HQIV N-body (PySCo)

Action-derived Horizon-Quantized Informational Vacuum (HQIV) physics for N-body simulations, built on top of PySCo. Primary test case: the Bullet Cluster — gas–galaxy offset and weak-lensing κ-map without dark matter.

This is the submodule for the HQIV repo. It is the single source of truth for N-body work: physics modules, run scripts, tests, and archived analysis.

n-body/
├── hqiv_modifications/   # φ field, inertia, vorticity, G_eff, Bullet ICs
├── scripts/              # run_bullet.py, postprocess_lensing.py, …
├── tests/                # unit tests (paper cross-check)
├── docs/                 # PAPER_VERIFICATION.md
├── analysis/             # committed reports + figures (no .npz)
├── runs/                 # run matrix and current results
└── pysco/                # PySCo submodule (git submodule)

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Physics (paper/main.tex)

Module	Equation	File
Modified Einstein	G_μν + γ(φ/c²)g_μν = (8πG_eff/c⁴)T_μν	background via background_wrapper.py
Inertia factor	f = max(a_loc/(a_loc + φ/6), f_min)	inertia_factor.py
Varying G	G_eff = G₀(H/H₀)^α, α≈0.6	g_eff.py
φ field	φ = 2c²/Θ_local	phi_field.py
Vorticity source	∂ω/∂t + (v·∇)ω = (∂f/∂φ)(k×∇φ)·ê_ω	vorticity_source.py
Bullet ICs	Two NFW halos + gas/galaxy split	bullet_ic.py

Full paper-to-code audit: docs/PAPER_VERIFICATION.md.

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Quick start

1. Clone with submodules

git clone --recursive https://github.com/HQIV/n-body.git
cd n-body
git submodule update --init pysco

Or from the parent HQIV repo:

git submodule update --init n-body
cd n-body && git submodule update --init pysco

2. Install dependencies

pip install numpy scipy matplotlib
pip install pysco-cosmo   # or use the pysco/ submodule

3. Smoke test

python3 scripts/run_bullet.py --resolution 64 --npart 1e5 --steps 100
python3 tests/test_hqiv.py -v

4. Production run (detached, resource-capped)

Heavy 512³ runs can exhaust RAM and freeze the IDE. Use the nohup wrapper:

BULLET_MEM_GB=8 BULLET_CPU_PCT=70 \
  scripts/run_bullet_nohup.sh output/bullet_5mpc_512 logs/bullet.log

5. Lensing post-process

python3 scripts/postprocess_lensing.py \
  --output output/bullet_5mpc_512/ \
  --snapshot snapshot_00040.npz

# Multi-snapshot report (needs snapshots on disk under output/)
python3 scripts/compile_bullet_report.py

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Current results (May 2026)

512³, 5 Mpc box, γ=0.40 (bullet_5mpc_512)

Snapshot	z	offset lensing–gas [kpc]
10	0.96	2 505
20	0.92	568
30	0.89	221
40	0.85	569

At z≈0.89 the offset reaches 221 kpc, within ~20% of the Clowe et al. (2006) observed ~180 kpc. Figures: analysis/bullet_5mpc_512/.

γ=0.432 best-fit scan (bullet_bestfit_gamma0432)

512³, 100 Mpc box, 8M gas + 1.6M galaxy particles. Final snapshot (z≈0.90) gave offset 25 921 kpc — the larger box and higher γ over-amplify the effect. See analysis/bullet_bestfit_gamma0432/lensing_summary.txt.

Paper figure run (64³, 20 steps, γ=0.40)

Used in paper/main.tex § Bullet Cluster. Produces lensing_comparison.png via postprocess_lensing.py. Fast (~minutes).

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Configuration

Edit hqiv_modifications/config_hqiv.ini:

[hqiv_parameters]
gamma = 0.40           # thermodynamic coefficient
alpha_G = 0.6          # G_eff exponent
f_min = 0.01           # inertia floor

[bullet_cluster]
z_obs = 0.3
M_main = 2.5e14        # Msun
v_collision = 4500       # km/s

See runs/README.md for the full run matrix.

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Relationship to parent HQIV repo

Location	Role
n-body/ (this repo)	Canonical N-body implementation
n-body_pysco_hqiv/ (legacy in parent)	Pre-submodule copy; use n-body/ going forward
hqiv-class/	CLASS / HiCLASS CMB patches (companion submodule)
horizon_modes/python/	Lattice background table feeding CLASS

The N-body and CLASS branches share the same HQIV background parameters (H₀=73.2, baryon-only Ω_m, γ=0.40, α=0.6) but test different observables: Bullet offset/lensing vs CMB peaks and σ₈.

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License

MIT — see parent HQIV repo. PySCo: see pysco/LICENSE.

References

1. Ettinger (2026), HQIV paper — paper/main.tex in parent repo
2. Clowe et al. (2006), Bullet Cluster weak lensing
3. PySCo: Breton et al., https://github.com/mianbreton/pysco