Gravitational waves generated by second order effects during inflation
  The generation of gravitational waves during inflation due to the non-linear coupling of scalar and tensor modes is discussed. Two methods describing gravitational wave perturbations are used and compared: a covariant and local approach, as well as a metric-based analysis based on the Bardeen formalism. An application to slow-roll inflation is also described. 
Bob Osano, Cyril Pitrou, Peter Dunsby, Jean-Philippe Uzan, and Chris   Clarkson