Signature of Gravity Waves in Polarization of the Microwave Background

Using spin-weighted decomposition of polarization in the Cosmic Microwave Background (CMB) we show that a particular combination of Stokes $Q$ and $U$ parameters vanishes for primordial fluctuations generated by scalar modes, but does not for those generated by primordial gravity waves. Because of this gravity wave detection is not limited by cosmic variance as in the case of temperature fluctuations. Numerical evaluation in inflation-based models shows that the expected signal for tensor to scalar ratio of unity is of the order of 0.5 $\mu K$, which could be directly tested in future CMB experiments.