Singular Instantons and Extra Dimensions

Hawking and Turok (HT) have recently proposedthat an open universe can be created from nothing. Theinstanton describing this process is singular, andtherefore its validity has been subject to question. In particular, Vilenkin has shown that aninstanton with the same singular structure as Hawkingand Turok's would lead to the unsuppressed decay of flatspace. However, Vilenkin's solution can be seen as the dimensional reduction of a five-dimensionalnonsingular instanton. In this context the unsuppressedinstability of flat space can be traded formetastability with a low decay rate, provided that the size of the extra dimension is large comparedwith the Planck scale. Implications for the HT model arediscussed.