A CMB Millikan experiment with cosmic axiverse strings

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 $$ \mathcal{O} $$ O (αem). 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.

Polarimetric signatures of the photon ring of a black hole that is pierced by a cosmic axion string

“…A similar way to measure (or constrain) A via the same effect was recently proposed in different setups [44] (see also [102]). 13 The value of A that we can expect from theoretical considerations depends on the theoretical model that gave rise to the anomaly; see e.g.…”

“…We shall argue that global hyper-light axion-type cosmic strings that pierce a black hole can give rise to characteristic polarization patterns in the photon ring of the black hole. These patterns emerge due to the coupling φFF between the Goldstone boson (axion) φ and the photon as this coupling leads to a change of the polarization direction of linearly polarized photons when they circle the black hole (and the string) [44]. The change of the polarization direction is generated by a topological Aharonov-Bohm type effect.…”

“…The change of the polarization direction is generated by a topological Aharonov-Bohm type effect. It only depends on the total angle that a photon has passed when circling the black hole (and the string) and on the coefficient C in the coupling C 2 φ v FF [44]. 4 In models with charged fermions and Yukawa interactions between φ and the fermions, the value of C is set by the coefficient of the mixed anomaly of electromagnetism and the (spontaneously broken) symmetry that gave rise to the cosmic string.…”

“…provide us with a new way of determining the value of this anomaly coefficient. Recently, a potential way to measure the anomaly coefficient via the same method was proposed in different setups in [44]. 5 We shall work out polarization patterns in the photon ring of a pierced black hole both in time-averaged setups and, in a simple example, in setups with time dependent light sources.…”

“…13 The value of A that we can expect from theoretical considerations depends on the theoretical model that gave rise to the anomaly; see e.g. [44] which includes a short summary of what values of A we can expect in various models where the anomaly is obtained by integrating out fermions in the UV (see also [109] for a discussion of this kind for minimal GUT models). Knowledge of A could help us to learn a lot about the UV model that is realized in nature.…”