4.2 The Model 49 Fig. 4.2: The potential given by Eq. (4.5). The zero temperature potential is To obtain the VEV of φ , we find the minimum of the zero temperature potential. The VEV is Arbitrary Units V ( φ, 0 , ψ ) = V$_{0}$ − 1 2 m $^{2}$φ$^{2}$+ λ φ 2 n +4 M 2 n P + 1 2 m 2 $_{ψ}$ψ 2 (4.6) 〈 φ 〉 =   mM n P √ (2 n + 4) $_{λ}$  1 n +1 (4.7) background theory, such cosmic strings may well be unstable. Thus, we ignore them.