زینب کلانتری (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: Recent rapid progress in time domain surveys makes it possible to detect various types of explosive transients (Gamma-ray bursts, Gravitational wave, supernovae, and Fast radio bursts) in the Universe in large numbers, some of which will gravitationally lensed into multiple images. Although a large number of strongly lensed distant galaxies and quasars have already been discovered, strong lensing of explosive transients opens up new applications, including improved measurements of cosmological parameters and powerful probes of small scale structures of the Universe, thanks to their rapidly evolving light curves as well as their compact sizes. Recently we have witnessed first discoveries of strongly lensed supernovae, and strong lensing events of other types of explosive transients are expected to be observed soon. In this presentation, I will review a recent paper on the current state of research on strong gravitational lensing of explosive transients and discuss future prospects.
یکشنبه 24 آذر 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
مهدی ترابیان (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: The swampland conjectures that are motivated by string theory offer a list of criteria that every field theory consistent with quantum gravity must admit. In particular, they constrain the inflating phases of the early and late universe. In this talk, we discuss the recently proposed trans-Planckian censorship conjecture and its consequences for inflationary cosmology. It excludes many models of inflation. Specifically, it implies that inflation must be low scale that implies severe fine-tuning of initial conditions and no primordial gravitation waves. We discuss how multiple inflationary models can explain the present horizon problem, alleviate the initial condition problem and accommodate possible detection of gravitational waves.
یکشنبه 17 آذر 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
مهدی ترابیان (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: The swampland conjectures that are motivated by string theory offer a list of criteria that every field theory consistent with quantum gravity must admit. In particular, they constrain the inflating phases of the early and late universe. In this talk, we discuss the recently proposed trans-Planckian censorship conjecture and its consequences for inflationary cosmology. It excludes many models of inflation. Specifically, it implies that inflation must be low scale that implies severe fine-tuning of initial conditions and no primordial gravitation waves. We discuss how multiple inflationary models can explain the present horizon problem, alleviate the initial condition problem and accommodate possible detection of gravitational waves.
یکشنبه 10 آذر 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
امیرحسین دهقانی (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: Detection of light blending in a gravitational field, previously predicted by general relativity, stimulated a new line of research for astronomers. In 1986, Paczynski, regarding the dark matter problem, investigated the possibility of observing lensing of a source star light by gravitational field of another star in the Milky Way galaxy. In a simple microlensing event, the only observable parameter that has physical information is the Einstein crossing time; a characteristic time scale of microlensing events. This parameter is a function of lens mass, lens and source distance and their relative velocity. Several methods have been proposed to overcome this degeneracy problem. In this talk, after presenting a brief review of basic formalism and physics, I will show different methods of microlensing degeneracy breaking and will finally present my proposal which is mainly based on Gaia observations. The results show a continuum in mass distribution of lenses with some candidates for neutron stars and black holes.
یکشنبه 3 آذر 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
مهدی ترابیان (دانشکده فیزیک دانشگاه صنعتی شریف)
Abstract: The swampland picture that has been emerging from string theory offers a list of criteria that every field theory consistent with quantum gravity must admit. These conditions have interesting implications for the early Universe cosmology and strongly constrain cosmological models. In this talk, we discuss the trans-Planckian censorship conjecture and its consequences for inflationary cosmology. In particular, it implies that the vanilla models of inflation are rather low scale and suffer from severe fine-tuning of initial conditions. We discuss how multiple inflationary models could alleviate the initial condition problem and accommodate a possible detection of gravitational waves in future.
یکشنبه 26 آبان 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
Naresh Dadhich
(Inter-University Center for Astronomy and Astrophysics (IUCAA), M.A. Ansari Chair in Theoretical Physics at Centre for Theoretical Physics, Jamia Millia Islamia, Delhi)
لطفا به مکان جدید سمینار توجه فرمایید!
Abstract: It is 50 years since Penrose proposed in 1969 the purely geometric process of extracting rotational energy from a rotating black hole. It was indeed a very novel and remarkable process but it was soon realized that it cannot be efficient enough for astrophysical application in powering high energy objects like quasars and AGNs. The difficulty was in the requirement that relative velocity between the two fragments must be greater half the velocity of light, which was astrophyically untenable. This difficulty was beautifully overcome by the speaker and his coworkers in 1985 when they considered magnetic version of the process and had shown that efficiency of energy extraction could even exceed 100%. Now the fully relativistic magnetohydrodynamics simulations wonderfully bear out this prediction and MPP is being considered the most favored process for powering the central engine of high energy objects. The speaker will review the fifty years journey.
یکشنبه ۱۹ آبان ۱۳۹۸، ساعت ۱۵:۰۰
دانشکده فیزیک، طبقه چهارم – تالار پرتوی
مریم آقایی (دانشکده فیزیک دانشگاه صنعتی شریف )
Abstract: Gravitational waves are exciting messengers for the history of the universe as they travel through the space with the speed of light, and have almost no interaction. These valuable messengers can be originated in the early universe or in emerging heavy objects like black holes. Primordial gravitational waves are the direct results of primordial tensor perturbation, applied to the metric which describes our universe. While traveling toward us, these waves experience different cosmological epochs. As the cosmological epochs (namely Inflation, reheating, radiation dominated and matter dominated) are the background for the primordial gravitational waves, they have significant effects on the GWs evolution. In this talk I will speak about the primordial gravitational waves equation of motion and its solutions in different epochs. We will keep this question in our mind that: are they detectable?
یکشنبه 12 آبان 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
علیرضا وفایی صدر (پژوهشکده فیزیک پژوهشگاه دانش های بنیادی – IPM )
Abstract: Anomaly/outlier detection is one challenging area in machine learning, especially for large datasets in high dimensions. How can one build a trap for a new type of animal if one knows nothing at all about that animal?
We have explored a general anomaly detection framework based on dimensionality reduction and unsupervised clustering (DRAMA). This approach identifies the primary prototypes in the data with anomalies detected by their large distances from the prototypes, either in the latent space or in the original, high-dimensional space. In this talk we will know more about the anomaly detection techniques and compare them to DRAMA in a wide variety of challenges. The talk is based on: https://arxiv.org/abs/1909.04060
یکشنبه 28 مهر 1398، ساعت 15:00
دانشکده فیزیک، طبقه اول کلاس فیزیک 3
