LATTICEEASY: A program for lattice simulations of scalar fields in an expanding universe

We describe a C++ program that we have written and made available for calculating the evolution of interacting scalar fields in an expanding universe. The program is particularly useful for the study of reheating and thermalization after inflation. The program and its full documentation are available on the Web at http://www.science.smith.edu/departments/Physics/fstaff/gfelder/latticeeasy/. In this paper we provide a brief overview of what the program does and what it is useful for. Program summary Program title: LATTICEEASY Catalog identifier: AEAW_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEAW_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 2579 No. of bytes in distributed program, including test data, etc.: 34 521 Distribution format: tar.gz Programming language: C++ Computer: Any Operating system: Any RAM: Typically 4 MB to 800 MB Classification: 1.9 Nature of problem: After inflation the universe consisted of interacting fields in a high energy, nonthermal state [1]. The evolution of these fields can not be described with standard approximation techniques such as linearization, kinetic theory, or Hartree expansion, and must thus be simulated numerically. Fortunately, the fields rapidly acquire large occupation numbers over a range of frequencies, so their evolution can be accurately modeled with classical field theory [2]. The specific fields and interactions relevant at these high energies are not known, so different models must be tested phenomenologically. Solution method: LATTICEEASY solves the equations of motion for interacting scalar fields in an expanding universe. The user describes a particular theory by entering the field potential and its derivatives in a "model file" and the program then uses a staggered leapfrog method to evolve the field equations and Friedmann equation for the fields and the expansion of the universe. Restrictions: In its current form LATTICEEASY only includes scalar fields and does not include metric perturbations. Running time: The running time can range from minutes to weeks.