MultiModeCode: An efficient numerical solver for multifield inflation

TL;DR

MultiModeCode is a new, efficient Fortran package designed for large-scale numerical analysis of multifield inflation models, enabling detailed exploration of their perturbations and spectra.

Contribution

It is the first publicly available code capable of efficiently generating large sample sets for models with around 100 fields, including perturbation calculations and slow-roll predictions.

Findings

Successfully explores toy models of multifield inflation

Provides detailed power spectra including isocurvature modes

Addresses numerical stability near the adiabatic limit

Abstract

We present MultiModeCode, a Fortran 95/2000 package for the numerical exploration of multifield inflation models. This program facilitates efficient Monte Carlo sampling of prior probabilities for inflationary model parameters and initial conditions and is the first publicly available code that can efficiently generate large sample-sets for inflation models with $\mathcal O(100)$ fields. The code numerically solves the equations of motion for the background and first-order perturbations of multi-field inflation models with canonical kinetic terms and arbitrary potentials, providing the adiabatic, isocurvature, and tensor power spectra at the end of inflation. For models with sum-separable potentials MultiModeCode also computes the slow-roll prediction via the $\delta N$ formalism for easy model exploration and validation. We pay particular attention to the isocurvature perturbations as the system approaches the adiabatic limit, showing how to avoid numerical instabilities that affect some other approaches to this problem. We demonstrate the use of MultiModeCode by exploring a few toy models. Finally, we give a concise review of multifield perturbation theory and a user's manual for the program.