physics permitted to be fictitious and not real.

یکشنبه 21 اسفند 1401، ساعت 17:30

Sunday 12 March 2023 – 17:30 Tehran Time

سمینار به صورت هیبریدی (حضوری و مجازی) برگزار می شود

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Seminar 1: Data Analysis of Gaia and OGLE projects and determination properties of gravitational microlensing

Saeed Mazhab

Department of Physics, Sharif University of Technology

Seminar 2: Detecting primordial Black Holes with the help of 21-cm cosmology

Sajad Tabasi

Department of Physics, Sharif University of Technology

Abstract 1: In this project, we intend to examine the data related to gravitational microlens observed in the OGLE project and compare them with the data related to the GAIA project. Based on the comparison of the data recorded in both projects, we estimate the exact coordinates of the gravitational microlens and by comparing the light curves and matching the existing catalogs, according to the parallax, we can calculate the distance, mass and specific motion of the observed gravitational microlenses. In this process, we can determine how many different masses the observed microlens is due to, and among the investigated cases, objects with very high masses are considered black holes. Finally, by repeating this process and collecting enough data, the general result can be statistically analyzed. A similar research has been conducted in the past years by Dr. Sohrab Rahvar and Amirhossein Dehghani, and the results of this research can be used in the statistical analysis stage.

Abstract 2: Over time, the temperature of the intergalactic medium is highly sensitive to the physical mechanisms within it. Thus, any physical phenomenon can increase or decrease the temperature. Since primordial black holes can accrete and pull in the surrounding gas and cause them to radiate, if they are abundant enough, they will have the ability to seriously affect the temperature of the intergalactic medium. In this seminar, I will first show plots of the linear power spectrum by considering primordial black holes as dark matter and then its effect on the temperature of the intergalactic medium. In addition, I will explain how much the temperature of the medium increases for different masses of these black holes and for what masses the whole dark matter can be considered as primordial black holes. Finally, I will elaborate on a more realistic model and consider the nonlinear power spectrum, and in this regime, we can measure the effect of primordial black holes on the power spectrum. It is so important to see whether our radio observatories can detect such a power spectrum or if we should wait for the future!

یکشنبه 14 اسفند 1401، ساعت 17:30

Sunday 5 March 2023 – 17:30 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Asiyeh Yaghoobi

Department of Physics, Ferdowsi University of Mashhad

Formation of Multiple Stellar Populations in globular clusters

Abstract: Globular clusters (GCs) can provide useful information from the early star-forming subsystems to study the formation history of their parent galaxies. They are composed of low-metal ([Fe/H] <1), low-mass (< 0.8 Msun), and old (> 10 Gyr) stars. Traditionally, it was assumed that all stars in GCs have similar chemical compositions to the initial cloud from which GCs and host galaxies have already been formed. However, recent observations have revealed that stars within GCs display anomalous variations in light elements (e. g. He, C, N, O, Na, Al, and sometimes Mg) that are not expected to occur inside the low-mass stars of GCs. These populations with anomalous abundance variations in GCs are known as “Multiple Stellar Populations” (MSPs) or second generation (SG), and their formation is still shrouded in mystery. In this talk, I investigate the formation of MSPs within young clusters using 3D hydrodynamical simulations in the context of the asymptotic giant branch (AGB) scenario. I will start by introducing the AGB model and then explain some proposed problems for this scenario, such as ram pressure and ionizing radiation. After that, I will present our most important results based on the two papers below. Our results imply that the formation of SG stars in the AGB scenario needs to occur within massive star clusters located in dense environments, consistent with properties of the ambient surrounding of GCs in star-forming disk galaxies observed at redshift z > 2.

https://ui.adsabs.harvard.edu/abs/2022MNRAS.517.4175Y/abstract
https://ui.adsabs.harvard.edu/abs/2022MNRAS.510.4330Y/abstract

یکشنبه 7 اسفند 1401، ساعت 17:00

Sunday 26 February 2023 – 17:00 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Mohammad Reza Ebrahimi

Department of Physics, Sharif University of Technology

Simulation of Structure Formation in the presence of Fuzzy Dark Matter using Gevolution Code 

Supervisor: Ali Akbar Abolhasani

 Abstract: In this thesis, the formation of cosmic structures in the presence of fuzzy dark matter (FDM) is investigated using the Gevolution code. In the FDM model, dark matter is composed of ultralight bosons. In fact, the mentioned bosons are the remnants of a scalar field present in the early universe. Simulations of structure formation in the presence of FDM that have been carried out so far (for example with Gadget and RAMSES) are based on Newtonian gravity, which have a number of limitations. The Gevolution code is an N-Body simulation that, unlike other existing simulations, is based on General Relativity, which can lead to interesting results. In fact, in this simulation, the weak field limit in General Relativity is looked at and metric degrees of freedom are calculated. Other advantages of Gevolution code are higher simulation speed and less hardware limitations. In addition, in this code, a number of theories can be studied.

 یکشنبه  26 بهمن 1401، ساعت 17:00

Sunday 19 February 2023 – 17:00 Tehran Time

تالار جناب  – آمفی تئاتر دانشکده فیزیک

Melika Sarrami

Department of Physics, Sharif University of Technology

Non-linear Structure formation and halo statistics in the fuzzy dark matter model 

Supervisor: Shant Baghram

 Abstract: Fuzzy dark matter ( FDM ) or wave dark matter is an alternative theory designed to solve small-scale problems faced by the standard cold dark matter, such as the missing satellites problem, core cusp problem, and too big to fail problem. FDM is made up of ultra-light axions having mass ∼  that typically have de Broglie wavelength of several kpc, so in small-scales behave like a wave.

For this thesis, I aim to explore how the sub-halo structure and its features change with FDM particles masses which may be able to solve small-scale problems. We derived the halo mass function for FDM by solving the excursion set problem with a mass-dependent barrier function; then, we compared the approach and results of the ΛCDM model and halo statistics that stem from it with the FDM model. FDM model suppresses the small-scale structure of dark matter that results in a cut-off at the lower end of the halo mass function. Markovian processes have an analytical solution, but most are non markov, meaning steps are correlated. Then for this problem, we can approximate it using up-crossing in large mass structure. This approach could be intelligent because Non-Markov trajectories are more smooth than Markovian trajectories, which makes it possible to use this approximation. At the end of the project, we conduct progenitor history for FDM in different redshifts with various mass ranges using the top hat window function.

چهارشنبه  26 بهمن 1401، ساعت 10:00

Wednesday 15 February 2023 – 10:00 Tehran Time

تالار پرتوی  – دانشکده فیزیک

Elahe Khalouei

Research Center for Gravitational-Wave Universe (GWUniverse), Seoul National University

Primordial Black Holes and Gravitational waves

سمینار به صورت هیبریدی (حضوری و مجازی) برگزار می شود

Abstract: Detection of Gravitational waves is a new tracer to search Primordial Black Holes (PBHs). In this talk, I will overview the primordial black holes, focusing on gravitational waves.

یکشنبه 2 بهمن 1401، ساعت 17:00

Sunday 22 January 2023 – 17:00 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Bahar Nosrati

Department of Physics, Shahid Beheshti University

Cosmic voids are emptier in the presence of symmetron’s domain walls

سمینار به صورت هیبریدی (حضوری و مجازی) برگزار می شود 

Abstract: The symmetron field has an environment (density) dependent behavior which is a common feature of the models with the screening mechanism and results in a rich phenomenology. This model can produce domain walls between regions with different densities. We consider this aspect and study the physics of domain walls in between (underdensity) voids and (overdensity) halo structures. The (spherical) domain walls exert a repulsive force on a test mass outside of the wall while a test mass inside of the wall sees no force. This makes the structures outside the voids go further to a larger radius. Effectively, this means the voids are becoming larger in this scenario in comparison to the standard model of cosmology. Interestingly, this makes voids emptier which may shed light on Peebles’ void phenomenon.

یکشنبه 25 دی 1401، ساعت 17:00

Sunday 15 January 2023 – 17:00 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Javad Tabatabaei

Department of Physics, Sharif University of Technology

Local Limit of Nonlocal Gravity: A Teleparallel Extension of General Relativity

سمینار به صورت هیبریدی (حضوری و مجازی) برگزار می شود  

Abstract: In this talk we explore a general constitutive framework for a teleparallel extension of the general theory of relativity. This approach goes beyond the teleparallel equivalent of general relativity (TEGR) by broadening the analogy with the electrodynamics of media. This theory is a local constitutive extension of TEGR that is in fact the local limit of nonlocal gravity (NLG). We will also explore the cosmological implications of this theory for flat cosmology in background and perturbation levels. The observational problems we wish to approach using this theory are also discussed.

یکشنبه 11 دی 1401، ساعت 17:00

Sunday 1 January 2023 – 17:00 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest

Borna Salehian

The Abdus Salam International Centre for Theoretical Physics (ICTP)

Dissipative effects during inflation 

سمینار به صورت هیبریدی (حضوری و مجازی) برگزار می شود

Abstract: I will talk about a model in which inflaton dissipates considerable amount of its energy into other fields with the expense of slowing down. In this scenario perturbations are dominated by the noise resulting from the extra fields rather than quantum fluctuations. I will also spend some time on comparison with different models that one can find in the literature.

یکشنبه 4 دی 1401، ساعت 17:00

Sunday 25 December 2022 – 17:00 Tehran Time

Hybrid Seminar

آمفی تئاتر دانشکده فیزیک-تالار جنابPhysics Department – Amphitheater- Jenab Hall/

https://vc.sharif.edu/ch/cosmology

گزینه ورود به صورت مهمان – Enter as a Guest