"The current universe appears to be unreasonably homogeneous and flat. The standard explanation for this is ""inflation"": a phase of accelerated growth in the very early universe. Inflation must come to an end so that stars and galaxies can form; this transition is called ""reheating"". Straightforward calculations and simulations during reheating are intractable due to the timescales involved and formation of large overdensities. We showed that during reheating, these difficulties can be avoided by approximating the Klein-Gordon equations with the Schrodinger-Poisson equations. This allows the adoption of existing techniques and code from ultralight dark matter. We were able to do the first simulations of large overdensities forming during reheating. By showing how to do calculations during reheating, we have opened up new possibilities in the study of the early universe."