Gravitational Microlensing and Degeneracy Problem: New Method for Breaking the Degeneracy Using Gaia Catalogue
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.
The trans-Planckian Censorship and the Initial Condition Problem
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.
50 years of energy extraction from rotating black hole: Magnetic Penrose Process
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.
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?
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
Abstract: We determine the quasinormal mode (QNM) frequencies of a black hole with quadrupole moment in the eikonal limit using the light-ring method. The generalized black holes that are discussed in this work possess arbitrary quadrupole and higher mass moments in addition to mass and angular momentum. Static collapsed configurations with mass and quadrupole moment are treated in detail and the QNM frequencies associated with two such configurations are evaluated to linear order in the quadrupole moment. For a rotating system we consider the Hartle-Thorne spacetime. This collapsed system as well as its QNMs are characterized by mass M, quadrupole moment Q and angular momentum J. When the quadrupole moment is set equal to the relativistic quadrupole moment of the corresponding Kerr black hole, the Hartle-Thorne QNMs reduce to those of the Kerr black hole to second order in angular momentum J.
Abstract: First, I am going to briefly review some of the Swampland conjectures which aim to find the common properties shared among all field theories consistent with quantum gravity. After that, I will talk about the motivations for, and consequences of a newly proposed conjecture, the Trans-Planckian Censorship Conjecture, and how it fits to the existing net of the Swampland conjectures. Finally, I am going to discuss the cosmological implications of this conjecture to inflationary models. This talk is based on arXiv preprints 1909.11063 and 1909.11106.
Three-dimensional map of the hot Local Bubble using diffuse interstellar bands
نقشه سه بعدی حباب محلی با استفاده از باندهای پخشی میان ستاره ای
امین فرهنگ (پژوهشکده نجوم پژوهشگاه دانش های بنیادی – IPM )
Abstract: Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10^6 K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers (5780A and 5797A), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of 5797A\5780A inside the Bubble compared to the outside. This finding proves that the carrier of the 5780A DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the 5797A DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the 5780A DIB must be larger than that of the 5797A DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.
