Gravitational waves from inflation
  The production of a stochastic background of gravitational waves is a
fundamental prediction of any cosmological inflationary model. The features of
such a signal encode unique information about the physics of the Early Universe
and beyond, thus representing an exciting, powerful window on the origin and
evolution of the Universe. We review the main mechanisms of gravitational-wave
production, ranging from quantum fluctuations of the gravitational field to
other mechanisms that can take place during or after inflation. These include
e.g. gravitational waves generated as a consequence of extra particle
production during inflation, or during the (p)reheating phase. Gravitational
waves produced in inflation scenarios based on modified gravity theories and
second-order gravitational waves are also considered. For each analyzed case,
the expected power-spectrum is given. We discuss the discriminating power among
different models, associated with the validity/violation of the standard
consistency relation between tensor-to-scalar ratio $r$ and tensor spectral
index $n_{\rm T}$. In light of the prospects for (directly/indirectly)
detecting primordial gravitational waves, we give the expected present-day
gravitational radiation spectral energy-density, highlighting the main
characteristics imprinted by the cosmic thermal history, and we outline the
signatures left by gravitational waves on the Cosmic Microwave Background and
some imprints in the Large-Scale Structure of the Universe. Finally, current
bounds and prospects of detection for inflationary gravitational waves are
summarized.
