asevolution: a relativistic N-body implementation of the (a)symmetron

TL;DR

The paper introduces asevolution, a relativistic N-body simulation code for the (a)symmetron scalar field, capable of modeling non-linear structures and relativistic effects in cosmology, with validation against existing codes and analysis of domain wall stability.

Contribution

It presents a new cosmological N-body code that incorporates relativistic corrections and scalar field dynamics for the (a)symmetron model, including a novel potential modification and domain wall analysis.

Findings

Relativistic corrections have a small effect in test simulations.

Dynamic scalar fields cannot be accurately modeled with the quasi-static approximation at small masses.

Unstable domain walls are more prominent in the asymmetron scenario.

Abstract

We present asevolution, a cosmological N-body code developed based on gevolution, which consistently solves for the (a)symmetron scalar field and metric potentials within the weak-field approximation. In asevolution, the scalar field is dynamic and can form non-linear structures. A cubic term is added in the symmetron potential to make the symmetry-broken vacuum expectation values different, which is motivated by observational tensions in the late-time universe. To study the effects of the scalar field dynamics, we also implement a constraint solver making use of the quasi-static approximation, and provide options for evaluating the background evolution, including using the full energy density averaged over the simulation box within the Friedmann equation. The asevolution code is validated by comparison with the Newtonian N-body code ISIS that makes use of the quasi-static approximation. There is found a very small effect of including relativistic and weak-field corrections in our small test simulations; it is seen that for small masses, the field is dynamic and can not be accurately solved for using the quasi-static approximation; and we observe the formation of unstable domain walls and demonstrate a useful way to identify them within the code. A first consideration indicates that the domain walls are more unstable in the asymmetron scenario.