Sohrab Rahvar
Department of Physics, Sharif University of Technology
Abstract: This talk will be based on
1) https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.084001
2) https://academic.oup.com/mnras/article-abstract/507/1/914/6339282?redirectedFrom=fulltext
یکشنبه 4 مهر 1400، ساعت 19:00
Sunday 26 September 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Amir Yarahmadi
Department of Physics, Sharif University of Technology
Abstract: The inflation paradigm solves the problems of flatness and homogeneity of the standard cosmological model by considering the exponential expansion for the early universe. It also provides a mechanism for generating density perturbations, which are the core of large-scale structures. One of the successes of inflation is the anticipation of a range of irregular scale perturbations which is well in line with the observations of cosmic background radiation. But after a period of exponential expansion, there will be a cold, empty universe of matter, so there needs to be a process that, after inflation, leads to a hot, standard universe of particles. The two main proposals for achieving standard cosmology are the two theories of warming and hot inflation. In post-inflation reheating theory, considering the coupling between the inflation field and the field as a representative of the standard model field, and considering the effect of the inflation oscillation field, an explosion of particles in the thermal equilibrium can be had. In this study, we examine the effects of producing particles of an extra degree of freedom during and after inflation.
یکشنبه 7 شهریور 1400، ساعت 15:00
Sunday 29 August 2021 – 15:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Parinaz Sheikhansari
Department of Physics, Sharif University of Technology
Yasaman Khalaj Masoumi
Department of Physics, Sharif University of Technology
Abstract 1: The standard cosmological model has been successful in describing many observations from widely different epochs of the Universe, from primordial nucleosynthesis to the accelerating expansion of the present day. With the improvement of the number and the accuracy of observations, some inconsistencies among key cosmological parameters of the model have emerged. One of these discrepancies is between the early and late time measurements of H0.
Some alternative models are proposed to alleviate H0. The question is how these models predict other cosmological observations. In this work, we measured bulk flow in the LCDM model and the uLCDM model to see how bulk flow changes in uLCDM and investigate whether it can reconcile H0 tension.
Abstract 2: The simplest ΛCDM model provides a good fit to a large span of cosmological data. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the 4σ to 6σ disagreement between predictions of the Hubble constant. There are many proposals beyond the ΛCDM model to reconcile the tension, like early dark energy (EDE model), primordial magnetic field, mergers of primordial black holes in extreme clusters. The model which we have been focused on is the mergers of primordial black holes in extreme clusters. In this talk, I will discuss several models which proposed PBHs are clustered, therefore, the gravitational waves emitted by collisions of such PBH binaries could resolve the Hubble tension
یکشنبه 31 مرداد 1400، ساعت 19:00
Sunday 22 August 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Mahdi Abdollahi
Department of Physics, Sharif University of Technology
Amirhossein Mohammadi
Department of Physics, Sharif University of Technology
Abstract 1: The importance of using fast and automatic methods to classify variable stars for a large amount of data is undeniable. There have been many attempts at classifying Variable Stars by traditional algorithms, which require long pre-processing times. In recent years, neural networks as classifiers have come to notice. This paper proposes the Hierarchical Classification technique, which contains several models with the same network structure. We use two pre-processing methods, which produce input data by using light curves and the period. We use the OGLE variable stars’ database to train and test the performance with different models based on the Hierarchical Classification technique. Further, we use Convolutional Neural Networks and Recurrent Neural Networks in the network structure. We see these neural networks work faster than traditional methods and have more accurate predictions. We obtain the best accuracy of 98% for class classification and 92% for subclasses classification.
Abstract 2: Accretion disks are disk-like flows of materials that are orbiting a central object in a gravitational potential. We can see these disks as a fluid and discuss their dynamics which we will do in steady and decaying states. The central body of the accretion disks can be stars which, we will discuss the effect of the accretion disk on the luminosity of the star in the steady-state.
یکشنبه 24 مرداد 1400، ساعت 19:00
Sunday 15 August 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Mohammad Amin Hejazi
Department of Physics, Sharif University of Technology
Abstract: Primordial black holes (PBH) are at the forefront of theoretical cosmology since they can explain dark matter, gravitational waves, and more. The enhanced power spectrum of small scales can result in the production of primordial black holes that could be dark matter candidates. In this talk, we discuss step-like features of the inflationary potential resulting in an increase in the power spectrum of curvature perturbations by several orders of magnitude. As part of this research, we apply a number of these features at the same distances periodically in the inflationary slow-roll potential. Furthermore, we apply the floquet theory to better understand how these features optimize power spectrum enhancements. Finally, we estimate the floquet exponent using both analytical and numerical methods in different ranges of feature parameters.
یکشنبه 17 مرداد 1400، ساعت 19:00
Sunday 8 August 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Ehsan Ebrahimian
Department of Physics, Sharif University of Technology
Abstract: Many previous studies have shown that the direction of the angular momentum of structures is aligned with their environment. Some studies tried to explain this with the Tidal Torque Theory (TTT). The TTT is the accepted theory for explaining the origin of the angular momentum of structures. However, we suggested another mechanism that could change the angular momentum of the dark matter halos. This mechanism is called the Tidal Locking Theory (TLT). It is similar to the mechanism that locked the Moon to the Earth via tidal forces. We investigate the IllustrisTNG simulation to find evidence of the TLT and how it affects the spin alignment of the halos. We suggest that the mad halos, halos with high velocity, provide clear evidence of the TLT. Therefore, we recommend that to understand the spin-LSS alignment better, one must look at the halos’ speed and not at their masses.
یکشنبه 10 مرداد 1400، ساعت 19:00
Sunday 1 August 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Samaneh Sarbaz
Department of Physics, Sharif University of Technology
Abstract: In the gravitational microlensing phenomenon, multiple responses are obtained for the parameters of the gravitational lens. There are many ways to solve this problem. One of the recent techniques proposed is by the use of source starlight spectroscopy, in which shifts in the absorption lines of the source star can be considered as the reason for the relative motion of the lens. In this research, our objective is to study the frequency shifts in binary lenses. Therefore, after reviewing the frequency shift in single gravitational lens systems, we will study this phenomenon in binary systems.
In binary lenses, which are mainly used to detect extrasolar planets, the problem of degeneracy is causing multiple responses, and we want to use this technique to detect extrasolar planets better than before.
یکشنبه 3 مرداد 1400، ساعت 19:00
Sunday 25 July 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Nonlinear astrophysical probes of gravity beyond General Relativity
David F. Mota
Institute of Theoretical Astrophysics, University of Oslo
Abstract:x Extending General Relativity by adding extra degrees of freedom is a popular approach to explain the accelerated expansion of the universe and to build high energy completions of the theory of gravity. The presence of such new degrees of freedom is, however, tightly constrained from observations and experiments. The viability of a given modified theory of gravity therefore strongly depends on the existence of screening mechanisms that suppresses the extra degrees of freedom in certain scales and regimes. I describe how one can use nonlinear structure formation to probe extensions to General Relativity, and will present a set of astrophysical observables that could give smoking guns of screening mechanism.
یکشنبه 27 تیر 1400، ساعت 19:00
Sunday 18 July 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest
Mohammadreza Ayromlou
Max Planck Institute for Astrophysics
Abstract: I first describe the standard theory of galaxy formation and evolution and introduce different kinds of galaxy formation modeling, including semi-analytical models and hydrodynamical simulations.
I then introduce a local background environment (LBE) estimator to quantify environment locally for all galaxies within cosmological simulations. Analyzing the LBE properties, I show that there should be no boundary for dark matter haloes when modeling galaxy evolution. I use the time-evolving LBE of galaxies to develop a method to better account for environmental processes within the Munich semi-analytical model of galaxy formation, L-Galaxies. Using this new method, I remove the artificial halo boundary and extend environmental processes to all galaxies in the simulation. I recalibrate the updated model using a Markov Chain Monte Carlo (MCMC) method and a few observational constraints. By comparing our results to data on galaxy properties in different environments from different surveys (e.g. SDSS, HSC), I demonstrate that the updated model significantly improves the agreement with the observations. Overall, in the vicinity of massive dark matter haloes, the new model produces stronger environmental dependencies, better recovering observed trends with halocentric distance up to scales much beyond the halo virial radius. This is likely to influence the correlations between galaxies up to tens of Megaparsecs. This presentation is based on the following papers: arXiv 1903.01988, 2004.14390, 2011.05336
یکشنبه 20 تیر 1400، ساعت 19:00
Sunday 11 July 2021 – 19:00 Tehran Time
اتاق سمینار مجازی –Virtual Seminar Room
https://vc.sharif.edu/ch/cosmology
گزینه ورود به صورت مهمان – Enter as a Guest