Institute for Advanced Study/Princeton University Early Universe/Cosmology Lunch Discussion

Abstract 1: 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 improve our understanding of the particle nature of dark matter.
In particular, galaxy-scale strong gravitational lensing provides a unique way to detect and characterize dark matter on small scales. I will present advances in the analysis of gravitational lenses and identification of small-scale clumps using machine learning. I will introduce the convergence power spectrum as a promising statistical observable that can be extracted from strongly lens images and used to distinguish between different dark matter scenarios, showing how different properties of the dark matter get imprinted at different scales. I will also discuss the different contribution of substructure and line-of-sight structure to perturbations in strong lens images.

Abstract 2: The phenomenology of dark energy may be explained by an extra scalar particle mediating a fifth force at cosmological scales. However, stringent limits on the existence of extra forces have been placed by solar-system experiments: viable models must therefore be able to screen (i.e. suppress) the fifth force in environments where it is known to be small.
Several screening mechanisms have been proposed. However, progress in understanding their behaviour has been hindered by the complexity of the field equations involved, which are nonlinear in nature and characterised by a large hierarchy of scales. This is especially true of Vainshtein screening, where the fifth force is suppressed by high-order derivative terms becoming dominant within a large Vainshtein radius.
I will present \phi-enics, a numerical code based on the finite element method that can be used to overcome this problem. I will use \phi-enics to perform a theoretical test of Vainshtein screening, by comparing the behaviour of a set of massive Galileon theories exhibiting screening with their UV completion. We show that the screening does not survive the extension, casting doubt over whether Vainshtein screening can take place within the limits of validity of the theories that invoke it.