Overview | NuShells

Overview

A spherical shell N-body code for simulating collisionless neutrino dynamics with long-range Yukawa interactions mediated by a scalar field. Shells evolve under free-streaming and long-range forces using a kick-drift-kick (KDK) leapfrog integrator.

View it on GitHub

Implemented Features

Grid and ICs

✅ Geometric radial grid — shells placed on geomspace(Rmin, Rmax)
✅ Reflecting boundary conditions at infinity — shells softened at soft and bouncing at Rmax
✅ Fermi-Dirac initial conditions — momentum samples drawn from FD distribution, angular momentum assigned via random sampling
✅ Generic perturbation profile — Gaussian, top-hat, poly, exponential and compensated

Evolution

✅ KDK leapfrog integrator — symplectic kick-drift-kick time stepping
✅ Adaptive timestep: condition dt on acceleration magnitude
✅ Effective mass update — consistent mass/energy at each potential update
✅ Scale factor evolution — matter-dominated Universe only
✅ Enclosed mass tracking — cumulative weighted sum for gravity

Forces

✅ Neutrino free-streaming force — angular momentum term ell^2 / (eps * r^3)
✅ Yukawa force solver — inner and outer shells contribtions at each R
✅ Gravity force solver — Newtonian gravity force using _cumMass and beta
✅ Scalar field Poisson solver — iterative solve for phi(r) at each timestep: naive and anderson methods
✅ Phi interpolation — interpolates phi from pre-drift positions as initial guess for iteration

I/O

✅ Text output — shell state saved with numpy.savetxt
✅ HDF5 output — shell state saved with header metadata using h5py
✅ Measurements — shell state saved with linear or log-spacing in a(t)

Utilities

✅ Driver — driver script to set params and run the simulation
✅ Profiler — wall-clock profiling decorator (@timed)
✅ Logger — info and debug verbosity levels
✅ Radial density — bins shells by weight into radial number density n(r)
✅ Visualisation — scripts to visualise circles or neutrino delta

To do list

Convergence tests in the iteration: Naive vs anderson check
Boundary term: Add contribution from infinity for phi and Yukawa force
Alternative IC sampling: Explore PSD-proportional radial sampling