  We study the generation and evolution of second-order energy-density
perturbations arising from primordial gravitational waves. Such "tensor-induced
scalar modes" approximately evolve as standard linear matter perturbations and
may leave observable signatures in the Large-Scale Structure of the Universe.
We study the imprint on the matter power-spectrum of some primordial models
which predict a large gravitational-wave signal at high frequencies. This novel
mechanism in principle allows us to constrain or detect primordial
gravitational waves by looking at specific features in the matter or galaxy
power-spectrum, thereby allowing to probe them on a range of scales unexplored
so far.
