Past exam of the mathematics course of the University of Cambridge / 2009 / iii / Paper 55 / 1 / b / 2 / Solution

The scale factor of the cosmological domain wall network is $a\propto t^2$, so $\ddot{a}>0$: the expansion accelerates. Equivalently the Friedmann acceleration equation has $\rho +3P=\rho (1+3w)=-\rho <0$ for $w=-2/3$.