Abstract
We have studied clustering dark energy in the effective field theory framework. In particular, we studied the class of dark energy theories parametrized by two free parameters, which are the equation of state, w and the sound speed squared, cs2 . These theories are known as k-essence. It is known that k-essence suffers from an instability that forms in finite time for small speed of sound when non-linearities are included in the equation of motion. We used and built upon the existing k-evolution code to study the instability phenomenon in the fluid picture to better understand the nature of these models. This was possible since k-essence is fundamentally a perfect fluid. The fluid quantities are calculated directly from the existing dark energy fields. This is now a new feature in the k-evolution code, and the fluid quantities can be written to file like the other existing fields. We also modified the existing code to make the dark energy scalar field solver more consistent and accurate. This allows for consistent higher order solvers and a more accurate study of the instability phenomenon also known as blow-up. Lastly, we made a Python class, that uses the free Mayavi plotting package to visualize the data cubes produced by our simulations in 3D. This class played a big part for our results, both for visualizing the fluid quantities and the dark energy instability. We observed that there was a strong correlation between energy density and velocity divergence, which suggests that k-essence dark energy clusters in regions of large energy density and spreads out in regions of small energy density. The class is publicly available and can be used, generally, to visualize any type of scalar field on a regular cubic grid.
Supervisors:
Professor David F. Mota, Institute of Theoretical Astrophysics, UiO
Postdoctoral Fellow Farbod Hassani, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor Luc Rouppe van der Voort, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Dr. Guilherme Brando de Oliveira, M.P.I Astrophysical and Cosmological Relativity