  Within an effective field theory derived from string theory, the universal
axion has to be coupled to the the gravitational Chern-Simons (gCS) term.
During any era when the axion field is varying, the vacuum fluctuation of the
gravitational wave amplitude will then be circularly polarised, generating an
expectation value for the gCS term. The polarisation may be observable through
the Cosmic Microwave Background, and the vacuum expectation value of the gCS
term may generate the baryon asymmetry of the Universe. We argue here that such
effects cannot be computed without further input from string theory, since the
`vacuum' in question is unlikely to be the field-theoretic one.
