Events
Astrophysics and Cosmology Seminar
Applications of Gravitational Lensing: Revealing the Structure of Quasars
Dr. Carina Fian
Hctpa/Tau
Wed, 03 Mar 2021, 11:10
Sacta-Rashi Building for Physics (54), room 207
Abstract: There is no chance of spatially resolving the inner parts of active
galactic nuclei (AGNs) with current optical telescopes, therefore
observations of gravitationally lensed quasars and galaxies
provide a powerful alternative to study their inner parts and
distribution of mass. Compact objects such as stars in the lens galaxy
induce uncorrelated changes in the flux of the images. This
microlensing effect can help us overcoming these difficulties as it is
sensitive to the size of the emitting region and thus, can be used to
extract information about both the source and the lens. Photometric
monitoring and spectroscopic observations in different epochs are a
powerful technique to measure the dimensions of the different emitting
regions in a gravitationally lensed quasar and to probe their
kinematics. In this talk I will show how to use gravitational
microlensing to reveal the size and geometry of AGNs on different
scales, starting from the broad-line region (BLR), followed by the
accretion disk, down to the smallest region of the AGN - the central
supermassive black hole (SMBH).
Biological and soft-matter physics
Growth laws for unicellular organisms from ribosome biogenesis
Dr. Sarah Kostinski
School Of Chemistry, Tel-Aviv University
Thu, 04 Mar 2021, 12:10
ZOOM only - Meeting ID: 874 2021 0979
Abstract: Ribosomes generate all proteins in a cell, including their own ribosomal proteins. Previous work on bacteria showed that ribosomal protein production imposes a bound on cellular growth rates, since cell doubling requires a commensurate doubling of ribosomes. However, ribosomes are made not only of protein, but also of ribosomal RNA. We obtain a new speed limit on cell growth which originates in the generation of ribosomal RNA [1]. A comparison with E. coli data reveals that the bacterial ribosome’s 1:2 protein-to-RNA mass ratio uniquely maximizes cellular growth rates as permitted by both bounds. This observation leads to a growth law involving RNA polymerases, and an invariant of bacterial growth. Similar arguments for Eukarya lead to several new growth laws [2]. Despite the greater complexity of that domain of life, the predictions are consistent thus far with available data for the model organism S. cerevisiae.
[1] S. Kostinski and S. Reuveni, “Ribosome composition maximizes cellular growth rates in E. coli,” Phys. Rev. Lett. 125, 028103 (2020).
[2] S. Kostinski and S. Reuveni, “Growth laws and invariants from ribosome biogenesis in lower Eukarya,” Phys. Rev. Research 3, 013020 (2021).
Join Zoom Meeting
https://us02web.zoom.us/j/87420210979
Meeting ID: 874 2021 0979
Condensed Matter Seminar
Thermalization and quantum chaos in many-body interacting systems
Prof. Fausto Borgonovi
Catholic University Brescia Italy
Mon, 08 Mar 2021, 11:30
Zoom
Abstract: The problem of thermalization in isolated quantum many-body systems is discussed in the frame of recent experiments and old and new theoretical results. In particular, the role of quantum chaotic signatures in the process of relaxation is stressed. Starting from a very paradigmatic model of a finite number (N) of interacting particles we will demonstrate that the number of states participating in the evolution after a quench increases exponentially in time, provided the eigenstates are chaotic and delocalized in the energy shell. The rate of the exponential growth is defined by the width $\Gamma$ of the local density of states (LDoS). In finite systems, the exponential growth eventually saturates due to the finite volume of the energy shell. We estimate the time scale for the thermalization and show that it is N times larger than the characteristic Heisenberg time $\hbar/\Gamma$.
Last but not least we address the question of the relevance of thermalization to the increase of correlations. Specifically, we study how, in the process of thermalization, the correlations between occupation numbers increase in time resulting in the emergence of the Bose-Einstein distribution.
Join Zoom Meeting
https://us02web.zoom.us/j/85072780943?pwd=QWIwa2ZsQVU1SE9rTkVEaitEOGRUdz09
Physics Colloquium
Discovering New Physics with Cosmological Data Sets
Cora Dvorkin
Harvard
Mon, 08 Mar 2021, 16:10
Zoom
Abstract: Measurements of the Cosmic Microwave Background and the large-scale structure of the universe have made it possible to determine with great precision the universe's inventory, as well as properties of its initial conditions. However, there are profound questions that remain unanswered. Cosmological observations and galaxy dynamics seem to imply that 84% of all matter in the universe is composed of dark matter, which is not accounted for by the Standard Model of particles. The particle nature of dark matter is one of the most intriguing puzzles of our time. The wealth of knowledge which is and will soon be available from cosmological surveys will reveal new information about our universe. I will discuss how we can use new and complementary data sets to identify new physics at different scales.
*** This colloquium is joint with all the universities in Israel. We invite you to log in at 16:00 for socializing. The talk itself will start at 16:10. ***
Biological and soft-matter physics
Modelling cellular spreading and emergence of motility in the presence of curved membrane proteins and active cytoskeleton forces
Prof. Nir Gov
Department Of Chemical And Biological Physics, Weizmann Institute Of Science
Thu, 11 Mar 2021, 12:10
ZOOM only - Meeting ID: 874 2021 0979
Abstract: Eukaryotic cells adhere to extracellular matrix during the normal development of the organism, forming static adhesion as well as during cell motility. We study this process by considering a simplified coarse-grained model of a vesicle that has uniform adhesion energy with a flat substrate, mobile curved membrane proteins and active forces. We find that a high concentration of curved proteins alone increases the spreading of the vesicle, by the self-organization of the curved proteins at the high curvature vesicle-substrate contact line, thereby reducing the bending energy penalty at the vesicle rim. This is most significant in the regime of low bare vesicle-substrate adhesion. When these curved proteins induce active protrusive forces, representing the actin cytoskeleton, we find efficient spreading, in the form of sheet-like lamellipodia. Finally, the same mechanism of spreading is found to include a minimal set of ingredients needed to give rise to motile phenotypes.
Join Zoom Meeting
https://us02web.zoom.us/j/87420210979
Meeting ID: 874 2021 0979
All events
Google calendar
