The Einstein equations for an FRW universe determine the evolution of
the scale factor . In section 6.2.1 we derive the
particular combination of these equations used by the program and then
convert this equation to program variables. There is a difficulty that
arises in solving this equation using the staggered leapfrog algorithm
employed by LATTICEEASY, though, because the equation contains first
derivative terms. This problem is explained in detail in section
6.4. Section 6.2.2 presents a revised formula
that solves the problem. Finally, section 6.2.3 derives the
equations used for fixed power-law expansion.