arXiv subjects

David W. Neilsen

Publications and source records attributed to David W. Neilsen.

4 recordsLinked to original sources

arXiv2026

Dendro-GR at high mass ratios with high spins

The Laser Interferometer Space Antenna (LISA) launches in less than a decade; it will detect spinning high-mass-ratio binary black hole inspirals annually, alongside other third-generation gravitational wave detectors. High-mass-ratio systems occupy a regime where numerical-relativity simulations remain computationally expensive and technically demanding, especially with high spins at precessing orientations. This portion of parameter space thus remains undersampled, leading to significant bias in parameter estimation. We must close these gaps. Here we report key progress in Dendro-GR toward reducing the computational cost of high-mass-ratio binaries with spin. We evolve the first Dendro-GR binaries at mass ratio $q=24$ (nonspinning) and at $q=12$ with spins up to $χ=0.8$ on both black holes, spanning various configurations. These proof-of-concept runs show strong evidence that Dendro-GR can simulate in this regime and beyond. The simulations generate accurate gravitational waveforms through multipole modes up to $\ell=8$, remain stable, keep constraint violations low and largely constant, conserve horizon mass, and have high computational efficiency with relatively low wall-hour cost. These results establish our starting line for systematic exploration of the high-mass-ratio, high-spin binary black hole systems that are needed for gravitational wave analysis.

gr-qc↗

arXiv2002

Black-hole threshold solutions in stiff fluid collapse

Numerical studies of the gravitational collapse of a stiff (P=rho) fluid have found the now familiar critical phenomena, namely scaling of the black hole mass with a critical exponent and continuous self-similarity at the threshold of black hole formation. Using the equivalence of an irrotational stiff fluid to a massless scalar field, we construct the critical solution as a scalar field solution by making a self-similarity ansatz. We find evidence that this solution has exactly one growing perturbation mode; both the mode and the critical exponent, gamma ~ 0.94, derived from its eigenvalue agree with those measured in perfect fluid collapse simulations. We explain why this solution is seen as a critical solution in stiff fluid collapse but not in scalar field collapse, and conversely why the scalar field critical solution is not seen in stiff fluid collapse, even though the two systems are locally equivalent.

gr-qc↗

arXiv1999

Ultrarelativistic fluid dynamics

This is the first of two papers examining the critical collapse of spherically symmetric perfect fluids with the equation of state P = (Gamma -1)rho. Here we present the equations of motion and describe a computer code capable of simulating the extremely relativistic flows encountered in critical solutions for Gamma <= 2. The fluid equations are solved using a high-resolution shock-capturing scheme based on a linearized Riemann solver.

gr-qc↗

arXiv1999

Critical phenomena in perfect fluids

We investigate the gravitational collapse of a spherically symmetric, perfect fluid with equation of state P = (Gamma -1)rho. We restrict attention to the ultrarelativistic (``kinetic-energy-dominated'', ``scale-free'') limit where black hole formation is anticipated to turn on at infinitesimal black hole mass (Type II behavior). Critical solutions (those which sit at the threshold of black hole formation in parametrized families of collapse) are found by solving the system of ODEs which result from a self-similar ansatz, and by solving the full Einstein/fluid PDEs in spherical symmetry. These latter PDE solutions (``simulations'') extend the pioneering work of Evans and Coleman (Gamma = 4/3) and verify that the continuously self-similar solutions previously found by Maison and Hara et al for $1.05 < Gamma < 1.89 are (locally) unique critical solutions. In addition, we find strong evidence that globally regular critical solutions do exist for 1.89 < Gamma <= 2$, that the sonic point for Gamma_dn = 1.8896244 is a degenerate node, and that the sonic points for Gamma >Gamma_dn are nodal points rather than focal points as previously reported. We also find a critical solution for Gamma = 2, and present evidence that it is continuously self-similar and Type II. Mass-scaling exponents for all of the critical solutions are calculated by evolving near-critical initial data, with results which confirm and extend previous calculations based on linear perturbation theory. Finally, we comment on critical solutions generated with an ideal-gas equation of state.

gr-qc↗