Abstract
Abstract
The evidence for a non-vanishing isotropic cosmic birefringence in recent analyses of the
CMB data provides a tantalizing hint for new physics. Domain wall (DW) networks have recently
been shown to generate an isotropic birefringence signal in the ballpark of the measured value
when coupled to photons. In this work, we explore the axionic defects hypothesis in more detail
and extending previous results to annihilating and late-forming networks, and by pointing out
other smoking-gun signatures of the network in the CMB spectrum such as the anisotropic
birefringent spectrum and B-modes. We also argue that the presence of cosmic strings in the
network does not hinder a large isotropic birefringence signal because of an intrinsic
environmental contribution coming from low redshifts thus leaving open the possibility that
axionic defects can explain the signal. Regarding the remaining CMB signatures, with the help of
dedicated 3D numerical simulations of DW networks, that we took as a proxy for the axionic
defects, we show how the anisotropic birefringence spectrum combined with a tomographic approach
can be used to infer the formation and annihilation time of the network. From the numerical
simulations, we also computed the spectrum of gravitational waves (GWs) generated by the network
in the post-recombination epoch and use previous searches for stochastic GW backgrounds in the CMB
to derive for the first time a bound on the tension and abundance of networks with DWs that
annihilate after recombination. Our bounds extend to the case where the network survives until the
present time and improve over previous bounds by roughly one order of magnitude. Finally, we show
the interesting prospects for detecting B-modes of DW origin with future CMB experiments.