Observational signatures of the parametric amplification of
  gravitational waves during reheating after inflation
  We study the evolution of Gravitational Waves (GWs) during and after
inflation as well as the resulting observational consequences in a
Lorentz-violating massive gravity theory with one scalar (inflaton) and two
tensor degrees of freedom. We consider two explicit examples of the tensor mass
$m_g$ that depends either on the inflaton field $\phi$ or on its time
derivative $\dot{\phi}$, both of which lead to parametric excitations of GWs
during reheating after inflation. The first example is Starobinsky's $R^2$
inflation model with a $\phi$-dependent $m_g$ and the second is a
low-energy-scale inflation model with a $\dot{\phi}$-dependent $m_g$. We
compute the energy density spectrum $\Omega_{\rm GW}(k)$ today of the GW
background. In the Starobinsky's model, we show that the GWs can be amplified
up to the detectable ranges of both CMB and DECIGO, but the bound from the big
bang nucleosynthesis is quite tight to limit the growth. In low-scale inflation
with a fast transition to the reheating stage driven by the potential
$V(\phi)=M^2 \phi^2/2$ around $\phi \approx M_{\rm pl}$ (where $M_{\rm pl}$ is
the reduced Planck mass), we find that the peak position of $\Omega_{\rm
GW}(k)$ induced by the parametric resonance can reach the sensitivity region of
advanced LIGO for the Hubble parameter of order 1 GeV at the end of inflation.
Thus, our massive gravity scenario offers exciting possibilities for probing
the physics of primordial GWs at various different frequencies.
