PELYCAN

PEculiar veLocitY for Cosmological ANalysis

A modular Python toolkit to model, analyze, and compare pairwise velocities and dispersions.

Developed by the PAIRS team

pelycan implements one scientific pipeline end-to-end:

Input: cosmological parameters (h, Ωc h², lnAₛ) + N-body simulation data v₁₂(r)

Output: posterior constraints on those parameters

The physics chain, implemented in pelycan/model/: CAMB → P(k) → ξ(r) → ξ̄(r) → ∂ξ̄/∂a → v₁₂(r) = −H(a)·a²·r·∂ξ̄/∂a / [3·(1+ξ)] The theoretical v₁₂(r) is compared to simulation measurements via a Gaussian likelihood (diagonal errors or full covariance matrix). Two samplers run this: emcee for single-parameter chi² spot checks, Cobaya for production joint inference with MPI.

Features

• Predict mean pairwise velocity $v_{12}(r,a)$ and dispersion (coming soon!)
• Compare theoretical predictions to simulations
• Run parameter inference using Cobaya

Project Structure

pelycan/
|
├─ pelycan/
│  └─ model            # Core model code
|
├─ main/
│  └─ run_pelycan.py   # Quickstart script to run the model
|
├─ tests/              # Unit tests
|
└─ data/               # Post-processed data containing v12, sigma12 and errors

Installation (work in progress)

Currently, pelycan can be installed directly from source.

Standard installation from source:

Clone the repository and install with pip:

git clone https://github.com/PAIRSteam/pelycan.git
cd pelycan
pip install .

Development installation

To modify or contribute to the code, install in editable mode:

git clone https://github.com/PAIRSteam/pelycan.git
cd pelycan
pip install -e .

Running the Code (work in progress)

Quickstart: run the model for one cosmology/redshift and plot v₁₂(r), no simulation data needed (run from the repo root):

python main/run_pelycan.py

For a full walkthrough (loading simulation data, comparing linear vs. non-linear vs. quasi-linear variants), see main/tutorial.ipynb.

Tests (in progress)

pip install pytest
pytest tests/

Contributing

Please:

• Use feature branches for development
• Write clear commit messages and PRs
• Open issues for bugs, questions, or ideas

Citation

If you use this code, please cite the papers:

• Jaber M., Hellwing W.~A., Garcia-Farieta J.~E., Gupta S., Bilicki M., 2024, PhRvD, 109, 123528. doi:10.1103/PhysRevD.109.123528
• ...

Authors (under construction)

• Mariana Jaber, Github mjaber, INSPIRE Profile
• Antonela Taverna, Github antotaverna, ORCID Profile
• Rasmus Strid, Github R-Strid