Joint Seminar on cosmology and related topics

Joint Seminar is a seminar series that is held regularly in Tokyo and its vicinity. The topics are on cosmology and related areas. The seminar venue alternates among the universities and research institutes in the Kanto area, and this time it is held at RIKEN. Among the 3 speakers in the event, Dr. Misako Tatsuuma and Dr. Puttarak Jai-akson from iTHEMS will give talks, together with an external speaker Dr. Koki Tokeshi (ICRR, U. Tokyo). The time table of the event is as follows:

Date: October 10th (Thu), 14:30. (room will open at 14:00)
Place: RIKEN iTHEMS Wako Campus, Okochi Hall
Program:

14:00 Room open
14:30 Koki Tokeshi’s talk（ICRR, 45 min）
15:15 Break & free discussion（15 min）
15:30 Misako Tatsuuma’s talk（RIKEN iTHEMS, 45 min）
16:15 Break & free discussion（15 min）
16:30 Puttarak Jai-akson’s talk（RIKEN iTHEMS, 45 min）
17:15 Free Discussion（15 min）
17:30 Close (go to dinner)

Titles and abstracts:

1st Speaker: Koki Tokeshi (ICRR)
Title: Exact solutions in stochastic inflation
Abstract: The stochastic formalism of inflation, or stochastic inflation for short, enables us to study the dynamics of large-scale primordial fluctuations in a non-perturbative way. I will present a class of all the possible exact expressions for statistical quantities such as distribution and correlation functions of a test field in the expanding universe, given that the significance of exact solutions in cosmology cannot be overemphasised. To this aim, a sequence of isospectral Hamiltonians and an underlying symmetry called shape invariance are exploited.

2nd Speaker: Misako Tatsuuma (RIKEN iTHEMS)
Title: Numerical Simulations of the Strengths of Dust Aggregates in Planet Formation
Abstract: Planet formation is the growth process from sub-micrometer-sized dust grains to planets larger than 10,000 km. This growth process can be broadly divided into two phases: the initial growth to kilometer-sized planetesimals, which involves the adhesion of dust grains through intermolecular forces such as van der Waals forces and hydrogen bonds, and the subsequent growth to planets, driven by gravitational forces. Currently, no unified planetesimal formation theory can fully explain both observations of protoplanetary disks, where planet formation takes place, and the exploration results of small bodies in the solar system, such as asteroids and comets, which are considered remnants of planetesimals. Constructing such a theory is the ultimate goal of planet formation studies.
To investigate planetesimal formation based on the exploration results of small solar system bodies, we have used the discrete element method (DEM) to calculate the tensile and compressive strengths of dust aggregates, constructing their physically-based models that we have compared to the properties of small solar system bodies. This talk will focus on the methodologies used in these studies.

3rd Speaker: Puttarak Jai-akson (RIKEN iTHEMS)
Title: Null Surfaces Through The Looking-Glass
Abstract: Carrollian physics has recently become a prominent topic in theoretical physics, especially in gravitational studies and flat-space holography. Understanding Carrollian geometries and symmetries is essential to the modern interpretation of null surfaces, whether at finite distances or asymptotic infinities. In this talk, I will introduce Carrollian geometries as intrinsic to any generic null surface, and highlight the profound analogy between gravitational dynamics on null surfaces and Carrollian hydrodynamics. Additionally, I will discuss the phase space structure and symmetries associated with these geometries, shedding light on their implications for gravitational theories and potentially for holographic dualities.