PSPECTRE: a pseudo-spectral code for (P)reheating

PSPECTRE is a C++ program that uses Fourier-space pseudo-spectral methods to evolve interacting scalar fields in an expanding universe. PSPECTRE is optimized for the analysis of parametric resonance in the post-inflationary universe and provides an alternative to finite differencing codes, such as Defrost and Lattice Easy. PSPECTRE has both second- (Velocity-Verlet) and fourth-order (Runge-Kutta) time integrators. Given the same number of spatial points and/or momentum modes, PSPECTRE is not significantly slower than finite differencing codes, despite the need for multiple Fourier transforms at each timestep, and exhibits excellent energy conservation. Further, by computing the post-resonance equation of state, we show that in some circumstances PSPECTRE obtains reliable results while using substantially fewer points than a finite differencing code. PSPECTRE is designed to be easily extended to other problems in early-universe cosmology, including the generation of gravitational waves during phase transitions and pre-inflationary bubble collisions. Specific applications of this code will be described in future work.