  We show that, in the first order gravity theory coupled to axions, the
instanton number of the Giddings-Strominger wormhole can be interpreted as the
Nieh-Yan topological index. The axion charge of the baby universes is quantized
in terms of the Nieh-Yan integers. Tunneling between universes of different
Nieh-Yan charges implies a nonperturbative vacuum state. The associated
topological vacuum angle can be identified with the Barbero-Immirzi parameter.
