A CMB Millikan Experiment with Cosmic Axiverse Strings

Understand

We study axion strings of hyperlight axions coupled to photons.

• Hyperlight axions -- axions lighter than Hubble at recombination -- are a generic prediction of the string axiverse.
• These axions strings produce a distinct quantized polarization rotation of CMB photons which is $\mathcal{O}(\alpha_{\rm em})$.
• As the CMB light passes many strings, this polarization rotation converts E-modes to B-modes and adds up like a random walk.