COBI: Cosmic Birefringence Analysis Pipeline

A comprehensive Python package for cosmic birefringence analysis using Simons Observatory Large Aperture Telescope (LAT) and Small Aperture Telescopes (SAT) data.

Overview

COBI provides end-to-end functionality for detecting and characterizing cosmic birefringence—a parity-violating effect that rotates the polarization plane of CMB photons. This rotation can arise from physics beyond the Standard Model, including axion-like particles and Chern-Simons modifications to electromagnetism.

Key Features

🌌 Full-sky CMB Simulation: Generate realistic CMB maps with cosmic birefringence effects
🎨 Foreground Modeling: Galactic dust and synchrotron emission with spatial templates
📊 Power Spectrum Analysis: Pseudo-Cℓ computation with NaMaster mode-coupling correction
🔍 Quadratic Estimator: Reconstruct birefringence angle maps from EB correlations
📈 Maximum Likelihood Estimation: Fit calibration angles and birefringence amplitudes
🚀 MPI Support: Parallel processing for large-scale simulations
🔗 Cross-correlation: LAT-SAT joint analysis for systematic mitigation

Installation

Prerequisites

COBI requires several dependencies with C extensions. We recommend using conda:

# Clone the repository
git clone https://github.com/antolonappan/cobi.git
cd cobi

# Create conda environment
conda env create -f conda/environment_with_build.yml
conda activate cobi

# Install COBI
pip install -e .

Quick Start Examples

1. Simulate CMB with Birefringence

from cobi.simulation import CMB
import matplotlib.pyplot as plt

# Isotropic birefringence: constant rotation angle
cmb = CMB(
    libdir='./sims',
    nside=2048,
    model='iso',
    beta=0.35,  # rotation angle in degrees
    lensing=True,
    verbose=True
)

# Visualize EB power spectrum (signature of birefringence)
plt.loglog(cmb.get_cb_lensed_spectra(beta=0.35)['eb'], label='β=0.35°')
plt.xlabel('$\\ell$')
plt.ylabel('$C_\\ell^{EB}$')

2. Complete Sky Simulation

from cobi.simulation import LATsky

# LAT observation simulation with systematics
lat = LATsky(
    libdir='./sims',
    nside=2048,
    cb_model='iso',
    beta=0.35,
    alpha=[-0.1, -0.1, 0.2, 0.2, 0.15, 0.15],  # miscalibration per freq
    alpha_err=0.1,
    bandpass=True,
    noise_model='NC',
    verbose=True
)

# Get observed Q/U maps
QU_27 = lat.obsQU(idx=0, band='27-1')

3. Calibration Analysis

from cobi.simulation import LATskyC, SATskyC
from cobi.spectra import Spectra
from cobi.calibration import Sat4Lat

# Setup simulations
lat = LATskyC(libdir='./sims', nside=2048, cb_model='iso', beta=0.35)
sat = SATskyC(libdir='./sims', nside=2048, cb_model='iso', beta=0.35)

# Compute spectra
lat_spec = Spectra(lat, libdir='./spec', galcut=40, binwidth=5)
sat_spec = Spectra(sat, libdir='./spec', galcut=40, binwidth=5)

# Fit birefringence
calib = Sat4Lat(
    libdir='./calib',
    lmin=100,
    lmax=3000,
    latlib=lat_spec,
    satlib=sat_spec,
    sat_err=0.1,
    beta=0.35
)

calib.plot_getdist(nwalkers=100, nsamples=2000)

4. Quadratic Estimator Reconstruction

from cobi.quest import FilterEB, QE

# Setup filtering
filt = FilterEB(lat, mask, lmax=3000, sht_backend='ducc')

# Quadratic estimator reconstruction
qe = QE(filt, lmin=100, lmax=3000, recon_lmax=2048)

# Reconstruct birefringence map
beta_lm_recon = qe.qlm(idx=0) - qe.mean_field()

Documentation Contents

API Documentation

API Reference

Project Information

Author:: Anto I. Lonappan
Contributors:: Patricia Diego-Palazuelos, Carlos Hervías-Caimapo
License:: MIT
Repository:: https://github.com/antolonappan/cobi
Documentation:: https://cobi.readthedocs.io/

Citation

If you use COBI in your research, please cite:

@software{cobi2024,
  author = {Lonappan, Anto I.},
  title = {COBI: Cosmic Birefringence Analysis Pipeline},
  year = {2024},
  url = {https://github.com/antolonappan/cobi}
}

Indices and Tables

Note

This project is under active development. Features and API may change in future releases.