On the Tolman-Hawking wormhole

The Tolman-Hawking wormhole $a={(R^2+{\tau }^2)}^{\frac{1}{2}}$ can be obtained as a solution of the Einstein equations with a conformally coupled scalar field. However, this procedure gives rise to negative values of the effective gravitational constant. Working in a pure-gravity minisuperspace isotropic model, I propose a much simpler method to obtain this instanton based on the introduction of a cutoff in the scale factor which is equivalent to introducing a three-sphere with minimum constant radius in the Euclidean metric. The resulting wormhole model no longer has any negative effective gravitational constant.