CMB birefringence from ultra-light axion string networks

Understand

The polarization of Cosmic Microwave Background (CMB) photons is rotated as they pass through (ultralight-) axion string loops.

• Studying this birefringence can reveal valuable information about the axion-photon coupling and the structure of the string network.
• We develop an approximate analytic formalism and identify a kernel function that can be used to calculate the two-point correlation function for CMB birefringence induced by an arbitrary axion string network.
• Using this formalism, we evaluate the birefringence signal for some simple loop distributions (including scaling and network collapse).