Primordial gravitational waves from spontaneous breaking of discrete symmetries

Strong evidence in support of stochastic gravitational wave background has been found recently in pulsar timing array (PTA) measurements. I will discuss how this background can be interpreted in terms of primordial gravitational waves emitted by extended topological objects in the early Universe, i.e., domain walls. The latter generically arise in systems exhibiting spontaneous breaking of discrete symmetries. I will describe two types of domain walls depending on the nature of a vacuum expectation value responsible for the symmetry breaking. Conventional domain walls correspond to the case of a constant expectation value. Less commonly discussed is the situation, where the expectation value decreases proportionally to the Universe temperature, in which case one deals with the so called melting domain walls. I will demonstrate that gravitational wave signatures of melting domain walls are in a particular good agreement with pulsar timing experiments.Strong evidence in support of stochastic gravitational wave background has been found recently in pulsar timing array (PTA) measurements. I will discuss how this background can be interpreted in terms of primordial gravitational waves emitted by extended topological objects in the early Universe, i.e., domain walls. The latter generically arise in systems exhibiting spontaneous breaking of discrete symmetries. I will describe two types of domain walls depending on the nature of a vacuum expectation value responsible for the symmetry breaking. Conventional domain walls correspond to the case of a constant expectation value. Less commonly discussed is the situation, where the expectation value decreases proportionally to the Universe temperature, in which case one deals with the so called melting domain walls. I will demonstrate that gravitational wave signatures of melting domain walls are in a particular good agreement with pulsar timing experiments.