Alberto Mariotti (VUB Brussels)

I will introduce basics aspects of cosmic defects, how they arise in cosmological phase transitions, and their implications for cosmology and gravitational waves.
I will then revisit the case of metastable defects, both in axion models and in theories with symmetry-breaking patterns analogous to the electroweak sector of the Standard Model. Next, I will focus on metastable strings arising in a concrete dark sector model, which have recently been considered as candidates for explaining the gravitational-wave background observed by pulsar timing arrays (PTA). I will discuss how they can decay quantum mechanically, and I will then present an analysis of their classical stability, where I will show that classical instabilities can affect the parameter space relevant for explaining the PTA signal. I will conclude by outlining future directions in the study of metastable defects.