What would we learn by detecting a gravitational wave signal in the   cosmic microwave background anisotropy?
  Inflation generates gravitational waves, which may be observable in the low multipoles of the cosmic microwave background (cmb) anisotropy but only if the inflaton field variation is at least of order the Planck scale. Such a large variation would imply that the model of inflation cannot be part of an ordinary extension of the standard model, and combined with the detection of the waves it would also suggest that the inflaton field cannot be one of the superstring moduli. Another implication of observable gravitational waves would be a potential $V^{1/4}=2$ to $4\times 10^{16}\GeV$, which is orders of magnitude bigger than is expected on the basis of particle theory. It might emerge in a hybrid inflation model where most of the energy density comes from the Higgs sector of a GUT, but only if both the vacuum expectation values {\em and the masses } of the Higgs fields are of this order. 
David H. Lyth (Lancaster)