Time: TBA (within 15:45 - 17:00)

Location: Central Building / Mondi 3

Attendees of the seminar are invited to join us for socializing at downtown (ISTA) after the talks!

ISTA Speaker: Itamar Israeli

Title: The Angel Problem, the Firefighter Problem, and Everything In Between

Abstract: The Containment Game is a two-player perfect-information game, initialised with a finite set of occupied vertices in an infinite connected graph $G$. On the $t$-th turn, the first player, called \emph{Spreader}, replaces the occupied set with a collection of $g(t)$ vertices adjacent to it; the second player, called \emph{Container}, then removes $q$ unoccupied vertices from the graph. If the spreading process continues indefinitely, Spreader wins; otherwise, Container wins. For $g\equiv 1$ it is equivalent to the classical Conway's angel problem, while for $g=\infty$ it reduces to a solitaire game for Container, known as the \emph{Firefighter Problem}. 

In this talk I will give a glimpse into the known solutions to both classical problems, then outline our study of the question "how much can Spreader be weakened so that it is still as strong as possible?". Joint work with Ohad Noy Feldheim.

VSM Speaker:  Alejandro Estrada Llesta (Universität Wien)

Title: Light cones and what are they made of

Abstract: Theoretical predictions about the universe are typically defined in the four-dimensional spacetime. However, astronomical observations lie on the past light cone (LC), a null three-dimensional hypersurface in it. To compare with data, these predictions must be restricted to the LC, introducing inherently geometric features. Modeling and computing the LC's geometric properties is therefore key to describing the data we have. 

In this talk, we first define what the light cone is and build intuition for how its flat version can be used to transform the results from N-body cosmological simulations to observational coordinates. We then adopt a more general perspective, treating the LC as a hypersurface of spacetime, and ask what ingredients are need to describe its geometry. We find that the LC can be split into cross-sections whose geometry can be nearly completely characterized by a tetrad that can be propagated along the light rays. This provides a path to numerically sample the LC points and its tangent space. With this approach, we aim to provide a self-consistent framework that naturally includes all effects in observational data arising  light propagation through curved spacetime.