Large Eddy Simulations of Binary Neutron Star MergersTurbulent Amplification of the Magnetic Field

Abstract

This thesis is focused on the amplification of the magnetic field (MF) in binary neutron star (BNS) mergers, which occurs mainly during the turbulent phase triggered by the Kelvin-Helmholtz instability (KHI) at small scales. Capturing the small-scale dynamics of this stage is achieved by using Large Eddy Simulations (LES) techniques combined with the sub-grid scale (SGS) gradient model. We first developed the theory for non-relativistic magneto-hydrodynamics (MHD) evolution equations and then for general relativity MHD (GRMHD). We performed box test simulations and analyzed the results obtained from integrated quantities and by looking to the spectral energy obtained for all achieved scales of the system. Later on, we applied these techniques to BNS mergers and, for the first time, we reach numerical convergence and saturation for the turbulent amplification of the MF. We also show that the initial topology of the MF for each star is lost in the amplification of the MF process, giving comparable results for different initial seeds. The last chapter is focused on alternative theories of general relativity (GR), paying attention to the dynamics of screening mechanisms in BNS mergers. Scalar tensor (ST) theories are the most promising ones to explain the expansion of the Universe without the addition of a cosmological constant introduced by Einstein. For the first time, BNS mergers in k-essence theory show that the dipole scalar emission is screened, but the quadrupole scalar mode is not. Our results point at quadrupole scalar signals as large as (or even larger than) in Fierz-Jordan-Brans-Dicke (FJBD) theories with the same conformal coupling, for strong-coupling scales in the MeV range that we can simulate.