Hwei Jang Yo

Improved numerical stability of stationary black hole evolution calculations

We experiment with modifications of the BSSN form of the Einstein field equations (a reformulation of the ADM equations) and demonstrate how these modifications affect the stability of numerical black hole evolution calculations. We use excision to evolve both non-rotating and rotating Kerr-Schild black holes in octant and equatorial symmetry, and without any symmetry assumptions, and obtain accurate and stable simulations for specific angular momenta J/M of up to about 0.9M.

Cosmological dynamics with propagating Lorentz connection modes of spin zero

The Poincare gauge theory of gravity has a Lorentz connection with both torsion and curvature. For this theory two good propagating connection modes, carrying spin-0+ and spin-0−, have been found. The possible effects of the spin-0+ mode in cosmology were investigated in a previous work by our group; there it was found that the 0+ mode could account for the presently accelerating universe. Here, we extend the analysis to also include the spin-0− mode. The resulting cosmological model has three degrees of freedom. We present both the Lagrangian and Hamiltonian form of the dynamic equations for this model, find the late-time normal modes, and present some numerical evolution cases. In the late time asymptotic regime the two dynamic modes decouple, and the acceleration of the Universe oscillates due to the spin-0+ mode.

Dynamic scalar torsion and an oscillating universe

For the Poincare gauge theory of gravity we consider the dynamical scalar torsion mode in a cosmological context. We explore in particular the possibility of using dynamical torsion to explain the current state of the accelerating Universe. With certain suitable sets of chosen parameters, this model can give a (qualitatively) proper description of the current universe without a cosmological constant, and the universe described is oscillating with a period of the Hubble time.

Numerical testbed for singularity excision in moving black hole spacetimes

We evolve a scalar field in a fixed Kerr-Schild background geometry to test simple

Gravitational wave trains in the quasiequilibrium approximation: A model problem in scalar gravitation

(3+1)

Improved fast-rotating black hole evolution simulations with modified Baumgarte-Shapiro-Shibata-Nakamura formulation

-dimensional algorithms for singularity excision. We compare both centered and upwind schemes for handling the shift (advection) terms, as well as different approaches for implementing the excision boundary conditions, for both static and boosted black holes. By first determining the scalar field evolution in a static frame with a

From binary black hole simulation to triple black hole simulation

(1+1)

Modifications for numerical stability of black hole evolution

-dimensional code, we obtain the solution to very high precision. This solution then provides a useful testbed for simulations in full

Torsion cosmology and the accelerating universe

(3+1)

Symmetric teleparallel general relativity

dimensions. We show that some algorithms which are stable for nonboosted black holes become unstable when the boost velocity becomes high.