MULTIMODECODE: an efficient numerical solver for multifield inflation

Abstract

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