A CMB Millikan Experiment with Cosmic Axiverse Strings

TL;DR

This paper investigates how hyperlight axion strings produce a quantized polarization rotation of CMB photons, which can be detected through B-mode polarization measurements, revealing insights into fundamental physics and the early universe.

Contribution

It introduces a novel method to detect axion strings via CMB polarization rotation, linking topological properties to observable signals and providing a way to probe UV physics.

Findings

Polarization rotation from axion strings is within current experimental reach.

Rotation angle depends only on the anomaly coefficient, not on other axion parameters.

Potential detection of axion strings through CMB and gravitational lensing measurements.

Abstract

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. Using numerical simulations we show that the expected size of the final result is well within the reach of current and future CMB experiments through the measurement of correlations of CMB B-modes with E- and T-modes. The quantized polarization rotation angle is topological in nature and can be seen as a geometric phase. Its value depends only on the anomaly coefficient and is independent of other details such as the axion decay constant. Measurement of the anomaly coefficient by measuring this rotation will provide information about the UV theory, such as the quantization of electric charge and the value of the fundamental unit of charge. The presence of axion strings in the universe relies only on a phase transition in the early universe after inflation, after which the string network rapidly approaches an attractor scaling solution. If there are additional stable topological objects such as domain walls, axions as heavy as $10^{-15}$ eV would be accessible. The existence of these strings could also be probed by measuring the relative polarization rotation angle between different images in gravitationally lensed quasar systems.