B-modes from Post-inflationary Gravitational Waves Sourced by Axionic Instabilities at Cosmic Reionization

Understand

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.