B-modes from Post-inflationary Gravitational Waves Sourced by Axionic
  Instabilities at Cosmic Reionization
  We show that axion-like particles that only couple to invisible dark photons
can generate visible B-mode signals around the reionization epoch. The axion
field starts rolling shortly before reionization, resulting in a tachyonic
instability for the dark photons. This generates an exponential growth of the
dark photon quanta sourcing both scalar metric modes and gravitational waves
that leave an imprint on the reionized baryons. The tensor modes modify the
cosmic microwave background (CMB) polarization at reionization, generating
visible B-mode signatures for the next generation of CMB experiments for
parameter ranges that satisfy the current experimental constraints.
